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Background Increasing zinc (Zn) concentrations in maize grains could contribute to alleviating widespread human Zn deficiency in sub-Saharan Africa (SSA). However, trade-offs between grain Zn concentrations and maize yields have been observed. Scope Using data from researcher-managed, on-farm and on-station field trials in Kenya, Zambia and Zimbabwe, we aimed (i) to confirm whether this trade-off is found in current farming systems in SSA and (ii) to explore whether genotypic and management options, relevant for the African context, can increase both yields and grain Zn concentrations across several environments. Results An overall negative, but weak relation between maize yields and grain Zn concentrations was found. High yields and high grain Zn concentrations did not co-occur. The negative relation between grain Zn concentrations and yields cannot be bypassed by selecting one of the commercially available varieties used in this study. Nitrogen application increased yields, but had contrasting effects on grain Zn concentrations depending on variety and site. Grain Zn concentrations were positively related with soil organic carbon and P and K availability. Conclusions Attaining grain Zn concentrations above the HarvestPlus target of 38 mg kg−1, considered adequate for reducing human Zn deficiency, with current commercially available maize varieties and presented management options, is not possible without compromising yield levels. Increasing soil organic matter content and balanced application of N, P and K fertilisers could increase grain Zn concentrations. These practices likely will also increase yields and could be a viable option to bypass the trade-off between maize yields and grain Zn concentrations.

Agricultural soils are at risk of nitrogen (N) leaching especially during the fallow period in autumn and winter. Cover crops are grown to capture soil mineral N that otherwise would leach to the groundwater. They can serve as green manure providing mineral N to the cash crop in spring. We investigated whether mixing species of cover crops can enhance N capture and therefore reduce N leaching more effectively than pure stands in autumn without increasing the risk of N leaching in spring. We hypothesised that mixed species with complementary traits will capture more N and accumulate more biomass. It was also expected that residues from cover crops with higher biomass and lower C:N ratio would mineralise faster and subsequently increase N leaching in spring. In a 4-year field experiment, cover crops were grown between August and February in a rotation with different cash crops. We used eight cover crop treatments, including three pure stands: radish (Raphanus sativus), vetch (Vicia sativa) and oats (Avena strigosa), all possible 2- and 3-species mixtures and a fallow (no cover crop). Treatment effects on leaching losses were estimated by analysing N concentrations in samples of leached pore water below the rooting zone and by modelling the volume of water leached per plot. Most N leaching occurred in autumn and winter while the amount of N leached in spring was negligible due to the lower precipitation. N leaching in autumn correlated negatively with cover crop biomass, N uptake and root length density. Radish and oats were the most productive species and dominated mixtures. Compared to the fallow, radish and mixtures that con- tained radish reduced N leaching by 49–73% and were characterized by quick soil cover, high N uptake and low to moderate C:N ratio. Subsequently, residues from radish and mixtures containing radish mineralized quickly, resulting in an increase in soil mineral N in spring by 70–110% as compared to fallow. This mineral N did not leach in spring and was available to the subsequent cash crop. This study demonstrates the importance of species selection in cover crop mixtures and recommends the use of radish-based mixtures if the purpose is to reduce N leaching in autumn and provide mineral N in spring.

AIMS Fertilisation of crops with zinc (Zn) is considered important to enhance agricultural productivity and combat human deficiencies in sub-Saharan Africa. However, it is unclear on which soils Zn fertilisation can lead to higher yields and increased grain Zn concentrations. This study aimed to find soil properties that predict where soil Zn is limiting maize yields and grain Zn concentrations, and where these respond positively to Zn fertilisation. METHODS Zinc omission trials were set up at multiple farm locations in Kenya (n=5), Zambia (n=4) and Zimbabwe (n=10). Grain yields and tissue Zn concentrations were analysed from plots with a full fertiliser treatment as compared to plots where Zn was omitted. RESULTS Zinc uptake (R2 = 0.35) and grain Zn concentrations (R2=0.26) on the plots without Zn fertiliser could be related to a limited extend to soil Zn measured in extractions that measure labile Zn. A positive maize yield response to soil Zn fertilisation was found at only two out of nineteen locations, despite soil Zn levels below previously derived critical concentrations at most locations. Soil properties nor plant concentrations were able to explain maize yield response to Zn fertilisation. However, a positive response in Zn uptake and grain Zn concentrations to Zn fertilisation was found at the majority of sites. CONCLUSIONS We conclude that soil Zn fertilisation can increase maize grain Zn concentrations, especially in soils with low pH and organic carbon content. Predicting a yield response to Zn fertilisation based on soil properties remains a challenge.

Multi-element soil extractions such as Mehlich 3 (M3) have gained popularity in recent years, but comparing outcomes to other soil testing methods is not always straightforward. In this study, extraction mechanisms of M3, Olsen and neutral 1 M ammonium acetate (AA) soil tests were explored and transfer functions were derived between P-Olsen and P-M3 as well as between K-AA and K-M3. Soils from tropical and temperate areas were used to derive these P and K transfer functions and were evaluated separately. The application of these transfer functions for tropical soils was evaluated by using them as input for the Quantitative Evaluation of the Fertility of Tropical Soils (QUEFTS). AA and M3 generally extracted similar amounts of K, but relations between K-AA and K-M3 were different for tropical and temperate soils. For tropical soils, the transfer function did not require additional parameters besides K-M3 to predict K-AA, but for temperate soils inclusion of clay content and pH was needed. This difference between tropical and temperate soils was explained by clay mineralogy. The relation between P-Olsen and P-M3 in tropical soils was found to be dependent on pH, Al-M3, Fe-M3 and Ca-M3. P-Olsen and K-AA values, calculated with their respective transfer functions, were used as input for QUEFTS. The yields predicted with measured P-Olsen and Exch. K were used as benchmark. For 63 out of 81 soil samples, predicted maize yields with transfer functions deviated less than 10% from the benchmark. The largest deviations from the benchmark were found for low P-Olsen and K-AA values, which corresponds to QUEFTS maize yield predictions up to 3000 kg ha−1. We conclude that a M3 extraction results and soil pH can reliably be transferred to, and thus replace P-Olsen and K-AA determinations with the functions developed for tropical soils. The transfer functions can be used to generate input for the QUEFTS model with minor effects on yield predictions, thus expanding its applicability in cases where only M3 extraction results are available.

Plant species have different traits for mobilizing sparingly soluble phosphorus (P) resources, which could potentially lead to overyielding in P uptake by plant species mixtures compared to monocultures due to higher P uptake as a result of resource (P) partitioning and facilitation. However, there is circumstantial evidence at best for overyielding as a result of these mechanisms. Overyielding (the outcome) is easily confused with underlying mechanisms because of unclear definitions. We aimed to define a conceptual framework to separate outcome from underlying mechanisms and test it for facilitation and complementarity with respect to P acquisition by three plant species combinations grown on four soils. Our conceptual framework describes both mechanisms of complementarity and facilitation and outcomes (overyielding of mixtures or no overyielding) depending on the competitive ability of the species to uptake the mobilized P. Millet/chickpea mixtures were grown in pots on two calcareous soils mixed with calcium-bound P (CaP) and phytate P (PhyP). Cabbage/faba bean mixtures were grown on both acid and neutral soils mixed with P-coated iron (hydr)oxide (FeP) and PhyP. Wheat/maize mixtures were grown on all four soils. Rhizosphere carboxylate concentration and acid phosphatase activity (mechanisms) as well as plant P uptake and biomass (outcome) were determined for monocultures rhizosphere and species mixtures. Facilitation of P uptake occurred in millet/chickpea mixtures on one calcareous soil. We found no indications for P acquisition from different P sources, neither in millet/chickpea, nor in cabbage/faba bean mixtures. Cabbage and faba bean on the neutral soil differed in rhizosphere acid phosphatase activity and carboxylate concentration, but showed no overyielding. Wheat and maize, with similar root exudates, showed overyielding (the observed P uptake being 22% higher than the expected P uptake) on one calcareous soil. We concluded that although differences in plant physiological traits (root exudates) provide necessary conditions for complementarity and facilitation with respect to P uptake from different P sources, they do not necessarily result in increased P uptake by species mixtures, because of the relative competitive ability of the mixed species.

Aims: The benefits of arbuscular mycorrhizal fungi (AMF) on yield and phosphorus (P) uptake of crops have commonly been studied by inoculating a single mycorrhizal fungal species in pot experiments. Yet, how the native AMF community affects the performance of different maize varieties under field conditions remains obscure. Methods: In-growth cores with and without rotation were used in three soils that differed in P application to assess shoot biomass, P uptake, and mycorrhizal colonization of three maize varietal groups, encompassing four inbred lines, 12 hybrids, and four landraces. Results: Rotating cores drastically reduced mycorrhizal colonization, biomass and P uptake for each varietal group at every P level. Performance of plants at natural mycorrhizal colonization at 30 kg P ha−1 was equal to that of reduced-mycorrhizal plants at 60 kg P ha−1, suggesting the potential for adequate mycorrhizal management to save P fertilizer. Conclusion: There were no significant differences between varietal groups for mycorrhizal responsiveness, confirming that the ability to associate with and benefit from AMF has been maintained in modern breeding. Mycorrhizal plants both exhibited higher P acquisition efficiency and higher P use efficiency than reduced-mycorrhizal plants. Disadvantages of in-growth cores should be duly considered.

Aim Cereal/legume intercropping is known to increase yield, partly because of increased nitrogen (N) and phosphorus (P) acquisition. The aim of this paper was to investigate the role of common mycorrhizal networks (CMNs) in overyielding by the crop species mixture and to find out if the effect of a CMN depends on which of the two species was colonized by AM fungi.Methods Microcosms with two compartments were used, separated by a 30-μm nylon mesh. Both compartments contained either chickpea or millet, in monoculture or mixed. One or none of the two compartments was inoculated with the AMF species Funneliformis mosseae. The plant in the inoculated compartment was referred to as the AMF donor, and the plant in the neighboring, non-inoculated compartment as the AMF receiver. Results Inoculation in one compartment resulted in mycorrhiza formation in the other compartment, providing evidence for the formation of CMNs. Inoculation of chickpea in the mixture increased N and P acquisition and biomass of both chickpea (AMF donor) and millet (AMF receiver), whereas inoculation of millet increased biomass of chickpea (AMF receiver) only, but did not increase N or P acquisition by any of the two species. Chickpea as AMF donor had higher numbers of phosphate-solubilizing bacteria in its rhizosphere compared to chickpea as receiver. The shoot N:P ratio of chickpea as AMF donor was lower than as receiver. Conclusion Our study demonstrated asymmetry in nutrient gains by a mixture of cereal and a legume, dependent on which plant species was the AMF donor or receiver. This suggests that initiating mycorrhizal networks by legumes in intercropping could be an important factor contributing to the magnitude of the intercropping effect.

Organic inputs as a substitution for, or addition to, chemical fertilizers can potentially mitigate N losses. However, it is not well known how their effects on N leaching and runoff depend on application approaches. We conducted a global meta-analysis of 129 studies to compare the effects of organic inputs on N export via leaching and runoff. We compared three application approaches: chemical fertilizer N substituted by organic fertilizer with: 1) equal amounts of total N or, 2) equal amounts of mineral N and 3) additional organic fertilizer N on top of chemical fertilizer. The meta-analysis showed that organic inputs reduced overall N leaching and runoff by 15% and 29%, respectively, without compromising crop yield, and that this effect was significantly influenced by the application approach taken. Organic substitution of chemical fertilizer N with equal amounts of total N decreased both leaching and runoff by more than 30% and hardly affected crop yield. Substitution with equal amounts of mineral N generally increased crop yield by 6% but also increased N leaching by 21%. Organic inputs in addition to chemical fertilizer N did not affect leaching and runoff. The differences between application approaches were reinforced with increased treatment duration. The loss ratios of leaching and runoff were 14% and 4.5%, respectively, from chemical fertilizer, and 9.2% and 2.6%, respectively, from organic fertilizer. The optimal substitution rates differed between leaching (40–60%) and runoff (60–100%) when substitution was based on equal amounts of total N. We conclude that substitution of chemical for organic fertilizer at equal amounts of total N is most effective in reducing N export via leaching and runoff without compromising crop production.

Improving fertilizer recommendations for farmers is essential to increase food security in smallholder landscapes. Currently, blanket recommendations are provided across agro-ecological zones, although fertilizer response and nutrient use efficiency by maize crop are spatially variable. We aimed to identify factors that could help to refine fertilizer recommendation by analyzing the variability in fertilizer response (FR) and the agronomic nitrogen use efficiency (N-AE). A literature search for on-farm studies across Kenya and Sub-Sahara Africa (SSA), excluding Kenya, yielded 71 publications. The variability in FR was studied using a meta-analysis whereas key factors that influence FR and N-AE were studied with linear regression models. On average, the FR was 2, but it varied considerably from 1 to 28.5 (excluding outliers). In SSA, 18% of the plots were non-responsive plots with an FR

Intercropping is known to increase the efficiency of land use, but no meta-analysis has so far been made on the yield gain of intercropping compared to sole cropping in terms of absolute yield per unit area. Yield gain could potentially be related to a relaxation of competition, due to complementarity or facilitation, and/or to the competitive dominance of the higher yielding species. The contributions of competitive relaxation and dominance were here estimated using the concepts of complementarity effect (CE) and selection effect (SE), respectively. We compiled a dataset on intercropping of grain-producing crops from China, a hotspot of strip intercropping in the world. We quantified the yield gain and its components and analysed the contribution to yield gain of species traits (C3, C4, legume, non-legume), complementarity in time and nutrient input. Total yield in intercrops exceeded the expected yield, estimated on the basis of sole crop yields, by 2.14 ± 0.16 Mg ha−1 (mean ± standard error). Ninety percent of this yield gain was due to a positive CE while the remaining 10 % was due to SE. The net yield gain increased with temporal niche differentiation (TND) which is the proportion of the total growing period of the crop mixture during which species grow alone. The mechanism underlying yield gain shifted from competitive dominance of the higher yielding species when there was more overlap in growth period between the two species, to competitive relaxation when there was less overlap, while competitive relaxation remained the major component of the yield gain. The yield gain was substantially greater in intercrops with maize than in intercrops without maize, but there was no difference in yield gain between systems with and without legumes. The yield gain increased with nitrogen (N) input in maize/C3-cereal intercrops but not in cereal/legume intercrops, illustrating the ability of legumes to compensate for low N input, and highlighting the need for N input for high productivity in intercropping systems without legumes. Yield gain did not respond to phosphorus (P) input. We conclude that competitive relaxation is the main contributing factor to yield gain in the investigated Chinese intercropping systems, which were mostly relay strip intercropping systems. The underlying drivers of yield gain were related to presence of maize and species complementarity in time, but we did not find strong evidence for the selection effect.

Intercropping, the simultaneous production of multiple crops on the same field, provides opportunities for the sustainable intensification of agriculture if it can provide a greater yield per unit land and fertilizer than sole crops. The worldwide absolute yield gain of intercropping as compared with sole crops has not been analysed. We therefore performed a global meta-analysis to quantify the effect of intercropping on the yield gain, exploring the effects of crop species combinations, temporal and spatial arrangements, and fertilizer input. We found that the absolute yield gains, compared with monocultures, were the greatest for mixtures of maize with short-grain cereals or legumes that had substantial temporal niche differentiation from maize, when grown with high nutrient inputs, and using multirow strips of each species. This approach, commonly practised in China, provided yield gains that were (in an absolute sense) about four times as large as those in another, low-input intercropping strategy, commonly practised outside China. The alternative intercropping strategy consisted of growing mixtures of short-stature crop species, often as full mixtures, with the same growing period and with low to moderate nutrient inputs. Both the low- and high-yield intercropping strategies saved 16-29% of the land and 19-36% of the fertilizer compared with monocultures grown under the same management as the intercrop. The two syndromes of production in intercropping uncovered by this meta-analysis show that intercropping offers opportunities for the sustainable intensification of both high- and low-input agriculture.

This article provides a comprehensive and cross-disciplinary overview of the phosphorus cycle through the wastewater and agri-food system. While mineral phosphorus stocks are finite, the use of mined phosphorus is accompanied with many losses, leading to pollution of water bodies. Recovering phosphorus from human excreta can contribute to more efficient use of phosphorus to ensure its availability for food production in the future. Phosphorous can be recovered through different recovery technologies and consequently used in agriculture via different recycling routes. Each recycling route has its own particularities in terms of interactions with technologies, actors and the environment to bring the recovered phosphorus back into agriculture. In this literature review, we adopt a socio-ecological-technical approach to map three phosphorus-recycling routes, via municipal sewage sludge, struvite recovered from municipal wastewater and source-separated urine. We firstly show that improvements are still needed in all three routes for achieving high P recovery efficiency, and a combination of these recycling routes are needed to achieve maximum recovery of phosphorus. Second, we identify key issues for each recycling route that currently limit the use of recovered phosphorus in agriculture. We indicate where interaction between disciplines is needed to improve recycling routes and identify gaps in research on how recovered phosphorus accesses agriculture.

Litter decomposition is a key process in the carbon balance of soils. Commonly, plant litters occur in mixtures where the species differ in quality traits such as the nutrient concentration and organic carbon quality. Many studies explored if mixing litters retards or speeds up litter decomposition compared to species decomposing alone, with varying results. To identify consistent trends with an overarching quantitative synthesis, we test in a meta-analysis whether on average across studies, the mass loss of mixed litters of two plant species is faster than the average mass loss of single litters. We hypothesise that larger trait divergence of the litter quality of the species in a mixture results in a faster mass loss of the mixture than expected based on the single species. Furthermore we hypothesise that part of the variation in litter mixture mass loss can be explained by experimental design and environmental factors. Explanatory variables used were chemical litter trait dissimilarity in the C, N, P, lignin, cellulose, phenolics concentration as well as soil properties, ecosystem, climate, the duration of litter decomposition and the experimental design. Interactions were studied if supported by mechanistic hypotheses. In the majority of studies and on average, we found that the mass loss of mixed litters is equal to the weighted average of the mass loss of the constituent single litters. None of the hypothesised explanatory variables was consistently associated with litter mixture effects on the mass loss and explained variation in mass loss of significant models was invariably only a few percent of all variation. While further data exploration might elucidate further, interactive, patterns, many of these could not be explored due to lacking data. This meta-analysis therefore refutes the notion that mixing litters in general enhances rates of decomposition. We conclude that the effects of litter mixing are in many cases predictable from the decomposition rates of the individual species. According to our results, any interactive effects (positive or negative) between litter species are contextual, and cannot be generalized and predicted beyond the context in which the results were obtained.

Plant roots have a profound effect on soil microbial activity, particularly in the rhizosphere. Hence, it is important to understand the potential effects of genetically modified (GM) crops on soil microbial activity and related processes such as litter decomposition. In this study, we compared the effects of GM potato Modena on soil microbial activity and carbon (C) and nitrogen (N) mineralization to effects induced by Modena’s parental isoline (Karnico) and a conventional potato cultivar (Aventra). A field experiment was conducted at two sites to assess microbial catabolic diversity (using MicroRespTM) in the rhizosphere and in bulk soil, during flowering and senescence of the potato plants. In a laboratory experiment with soil and potato litter from the field experiment, we investigated whether the cultivars had modified the activity of soil microbial communities to such an extent that this affected C and N mineralization. Results of the field experiment showed no GM-induced effects on microbial catabolic diversity, while effects of field site location and sampling date were significant. Multivariate analysis including plant traits and soil characteristics revealed that microbial catabolic activities in rhizosphere soil were strongly correlated with soil organic matter and tuber sucrose content, whereas in bulk soil, they were primarily correlated with soil moisture. In the laboratory experiment, we found that Modena induced a "home-field advantage" in N mineralization, yet this effect was inconsistent across locations and was also observed for other cultivars. Based on our data and results from previous studies, we conclude that the effects of GM cultivar Modena on soil microbial activity and litter decomposition fall within the normal range of effects found for conventional potato cultivars.

The aim of this study was to assess the feasibility of controlling the potato cyst nematode Globodera pallida through biofumigation with glucosinolate-rich Brassica juncea genotypes. The main glucosinolate of B. juncea is 2-propenyl glucosinolate which is the precursor of 2-propenyl isothiocyanate. Toxicity of 2-propenyl isothiocyanate to encysted G. pallida was tested in vitro . Fifty percent reduction in hatching was found within 2 h of exposure to 0.002% 2-propenyl isothiocyanate. Based on the in vitro results, we hypothesized that biofumigation with B. juncea would reduce hatching of G. pallida in vivo and higher 2-propenyl glucosinolate levels would have a stronger effect. Plant genotype, sulfur fertilization and insect herbivory affected 2-propenyl glucosinolate concentration of B. juncea . However, increasing 2-propenyl glucosinolate concentration of B. juncea did not affect G. pallida hatching after biofumigation. The absence of a biofumigation effect was most likely due to lower concentrations of 2-propenyl isothiocyanate in vivo compared to in vitro conditions. These results show that it is difficult to reach sufficiently high levels of toxicity to reduce hatching of G. pallida under realistic conditions.

Soil amendment with biochar has been claimed as an option for carbon (C) sequestration in agricultural soils. Most studies on biochar/soil organic carbon (SOC) interactions were executed under laboratory conditions. Here we tested the stability of biochar produced in a traditional kiln and its effects on the stocks of native SOC under field conditions. The biochar was characterized using pyrolysis–gas chromatography–mass spectrometry, and then added to an Oxisol under savannah climate. This soil was amended with 0, 5, 10, 20 and 40 Mg ha− 1 of biochar in a randomized complete block design with four replications and cultivated with soybean over four cropping seasons (CSs; 120 days each). Soil samples from the 0–10 cm top layer were collected at the end of the first and fourth CSs and analyzed for CO2 emissions, isotopic C abundance (13C/12C ratio) and enzymatic activity (fluorescein diacetate and dehydrogenase). The biochar showed a low degree of thermal modification. Its relative decomposition rate was higher (k = 0.32–1.00 year− 1) than generally claimed (k = 0.005–0.0005 year− 1), and higher than the decomposition of native SOC (k = 0.22 year− 1). Addition of biochar did not affect the stocks of native SOC. Our findings highlight the need for critically reviewing the potential of locally produced biochar to sequester C.

Zinc (Zn) deficiency is a widely occurring constraint for rice production and for human nutrition. Scarcity of water is leading to a shift from flooded to aerobic rice production, which can have an impact on Zn deficiency in rice. Zinc bioavailability is a function of both soil and plant factors that can be altered by water management, particularly in relation to conditions in the rhizosphere. Biogeochemical modeling based on bulk soil conditions failed to predict the effect of water management on Zn bioavailability, but revealed that dissolved organic anions, pH, and redox conditions were major determinants. Rhizosphere sampling is needed to understand the difference in Zn mobilization and uptake between flooded and aerobic cultivation systems. Zinc bioavailability is not only affected by changes in the chemical properties of the soils, but also by biological processes such as mycorrhizal inoculation and root release of organic compounds into rhizosphere. Phytosiderophores and organic acids are two classes of Zn chelators secreted from roots that have been linked to the release of Zn from soil-bound forms and its subsequent uptake by plants. A shift to aerobic condition provides a favorable environment for activity of mycorrhizal fungi and enhanced mycorrhizal inoculation under aerobic conditions has been shown to increase plant Zn uptake. Aerobic rice genotypes with varying tolerance to Zn deficiency display a trade-off between mycorrhizal Zn responsiveness and root exudation of Zn chelator in the rhizosphere, which is probably due to a competition for carbon. Potential agronomic management practices in aerobic rice production systems are discussed, with an emphasis on their roles in improving bioavailability of Zn. Addition of Zn fertilizers by soil or foliar application have been shown to increase Zn concentration in cereal grains but the extent of the increase differs among crop species. The shift from flooded to aerobic condition can cause significant N transformations, which may consequently affect Zn mobilization and uptake. An appropriate N management strategy, including an effective combination of source, rate, application method, and timing, should consider the effects on soil pH. Application of P fertilizer should be done with careful consideration to the effect on Zn uptake. A reasonable cropping system (intercropping and crop rotation) could prevent Zn deficiency and offer an effective and sustainable pathway to Zn biofortification. Keeping these points in mind, this review describes our current knowledge of Zn bioavailability as affected by changes in soil–plant interactions caused by the transition from flooded to aerobic rice cultivation.

A decade ago, tunnels inside mineral grains were found that were likely formed by hyphae of ectomycorrhizal (EcM) fungi. This observation implied that EcM fungi can dissolve mineral grains. The observation raised several questions on the ecology of these ¿rock-eating¿ fungi. This review addresses the roles of these rock-eating EcM associations in plant nutrition, biogeochemical cycles and pedogenesis. Research approaches ranged from molecular to ecosystem level scales. Nutrient deficiencies change EcM seedling exudation patterns of organic anions and thus their potential to mobilise base cations from minerals. This response was fungal species-specific. Some EcM fungi accelerated mineral weathering. While mineral weathering could also increase the concentrations of phytotoxic aluminium in the soil solution, some EcM fungi increase Al tolerance through an enhanced exudation of oxalate. Through their contribution to Al transport, EcM hyphae could be agents in pedogenesis, especially podzolisation. A modelling study indicated that mineral tunnelling is less important than surface weathering by EcM fungi. With both processes taken together, the contribution of EcM fungi to weathering may be significant. In the field vertical niche differentiation of EcM fungi was shown for EcM root tips and extraradical mycelium. In the field EcM fungi and tunnel densities were correlated. Our results support a role of rock-eating EcM fungi in plant nutrition and biogeochemical cycles. EcM fungal species-specific differences indicate the need for further research with regard to this variation in functional traits.

Ectomycorrhizal fungi are hypothesized to enhance mineral weathering in forest soils. Several studies have shown an increased uptake of mineral-derived nutrients by trees when in symbiosis with ectomycorrhizal fungi. However, it is difficult to determine from these studies if the improved nutrient uptake is the result of increased weathering or better exploitation of the substrate by the ectomycorrhizal fungi. In a pot experiment, Pinus sylvestris (Scots pine) seedlings were grown with or without ectomycorrhizal fungi, and with or without the mineral muscovite as the only potassium (K) source or the mineral hornblende as the only magnesium (Mg) source. After 27 wk, all pools of non-mineral-bound K or Mg were determined. The ectomycorrhizal fungus Paxillus involutus increased weathering of muscovite but not hornblende. The other ectomycorrhizal fungi tested, Piloderma croceum and Suillus bovinus, did not increase weathering of either muscovite or hornblende compared with the nonmycorrhizal trees. The P. involutus-mediated mobilization of K from muscovite resulted in increased K content of root plus adhering hyphae, but not of shoots. In conclusion, ectomycorrhizal fungi may increase weathering of minerals in response to nutrient deficiencies, but this response is species specific.

Recently, it was hypothesized that ectomycorrhizas hyphae are involved in mineral tunneling. We evaluated the role of ectomycorrhizas in mineral weathering and the ecosystem influx of basic cations by correlating mineral tunnel density to ectomycorrhizas density in two forest productivity gradients. The gradients, two gentle slopes in northern Sweden, are the result of groundwater movement and are characterized by reduced productivity upslope due to lower nitrogen availability. As expected, ectomycorrhizas density in the O horizon was higher upslope, where nitrogen availability was lower and where the vegetation was dominated by ectomycorrhizas plant species. We consistently found that tunnel formation in mineral grains was more intense in nutrient-poor sites, indicating a higher contribution of fungi to ecosystem influx of potassium and calcium. ectomycorrhizas density was positively correlated with feldspar tunnel density in the upper 2 cm of the E horizon. This suggests that ectomycorrhizas are involved in mineral tunneling. We discuss the possible involvement of cricoid mycorrhizas and saprotrophic fungi in feldspar tunneling and the role of the weathering status of mineral grains as additional factors controlling mineral funneling.

Soil amendment with pyrogenic organic matter (PyOM), also named biochar, is claimed to sequester carbon (C). However, possible interactions between PyOM and native soil organic carbon (SOC) may accelerate the loss of SOC, thus reducing PyOM?s C sequestration potential. We combined the results of 46 studies in a meta-analysis to investigate changes in CO2 emission of PyOM-amended soils and to identify the causes of these changes and the possible factors involved. Our results showed a statistically significant increase of 28% in CO2 emission from PyOM-amended soils. When grouped by PyOM C (PyC):SOC ratios, the group of studies with a ratio >2 showed a significant increase in CO2 emissions, but those with a ratio

Aims - Intercropping increases aboveground and belowground crop productivity, suggesting potential for carbon sequestration. Here we determined whether intercropping affects decomposition of soil organic matter (SOM) and root litter. Methods - We measured in the laboratory and the field the breakdown of SOM, root litter of maize, wheat, or faba bean, litter mixtures, and a standard substrate (compost) in soils from a long term intercropping experiment. Results - Soil organic matter from intercrop plots decomposed faster than SOM from monocrop plots, but compost decomposed at similar rates in different soils. Faster SOM decomposition was associated with lower soil C:N ratio. Root litter mixtures of maize and wheat decomposed as expected from single litters, but litter mixture of maize and faba bean decomposed faster than expected, both in the laboratory and in the field. Root litter decomposed slowly in maize/wheat intercrop soil compared to the two monocropped soils in the laboratory, but the effect was absent in the field. Conclusions - Intercropping increases SOM decomposition, presumably through reduced SOM recalcitrance resulting from lower C:N ratio, higher litter input and better N retention. Depending on the crop combination, also non-additive effects of root litter mixing can enhance organic matter decomposition in intercropping soils.

Tunnels in feldspar grains, assumed to be created by fungal hyphae, were first discovered in a boreal podzol. In this paper, we further describe the phenomenon of mineral tunneling by determining the rate of feldspar tunneling across a north Sweden podzol chronosequence. The chronosequence is a result of ongoing land uplift, which started after the retreat of glaciers about 9000 years ago. The sequence comprises a series of soils that began developing on glacial tills 190–7800 years ago. Feldspar tunneling was concentrated in the uppermost 2 cm of the E horizon, and its frequency increased significantly with soil age. Although no tunnels were found in feldspar grains from the youngest soil (190 years), they were seen more frequently in soils aged 2000 years and older. This lag phase in tunnel formation of about 2000 years coincided with the disappearance of the easily weatherable potassium-(K) and calcium (Ca)-containing minerals biotite and hornblende and with the appearance of etch pits on feldspar grains. In the oldest soil (7800 y), about 25% of the feldspar grains in the upper 2 cm of the E horizon were tunneled. Within site variation in tunnel frequency was high, and we were able to exclude spatial variations in mineralogy and texture as a possible explanation. The shape of the tunnels, their depth distribution, and the fungal hyphae found inside them all offer support for the previous assumption that their formation is mediated by biological activity involving fungi. The results of this investigation also indicate that the bioavailability of Ca and K may be a factor in tunnel formation.

The aim of this study was to investigate the effect of tissue nitrogen concentration, as a consequence of nitrogen supply rate, on the susceptibility of tomato plants to three pathogens. We varied tissue N concentration by supplying N at different rates by adding nitrate in different, exponentially increasing amounts to the nutrient solution on which the tomato plants were grown. Separate experiments were carried out to test susceptibility of tomato plants to the bacterial speck-causing Pseudomonas syringae pv tomato, to the wilt agent Fusarium oxysporum f.sp. lycopersici and to tomato powdery mildew caused by Oidium lycopersicum. The effect of tissue N concentration appeared to be highly pathogen-dependent: there was no effect on susceptibility to F. oxysporum, but susceptibility to P. syringae and O. lycopersicum increased significantly with increasing N concentration. We have previously demonstrated the opposite for susceptibility to Botrytis cinerea: decreasing susceptibility with increasing N concentration. The apparent contradictory effects are discussed in relation to the effect of N supply on both the nutritional value of the plant tissue to the pathogen and on the concentration of resistance-related compounds. We conclude that the effect of changing both characteristics on disease susceptibility is highly pathogen-specific and is probably dependent on differences in resource requirements of the pathogen or the sensitivity of the pathogen to plant resistance reactions or on both these factors.

The aim of this study was to investigate the effect of nitrogen availability on susceptibility of tomato leaves to the fungal pathogen Botrytis cinerea. Plants with varying nitrogen availability were grown by adding N daily in exponentially increasing amounts to a nutrient solution at different rates. Leaves of plants grown at low nitrogen availability had a high leaf C/N ratio (21 g g-1) and were about 2.5 times more susceptible to primary lesion formation by B. cinerea compared to plant grown at high nitrogen availability, which had a low leaf C/N ratio (11 g g-1). Leaf C/N ratio accounted for 95% of variation in susceptibility. This relationship between C/N ratio and susceptibility persisted when plants were grown with exponential P addition and optimal N supply, and was thus independent of plant growth rate or related factors. We could not explain the effect of nitrogen availability by variation in the most obvious N-based resistance compound α-tomatine because more susceptible leaves with a high C/N ratio contained more α-tomatine. These leaves also contained more soluble carbohydrates. The level of soluble carbohydrates correlated positively with susceptibility, independent of the growth method. We therefore suggest that the effect of N availability on susceptibility must be explained by variation in levels of soluble carbohydrates and speculate about the role of these carbohydrates in the infection process.

Aim Our aim was to improve the prediction of Zn bioavailability to wheat grown on low-Zn soils. The classical approach that directly relates Zn in a certain soil extract to Zn uptake has been shown to be inadequate in many cases. We tested a stepwise approach where the steps of the uptake process are characterized with, respectively, Zn solid-solution distribution, adsorption of Zn to root surface, Zn uptake into root and Zn translocation to shoot. Methods Two pot experiments were done with wheat grown on nine low-Zn soils varying widely in pH, clay and organic matter content. Soluble Zn concentrations in two soil extracts (DTPA and CaCl2) were measured. Free Zn ion concentrations in CaCl2 soil extracts were determined with the Donnan Membrane Technique. These Zn concentrations were then related to plant Zn uptake following both the direct and the stepwise approach. Results In the direct approach, Zn in the DTPA extract was a better predictor for shoot Zn uptake than Zn in the CaCl2 extract. In the stepwise approach, the relationship between Zn in CaCl2 extracts and the root surface adsorbed Zn was pH-dependent and nonlinear. Root surface adsorbed Zn was linearly related to root Zn uptake, and the latter was linearly related to the shoot Zn uptake. The stepwise approach improved the Zn uptake prediction compared to the direct approach and was also validated for different wheat cultivars. Conclusions The adsorption of Zn on the root surface is pH dependent and nonlinear with respect to the soil Zn concentration, and a useful proxy for bioavailable Zn over a wide range of soils.

Zinc (Zn) is an exceptional micronutrient regarding itsrelevance in biological systems because it is the onlytrace metal represented in all classes of enzymes(Broadley et al. 2007). A large diversity of essentialcellular functions and metabolic pathways are directlyinfluenced by Zn including function and structural sta-bilityofproteins,integrityofbiologicalmembranesandprotection against reactive oxygen species (Cakmak,2000). Nearly 2800 proteins in biological systems re-quire Zn for their activity and structural stability(Andreini et al. 2009).It is, therefore, not surprising that crop plants showrapid and particular disturbances in growth and devel-opmentwhentheyaregrowingonlowZnsoils.Atleastone-third of the cultivated soils globally is estimated tocontain too low amounts of bioavailable Zn (Sillanpaaand Vlek 1985) for optimal crop production. Low bio-availability of Zn not only reduces plant productivity, italsoimpairsnutritionalqualityoftheharvestedproductsby lowering Zn density in seeds/grains.These problems in crop production, food qualityand human health associated with Zn deficiency andthe environmental issues related to Zn were addressedat the 3rd International Zinc Symposium held inHyderabad, India, 10–14 October, 2011, with 195participants from 30 countries. The event was jointlyorganized by the International Zinc Association (IZA),the International Fertilizer Industry Association (IFA),Fertilizer Association of India (FAI) and HarvestPlus.The symposium consisted of 6 sessions devoted tothe following topics.& human nutrition and health,& soil and crop management,& zinc fertilizers and crop nutrition,& plant physiology& genetics and molecular biology& environmental issuesThis short article summarizes the main findings ofthis symposium. Details of the findings and conclu-sions can be found in the articles in this special issue.Low Zn supply by soils to crops is becoming moreprominent due to the use of high yielding cultivarsand/or application of macronutrient fertilizers unbal-anced by Zn fertilization. In cropping areas with in-tensified farming, additional management practices(e.g., Zn fertilization) are often required to avoid Zndepletion of soils to sustain crop productivity andnutritional quality of the harvested product. The deter-mination of soil bioavailable Zn, however, is not

To test the hypothesis that rhizosphere acidification would enhance the hydrolyzation of organic phosphates by increasing phosphatase activity. A Petri dish experiment with sterile agar and a pot experiment with a low P soil were used. In the Petri dish experiment, roots of each plant were cultured in two compartments, each of which contained agar with one of three nitrogen combinations: NH4+/N0 (N0 = nitrogen free), NH4+/NO3-, and NO3-/N0. Phytin was supplied as the sole phosphorus (P) source to all compartments. In the pot experiment, the soil in each pot was treated with N0, KNO3, or (NH4)2SO4) together with 0 or 75 mg kg−1 phytin-P. Dry weight, P concentration, and P content of roots were highest in the NH4+ compartments in the Petri dish experiment. In the pot experiment, dry weight, P concentration, and P content of both shoots and roots were higher with NH4+ than with NO3-. NH4+ treatments reduced rhizosphere pH, promoted the hydrolization of phytin, enhanced acid phosphatase activity in the rhizosphere, and increased phytin-P utilization relative to N0 and NO3- treatments. Phosphatase activity was negatively correlated with rhizosphere pH but was positively correlated with plant P content in both experiments. Rhizosphere acidification optimized the activity of acid phosphatase excreted by maize roots and promoted phytin mineralization. NH4+-induced acidification in the maize rhizosphere improved the growth of maize roots by improving P uptake from phytin; the improved growth, in turn, increased NH4+ uptake and acidification.

summary Rape (Brassica napus L.) plants are efficient users of rock phosphates. This has been ascribed to the excretion of malic and citric acids from the roots during phosphorus deficiency, resulting in a decrease of the pH in the rhizosphere and a consequent solubilization of the rock phosphate. In this paper the biosynthesis and excretion of organic acids were investigated, partly by comparing physiological reactions to P deficiency of rape with those of an inefficient user of rock phosphate from the same family, hedge mustard (Sisymbrium officinale Scop.). In the leaves of both species, both citrate levels and phosphoenolypyruvate carboxylase were nearly doubled in P–deficient plants, and the citrate/sugar ratio in the phloem exudate into EDTA solution was doubled as well. After exposure of the shoots of rape plants to 14CO2 in the light, the specific activities of citrate in the root and in the nutrient solution were ten times higher than those of malate, and also much higher than the respective activities of citrate in P–sufficient controls. It is suggested that citrate produced in the leaf is exported to the roots, where it is accumulated in the excretion region. Malic acid which is prevailing in the exudate is probably newly–synthesized within the excreting root segment. When no phosphate is supplied, this excretion zone is located 1–2 cm behind the root tips. However, when rock phosphate is applied locally this zone shifts along the root to that part which is in direct contact with the rock phosphate particles. No accumulation and excretion of organic acids could be observed with the roots of P–deficient Sisymbrium.

A simulation model is presented which describes uptake of a growth limiting nutrient from soil by a growing root system. The root surface is supposed to behave like a zero-sink. Uptake of the nutrient is therefore determined by the rate of nutrient supply to the root surface by mass flow and diffusion. Inter-root competition and time dependent root density are accounted for by assigning to each root a finite cylindrical soil volume that delivers nutrients. The radius of these cylinders declines with increasing root density. Experiments with rape plants grown on quartz sand were used to evaluate the model. Simulated nitrogen uptake agreed well with observed uptake under nitrogen limiting conditions. In case no nitrogen limitation occurred nitrogen uptake was overestimated by the model, probably because the roots did not behave like a zero-sink any more.

Plant Zn uptake from low Zn soils can be increased by Zn-mobilizing chemical rhizosphere processes. We studied whether inoculation with arbuscular mycorrhizal fungi (AMF) can be an additional or an alternative strategy. We determined the effect of AMF inoculation on growth performance and Zn uptake by rice genotypes varying in Zn uptake when nonmycorrhizal. A pot experiment was conducted with six aerobic rice genotypes inoculated with Glomus mosseae or G. etunicatum or without AMF on a low Zn soil. Plant growth, Zn uptake and mycorrhizal responsiveness were determined. AMF-inoculated plants produced more biomass and took up more Zn than nonmycorrhizal controls. Mycorrhizal inoculation, however, significantly increased Zn uptake only in genotypes that had a low Zn uptake in the nonmycorrhizal condition. We conclude that genotypes that are less efficient in Zn uptake when nonmycorrhizal are more responsive to AMF inoculation. We provide examples from literature allowing generalization of this conclusion on a trade off between mycorrhizal responsiveness and nutrient uptake efficiency.

Two modified designs of micro-pore-water samplers were tested for their capacity to collect unbiased soil solution samples containing zinc and citrate. The samplers had either ceramic or polyethersulfone (PES) suction cups. Laboratory tests of the micro-samplers were conducted using (a) standard solutions with zinc and citrate concentrations that can be found in the rhizosphere and (b) two soils with contrasting texture. The results showed that both suction cups were inert with respect to citrate and zinc: they did not affect the concentrations of the two ions in freshly prepared and directly sampled standard solutions. Both micro-suction cups removed most microorganisms by filtration, though the PES cups were more effective in this respect. This filtering by the suction cups allowed for relatively accurate analysis of citrate in solutions containing microorganisms. These capacities, together with their small size and small dead volume, make the two micro-pore-water samplers highly suitable for sampling of rhizosphere soil solutions although further testing with other organic acids and trace metals may be needed.

Low molecular weight organic anions (LMWOA) can enhance weathering of mineral grains. We tested the hypothesis that ectomycorrhizal (EcM) fungi and tree seedlings increase their exudation of LMWOA when supply of magnesium, potassium and phosphorus is low to enhance the mobilization of Mg, K and P from mineral grains. ¿ Ectomycorrhizal fungi and Pinus sylvestris seedlings were cultured in symbiosis and in isolation on glass beads with nutrient solution or with sand as a rooting medium, with a complete nutrient supply or with Mg, K, P or N in low supply. Concentrations of all dicarboxylic LMWOA in the rooting medium were measured. ¿ Nonmycorrhizal seedlings released predominantly malonate. Colonization with Hebeloma longicaudum decreased the amount of organic anions exuded, whereas Paxillus involutus and Piloderma croceum increased the concentration of oxalate but not the total amount of LMWOA. Phosphorus deficiency increased the concentration of LMWOA by nonmycorrhizal and EcM seedlings. Magnesium deficiency increased the concentration of oxalate by nonmycorrhizal and EcM seedlings, but not the concentration of total LMWOA. Paxillus involutus grown in pure culture responded differently to low nutrient supply compared with symbiotic growth. ¿ Ectomycorrhizal fungi did not increase the total concentration of LMWOA compared with nonmycorrhizal seedlings but, depending on the fungal species, they affected the type of LMWOA found.

The objectives of this paper were to determine (1) if lowland rice (Oryza sativa L.) plants respond similarly to low zinc (Zn) and phosphorus (P) availability by increased root exudation of low-molecular weight organic anions (LMWOAs) and (2) if genotypic variation in tolerance to low soil supply of either Zn or P is related to LMWOA exudation rates. Exudation of LMWOAs can increase bioavailability of both Zn and P to the plant, through partly similar chemical mechanisms. We used seven lowland rice genotypes and showed in two experiments that genotypes that grow relatively well on a soil with low Zn availability also grow well on a sparingly soluble Ca-phosphate (r = 0.80, P = 0.03). We measured exudation rates of LMWOAs on nutrient solution and found that both Zn and P deficiency induced significant increases. Among the LMWOAs detected oxalate was quantitatively the most important, but citrate is considered more effective in mobilizing Zn. Citrate exudation rates correlated with tolerance to low soil levels of Zn (P=0.05) and P (P = 0.07). In a low-Zn-field we found an increased biomass production at higher plant density, which is supportive for a concentration-dependent rhizosphere effect on Zn bioavailability such as LMWOA exudation. We, for the first time, showed that tolerance to low Zn availability is related to the capacity of a plant to exude LMWOAs and confirmed that exudation of LMWOAs must be regarded a multiple stress response.

Scarcity of water causes a shift from flooded to aerobic conditions for rice production in zinc deficient areas in Northern China. This shift alters soil conditions that affect zinc availability to the crop. This paper concerns the effect of aerobic compared to flooded conditions on crop biomass production, grain yield and zinc content. A field experiment was done with six rice genotypes (Oryza sativa L.) grown on a calcareous soil, both with (23 kg Zn ha¿1) and without Zn fertilization. Sampling was conducted at tillering and physiological mature stage. Zn concentration in the shoots was significantly lower at both stages in plants grown in the aerobic field. At maturity, Zn uptake, biomass production, grain yield and Zn-harvest index [grain Zn/(shoot + grain Zn)] were lower under aerobic cultivation. Rice genotypes including aerobic rice and lowland rice differ in degree of response to low Zn supply. A twofold difference was found among aerobic genotypes in grain yield and Zn uptake. Also Zn-harvest index varied significantly. Zn application affected neither grain yield nor grain Zn content, although it significantly improved biomass production in both systems in most genotypes. These results demonstrate that introduction of aerobic rice systems on calcareous soils may increase Zn deficiency problems.

To study variation in zinc efficiency (ZE) among current Chinese rice genotypes, a pot experiment was conducted with 15 aerobic and 8 lowland rice genotypes. Aerobic rice is currently bred by crossing lowland with upland rice genotypes, for growth in an aerobic cultivation system, which is saving water and producing high yields. A Zn deficient clay soil was used in our screening. Zn deficiency resulted in a marked decrease in shoot dry matter production of most genotypes after 28 days of growth. Genotypes were ranked according to their tolerance to Zn deficiency based on ZE, expressed as the ratio of shoot dry weight at Zn deficiency over that at adequate Zn supply. Substantial genotypic variation in ZE (50¿98%) was found among both lowland and aerobic genotypes. ZE correlated significantly (P

No rock at the Earth's surface escapes weathering. This process is the primary source of all the essential elements for organisms, except nitrogen and carbon. Since the onset of terrestrial life, weathering has been accelerated under the influence of biota. The study of biological weathering started at the end of the 19th century. Although the role of bacteria (Eubacteria, Archaea) has attracted a lot of interest, until recently the role of fungi has largely been neglected. More recently, however, fungal weathering has become an increasingly important focus of biogeochemical research.

Molecular identification techniques based on total DNA extraction provide a unique tool for identification of mycelium in soil. Using molecular identification techniques, the ectomycorrhizal (EM) fungal community under coniferous vegetation was analyzed. Soil samples were taken at different depths from four horizons of a podzol profile. A basidiomycete-specific primer pair (ITS1F-ITS4B) was used to amplify fungal internal transcribed spacer (ITS) sequences from total DNA extracts of the soil horizons. Amplified basidiomycete DNA was cloned and sequenced, and a selection of the obtained clones was analyzed phylogenetically. Based on sequence similarity, the fungal clone sequences were sorted into 25 different fungal groups, or operational taxonomic units (OTUs). Out of 25 basidiomycete OTUs, 7 OTUs showed high nucleotide homology (≥99%) with known EM fungal sequences and 16 were found exclusively in the mineral soil. The taxonomic positions of six OTUs remained unclear. OTU sequences were compared to sequences from morphotyped EM root tips collected from the same sites. Of the 25 OTUs, 10 OTUs had ≥98% sequence similarity with these EM root tip sequences. The present study demonstrates the use of molecular techniques to identify EM hyphae in various soil types. This approach differs from the conventional method of EM root tip identification and provides a novel approach to examine EM fungal communities in soil.

Phosphorus-deficient rape plants appear to acidify part of their rhizosphere by exuding malic and citric acid. A simulation model was used to evaluate the effect of measured exudation rates on phosphate uptake from Mali rock phosphate. The model used was one on nutrient uptake, extended to include both the effect of ion uptake and exudation on rhizosphere pH and the effect of rhizosphere pH on the solubilization of rock phosphate. Only the youngest zones of the root system were assumed to exude organic acids. The transport of protons released by organic acids was described by mass flow and diffusion. An experimentally determined relation was used describing pH and phosphate concentration in the soil solution as a function of total soil acid concentration. Model parameters were determined in experiments on organic acid exudation and on the uptake of phosphate by rape from a mixture of quartz sand and rock phosphate. Results based on simulation calculations indicated that the exudation rates measured in rape plants deficient in phosphorus can provide the roots with more phosphate than is actually taken up. Presence of root hairs enhanced the effect of organic acid exudation on calculated phosphate uptake. However, increase of root hair length without exudation as an alternative strategy to increase phosphate uptake from rock phosphate appeared to be less effective than exudation of organic acids. It was concluded that organic acid exudation is a highly effective strategy to increase phosphate uptake from rock phosphate, and that it unlikely that other rhizosphere processes play an important role in rock phosphate mobilization by rape.

Aims Zinc (Zn) and phosphorus (P) deficiency often occurs at the same time and limits crop production in many soils. It has been suggested that citrate root exudation is a response of plants to both deficiencies. We used white lupin (Lupinus albus L.) as a model plant to clarify if citrate exuded by roots could increase the bioavailability of Zn and P in calcareous soils. Methods White lupin was grown in nutrient solution and in two calcareous soils in a rhizobox. Rhizosphere soil solution was sampled to determine citrate, metals and P. Based on the measured citrate concentrations, a soil extraction experiment with citrate as extractant was done. Results Absence of Zn triggered neither cluster root formation nor citrate exudation of white lupin grown in nutrient solution, whereas low P supply did. The maximum citrate concentration (similar to 1.5 mM) found in the cluster rhizosphere soil solution of one soil mobilized P, but not Zn. In the other soil the highest citrate concentration (similar to 0.5 mM) mobilized both elements. Conclusions White lupin does not respond to low Zn bioavailability by increasing citrate exudation. Such a response was observed at low P supply only. Whether Zn and P can be mobilized by citrate is soil-dependent and the possible controlling mechanisms are discussed.

Phosphorus (P) is a main limiting factor for agricultural production, but overusing P fertilizer has brought serious environmental damages in China. Improving P acquisition efficiency of agricultural crops is an urgent topic. It has been proven repeatedly that arbuscular mycorrhizal fungi (AMF) and genetic diversity within one crop plant can play important roles in P uptake by crops. The main objective of this thesis was to understand the role of the arbuscular mycorrhizal symbiosis in P acquisition efficiency of different maize varieties. The specific objectives were to test: 1) how P uptake by maize varieties responds to colonization by the native AMF community in the field; and 2) whether AMF hyphae take up P for plants from phytate which is the most abundant organic P form in soil; 3) whether mixing maize cultivars can improve maize productivity and whether AMF can play a role in this system; and 4) how AMF species (or community) legacy affects successional maize growth. In this thesis, I combined field experiments and greenhouse experiments and made use of maize genetic diversity and (native) AMF to improve P (including inorganic and organic P) acquisition. The effects of one single AMF species on maize growth and nutrient uptake have been well studied, but how maize varieties respond to the native AMF community has been insufficiently studied. In Chapter 2, I focused on how maize varieties responded to the native AMF community by using rotated cores in the field, to compare mycorrhizal responsiveness among 20 maize varieties and the difference of the AMF native community of four maize varieties (two old landraces and two modern hybrids). The results indicated that, 1) increased P fertilizer significantly reduced mycorrhizal responsiveness in the field; 2) a complicated relationship exists between mycorrhizal responsiveness in the field and pot experiment; 3) there was no significant difference between old and modern maize varieties in terms of mycorrhizal responsiveness and colonization; 4) there were only small differences in AMF community composition among the four maize varieties. By comparing mycorrhizal responsiveness of maize varieties between in the pot experiment and in the field experiment (with in-growth cores), I found mycorrhizal responsiveness of maize varieties in the pot experiment was significantly larger than that in the field experiment. Thus, mycorrhizal responsiveness of varieties within one cereal plant species tested classically in pots may not present their realistically mycorrhizal responsiveness in field. Phytate is the most abundant form of organic P in soil. To explore the potential of phytate utilization by plants is agriculturally and environmentally essential. Increased P nutrition of mycorrhizal plants derived from phytate has been reported, indicating that phytate can be a potential P source. However, earlier studies assessed phytate use by using acid phosphatase rather than phytase, and did not consider that phytate adsorption could lead to phosphate release. Thus, I investigated the effect of mycorrhizal hyphae-mediated phytase activity on P uptake by maize in Chapter 3. I conducted a rhizobox experiment to explore phytate use by mycorrhizal hyphae for two maize varieties. The results showed that: 1) phytate addition increased phytase and acid phosphatase activity, and resulted in increased P uptake and plant biomass; 2) the increase in P uptake and biomass were correlated with the increase of phytase activity but not with the increase of acid phosphatase activity; 3) lower phytate addition rate increased, but higher addition rates decreased hyphal length density. I conclude that P from phytate can be used by mycorrhizal plants, but that the phytate contribution to plant nutrition is likely limited. Phytase activity is a more relevant indicator to assess phytate use. In addition, there was a significant interaction between maize varieties and AMF species in taking up P from phytate, which implies there is a possibility to combine different maize varieties to increase total yield using phytate. Besides, I used an empirical relationship to assess phosphate release due to phytate addition. My calculation implies that phosphate desorption cannot be ignored when assessing phytate use, particularly when a large amount of phytate is applied as a P source. In multispecies natural ecosystems, AMF can play a key role in enhancing plant productivity. However, their role in enhancing crop productivity in mixed cropping systems is still poorly understood. In Chapter 4, I conducted both greenhouse and field experiments to investigate whether mixing maize varieties with different P acquisition strategies could lead to overyielding, and what roles AMF play in this system with two maize varieties. The results showed that mixing maize varieties resulted in overyielding, both in P uptake and shoot biomass, but only when plants were mycorrhizal. At the same time, I found higher hyphal length density and higher AMF diversity in mixtures compared to the monocultures in the field experiment, and higher colonization rate and higher hyphal length density in mixtures in the pot experiment. Thus, I propose that overyielding by mixing maize varieties might be due to increased mycorrhizal performance leading to more P uptake. I also used the partitioning formula to calculate the contribution through the selection effect and complementarity effect to overyielding. I found that the increase of the total yield and P uptake in mixtures was largely due to complementarity effect, implying that relative overyielding and enhanced P uptake were not due to enhanced competitive ability by the larger variety. The results of Chapter 4 suggest that mixing mycorrhizal maize varieties might be beneficial for enhancing productivity and P uptake efficiency. Plant - soil feedback experiments have shown that AMF can play a crucial role in determining the direction and magnitude of that feedback. Most studies investigated plant - soil feedback dynamics between different plant species. However, it is unknown to what extent one variety of an agricultural crop can affect the performance of another variety of that same crop through plant - soil feedback. In Chapter 5, I carried out a two-phase experiment in a greenhouse, including conditioning phase and test phase to determine plant - soil feedbacks in the absence and presence of AMF species or community, to test the effects of AMF on feedback dynamics. The results in Chapter 5 showed that: 1) in the conditioning phase, both maize varieties were differentially influenced by different AMF species compared to non-mycorrhizal control; 2) in the feedback phase, non-mycorrhizal maize exhibited negative feedback dynamics for biomass and P-uptake; 3) on the feedback phase, mycorrhizal maize generally exhibited positive feedback dynamics for biomass and P-uptake. The interaction coefficient was largest with the mixture of three different AMF species. The interaction coefficient for shoot and P uptake were significantly correlated with the coefficient for mycorrhizal colonization. These results imply that different maize varieties are affected differently by different AMF species, thereby influencing the productivity of the subsequent maize variety. The results also raise questions how AMF influence rhizosphere biota and how maize varieties may select more beneficial AMF. In Chapter 6, I integrate the results from previous chapters. I discuss possible relationships between (negative) plant - soil feedback effect (due to pathogen) and the mycorrhizal effect on overyielding and improved P uptake due to mixing maize varieties (compared to the monoculture). I also discuss the linkage between phosphorus acquisition efficiency and mycorrhizal responsiveness within one crop species, and the relationship between plant genetic diversity and plant - soil feedback effects, and try to come up with a conceptual model how mixing maize varieties in the presence of AMF could be beneficial.

Summary In this PhD thesis I studied the influence of biochar discourses on the political practices in Brazil and the impact of biochar on soil organic carbon (SOC) stocks, thus contributing to the current debate on the potential of biochar to mitigate climate change. Biochar is the solid material obtained from the carbonization of biomass. The deliberate production and application to soil distinguishes biochar from other carbonized products, e.g. charcoal. Inspired by the aged charcoal found in the fertile Amazonian Dark Earth (ADE; also known as Terra Preta de Índio), the current application of biochar in soil is claimed to simultaneously address four global challenges: food production, climate change, energy supply and waste reduction (Chapter 1). Biochar is supposed to be an absorbent and stable material, which can be used to retain nutrients in the soil, increasing agricultural productivity, while sequestering carbon over extended periods of time. Therefore, biochar is claimed to be a means to mitigate global climate change. Furthermore, if biochar is produced in a modern pyrolysis plant, it also can co-produce bio-oil and syngas that could be used as energy. And if biochar is produced by carbonization of agricultural residue, biochar may reduce the quantity of solid waste that needs to be disposed of. In Chapter 2, I analysed the policy arrangement related to biochar along the four dimensions of the policy arrangement approach, which are actors, discourse, power and rules. I focused on Brazil, which is an important player in the international biochar debate. My analysis shows that scientists in research institutions are the dominant players in the network, while policymakers, businessmen and farmers are marginally positioned. Experts from Embrapa occupy central positions and thus exercise most power in the network. Moreover, experts linked to ADE have lost prominence in the network. The cause for this reduction was the shift from the ADE/biochar to the biochar/technology discourse. The latter discourse includes different coalitions, such as: ‘climate change mitigation’, ‘improvement of soil fertility’ and ‘improving crop residue management’. Although the biochar/climate coalition is dominant at international level, it is far less prominent in Brazil. Nationally the discourses of ‘improvement of soil fertility’ and ‘improving crop residue management’ have particularly prompted actors’ relationships and practices. However, the biochar/technology discourse is not (yet) institutionalized into formal rules in Brazil. As a consequence, the country lacks an established biochar policy field. Brazilian biochar practices focus on the carbonization of the available residues into biochar and on the application of biochar in soils to increase the SOC content and consequently the fertility of these soils. In this context, in Chapter 3 I tested in the field the potential of biochar produced in traditional kilns to increase the C contents of sandy savannah soils. My results show that biochar produced in traditional kilns is less thermally altered than that produced by industrial kilns and therefore rapidly decomposes. The decomposition rate of traditionally produced biochar was higher (decomposition constant k = 0.32-1.00 year-1) than generally assumed (k = 0.0005-0.005 year-1), and higher than the decomposition of native SOC (k = 0.22 year-1). In Chapter 4 I demonstrated in a short-term laboratory experiment that oilseed-derived biochar had a similar or higher decomposition rate than native SOC. My results show that all three tested oilseed biochars decelerate the decomposition of SOC in the biochar-amended soils, with biochar richer in aromatics having a stronger negative effect than biochar richer in aliphatics. Therefore, oilseed biochar directly increases soil C stocks and indirectly raises soil C sequestration in the short term through decreasing the decomposition of native SOC. In my research, the decomposition studies were performed using 13C isotope analysis. However, the 13C isotope analysis cannot be used when the differences of 13C isotope abundance between biochar and soil are not sufficiently large. Therefore, its use can be limited. In Chapter 5, I aimed at improving the benzene polycarboxylic acid (BPCA) method. I re-designed the protocols of the BPCA method and found a better and faster way to quantify and characterize the BPCAs derived from biochar, compared to the previous protocols. The improved method was then successfully tested and implemented in a laboratory in Brazil. Combining my findings with results of the literature, I conclude (Chapter 6) that there is no evidence that biochar is a reliable way for C sequestration in sandy soils under savannah environments. Biochar decomposition is highly variable, depending on charring conditions, soil and climate: (i) biochar produced by traditional kilns is less thermally degraded than those pyrolysed by industrial kilns; (ii) in sandy soils less biochar accumulates than in clay-silt soils; and (iii) warm-dry conditions raise the decomposition of biochar. These conclusions have a direct consequence for the development of policies on biochar, because we cannot ensure that biochar will sequester the same quantity of C for the same period at different geographical regions.

Explorations of soil microbial processes driven by dissolved organic carbon Angela L. Straathof June 17, 2015, Wageningen UR ISBN 978-94-6257-327-7 Abstract Dissolved organic carbon (DOC) is a complex, heterogeneous mixture of C compounds which, as a substrate, may influence various processes of the soil microbial community. Microbial respiration and volatile production are two such processes. These have both been linked to general disease suppression (GDS), a phenomenon in agricultural soils which inhibits pathogenic infestation in crops. The underlying hypothesis of this thesis is that the quality of DOC, via regulation of microbial processes, may be an important indicator of soil functions, including GDS. Properties of DOC quality include proportions of hydrophobic and hydrophilic fractions, and aromaticity. This thesis describes a high range in DOC fractions from various types of compost, which is often added to soil as an amendment to promote GDS. Differences in soil microbial respiration rates were attributed to differences in the composition of compost DOC added to soil in a laboratory incubation experiment. Compost DOC high in proportion of the hydrophilic (Hi) fraction promoted respiration rates. Depletion of the hydrophobic humic acid (HA) fraction was also observed. The relationship between DOC and microbial respiration was further explored in a survey of 50 arable soils. Both HA and Hi fractions of DOC that were found to be statistically, significantly related to respiration rates in these soils. Furthermore, in an assay measuring in vitro pathogen suppression by microbial volatile production, DOC concentration and microbial respiration were linked to growth suppression of Rhizoctonia solani, Fusarium oxysporum, and Pythium intermedium via multivariate regression modelling. This thesis provides evidence for the importance of DOC and DOC quality’s influence on microbial respiration and volatile production, thus supporting the hypothesis that DOC is a microbially-relevant soil chemical parameter, and potential indicator of general disease suppression in agricultural soils.

of the PhD thesis entitled “Chemical and biological rhizosphere interactions in low zinc soils” by Andreas Duffner Soil provides ecosystem services critical for life. The availability of micronutrients, such as zinc (Zn), in soils is an essential factor for normal healthy growth and reproduction of plants. Zinc deficiency is, however, a global problem in crop production due to low Zn bioavailability in soils to plants. The bioavailable Zn fraction in soils is controlled by several factors and is not directly related to the total Zn content of soils. The main objective of this thesis was the determination of factors which control Zn bioavailability in soils to plants and to assess approaches to improve the prediction of Zn plant uptake. Based on rhizobox experiments, in situ measurements in the rhizosphere as well as multisurface- and radial transport modeling approaches it was shown that the effect of root exuded citrate for increasing plant available Zn is soil specific and does not depend on a specific concentration of low molecular weight organic acids (e.g. citric acid) in the soil solution. Using various low Zn soils at the same time in an experimental setting improved the understanding of soil-responsiveness to root exuded citrate. Another insight was that multisurface models, which are widely used to assess the potential ecotoxicological risk in metal-contaminated soils, are also accurate to predict the Zn activity in soils with low Zn levels. The predictions were validated with the soil column Donnan Membrane Technique by using various soils with low Zn levels. It was predicted that soil organic matter is the dominant Zn sorbent and controlled the Zn activity also at low soil organic matter levels. Examples were shown how this modeling approach can be used to assess management options to increase bioavailable Zn to plants. Using soil extracted Zn fractions to directly predict the Zn plant uptake at low Zn levels was shown to be inaccurate. Using a stepwise approach where the steps of the uptake process were characterized with, respectively, Zn solid-solution distribution, adsorption of Zn to root surface, Zn uptake into root and Zn translocation to shoot made the prediction of Zn plant uptake more accurate. Root surface adsorbed Zn was shown to be a useful proxy for the bioavailable Zn. The framework of experimental and modeling approaches which were developed and applied in this thesis can also be used to study the plant-availability of other micronutrients at low concentration levels and how that is affected by various root exuded ligands.

Soil and plant responses to pyrogenic organic matter: carbon stability and symbiotic patterns Edvaldo Sagrilo Summary Pyrogenic organic matter (PyOM), also known as biochar, is the product of biomass combustion under low oxygen concentration. There is currently a growing interest in research on the use of PyOM as a soil amendment, inspired by the existence of highly fertile, PyOM-rich anthropogenic soils in the Amazon basin. The presence of PyOM in these so-called Amazonian Dark Earths (ADE) in quantities larger than in the non-anthropogenic surrounding soils is considered one of the main reasons for their high fertility. Soil additions of PyOM have been suggested to increase soil fertility and crop yields, simultaneously providing additional important environmental services. The offset of CO2 emissions through sequestration of a larger pool of recalcitrant soil organic carbon (SOC) is one of these services. This would at the same time sustain soil microbial activity, which is directly associated to soil quality, for instance, nutrient cycles and plant growth. This multiple win scenario suggests that the addition of PyOM to the soil would be the solution for the “carbon dilemma”. The dilemma states that the main biological benefits from soil organic matter are a consequence of its decay. Therefore, it is unlikely that increased C sequestration and the benefits from its decay can be simultaneously maximized. Rather than win-win, PyOM would then also be subjected to inevitable trade-offs. Additions of PyOM can modify the turnover rate of native SOC by either accelerating or decelerating its decomposition through a mechanism known as priming. Although positive priming by PyOM has been reported, negative priming has also been found. The higher amount of non-pyrogenic C in ADE, compared to non-anthropogenic surrounding soils has been considered evidence that PyOM can stabilize SOC in the long-term. A complicating issue in studies is that short-term increases in CO2 emission can be due to decomposition of labile PyOM fractions, erroneously suggesting positive priming of SOC. Addition of PyOM can also lead to modifications in the microbial activity and assemblages. Changes in microbial populations can have impacts on their functionality, favouring mutualistic root symbioses such as the arbuscular mycorrhizal fungal (AMF) symbiosis and the rhizobial symbiosis with legumes that is responsible for biological nitrogen fixation (BNF). Although soil amendments with PyOM can stimulate AMF and BNF, results are contrasting and mechanisms are not clear. Most studies of PyOM effects on SOC and on mutualistic root symbioses are from short-term experiments, often conducted in greenhouse or laboratory. Although such studies provide insights in potential factors driving changes in SOC and symbiotic relationships in PyOM-amended soils, they do not assess changes under realistic conditions over periods of time longer that one or a few cropping cycles. Therefore, there is still a gap in our understanding regarding the duration and magnitude of effects over time under field conditions and possible mechanisms involved. This thesis addresses these gaps. The aim of this research was to provide a better understanding of interactions between PyOM and SOC and the factors controlling symbiotic patterns in a tropical soil amended with PyOM. To reach this aim, I combined greenhouse and field studies. I also used meta-analytic methods in order to quantitatively synthesize data in literature. In Chapter 2, I combined the results of 46 studies in a meta-analysis. I investigated changes in CO2 emission patterns from an array of PyOM-amended soils and identified the causes of these changes and the possible factors involved. I showed an overall increase of 29% in CO2 emission from PyOM-amended soils. Such increases were only evident in soils amended with a PyOM-C (PyC):SOC ratio >2. These data are consistent with the hypothesis that increased CO2 emission after PyOM addition is additive and mainly derived from PyOM’s labile C fractions rather than from SOC. Therefore, positive priming is not a main driver of increases in CO2 emission in PyOM-amended soils. This PyC:SOC ratio provided the best predictor of increases in CO2 production after PyOM addition to soil. This meta-analysis indicates (i) the importance of taking into account the amount of applied PyC in relation to SOC for designing future decomposition experiments and that (ii) the recalcitrance of PyOM in soil-PyOM mixtures may be less than usually assumed. A technical problem of separating PyOM-induced priming on SOC from other non-additive interactions is the uncertainty regarding the origin of the respired CO2 (whether from SOC or PyOM). This issue can only be solved with the use of isotopes. In a field study (Chapter 3), I quantified changes in the PyOM and SOC stocks over four soybean cropping cycles (CC) in a sandy Ferralsol, previously supporting a vegetation with C4 plants, amended with different rates of PyOM (0, 5, 10, 20 and 40 Mg ha-1). The PyOM was produced from C3 woody species using traditional pyrolysis methods employed in Northeast Brazil. I used 13C isotopic analysis to discriminate the origin of the C in the soil and quantify the decomposition rates of native SOC and PyOM. I showed that decomposition of traditionally produced PyOM is faster (25-60% within first year) than normally assumed (10-20% within 5-10 years), which was higher than that of native SOC (5-14%). The data indicate preferential decomposition of PyOM compared to native SOC. The intensity of that effect depends on the rate of PyOM applied to the soil. Only on the longer term (>1 yr) addition of PyOM seems to stabilize SOC. In Chapter 4 I explored mechanisms controlling AMF activity and crop yield in PyOM-amended soils through the use of path analysis. I tested the effects of PyOM rates and P fertilization on soybean root colonization by AMF, soil P and plant performance over four cropping cycles (CCs). Data showed a major effect of CC and P, as well an interaction effect of PyOM x CC on mycorrhizal colonization. There was a linear decrease in root colonization by AMF in CC1 with increasing PyOM rates in contrast to a consistent linear increase in CC4. Plant performance was mainly affected by CC, but a significant interactive effect of PyOM x P was also observed on grain yield. Grain yield was highest at high PyOM rates (20 and 40 Mg ha-1) in the P-fertilized treatments in CC4. Soil pH increased in CC1 with increasing PyOM rates, but no effects were observed in CC4. Path analysis indicated that PyOM effects on root colonization by AMF were not mediated by changes in soil pH or P content. My data are consistent with the hypothesis that interference of PyOM in signalling processes is an important driver of change in AMF activity and that positive effects of PyOM on AMF and crop yield develop with time. In Chapter 5, I assessed the effects of PyOM application rates and P fertilization on BNF in soybean inoculated with Bradyrhizobium japonicum over four cropping cycles. Again I observed that CC had a significant main effect on most dependent variables, while PyOM was not a significant source of variation. There was a significant PyOM × CC interaction effect on shoot N concentration. In CC1 shoot N concentration after application of 5 Mg PyOM was significantly lower than that of plants grown on plots to which 10 or 20 Mg PyOM was applied. In CC4 shoot N concentration was not affected by PyOM. The major effect of CC was explained through changes in nutrient management, more specifically the addition of micronutrients in CC3 and CC4. Alleviation of micronutrient deficiency increased BNF and also resulted in a positive effect of P on BNF. I conclude that under conditions of adequate management, PyOM application does not improve BNF in soybean. In Chapter 6 (General Discussion) I synthesize the findings of the previous chapters and use data from additional greenhouse and litterbag field experiments to integrate the results. Data from Chapters 2 and 3 show that if any positive priming occurs due to PyOM addition, it is a small short-term event and does not lead to significant losses of native SOC in the long-term. This was confirmed by data from a 2 yr litterbag experiment, which showed no interaction between decomposition of PyOM and fresh organic matter. Stability of SOC has been considered an ecosystem property rather than a consequence of recalcitrance, but this definition has not yet been extended to PyOM. In this thesis I demonstrated that stability of PyOM can also be influenced by the soil environment. In order to link PyOM effects to SOC and on root symbioses, I performed path analysis integrating root colonization by AMF, SOC content and Ndfa in one model. We found no significant path coefficients linking AMF and BNF. The model indicated a significant positive path coefficient linking AMF root colonization and SOC in CC4, but not in CC1. The data suggest that PyOM may increase SOC stability through increased AMF activity. Soil aggregation and C sequestration are tightly correlated with abundance of AMF in the soil. I propose that the same mechanism through which AMF stabilizes native SOC may also positively influence PyOM stabilization in the long-term. In conclusion, I have shown that main beneficial effects of PyOM on AMF and crop yield develop with time, but in well-managed soils increased crop yield is not a direct consequence of increased AMF due to PyOM addition. Finally, although PyOM additions represent an effective form of sequestering C, positive effects of PyOM on crop yield are likely to occur after partial decomposition of PyOM. Therefore, although some benefits of adding PyOM can be simultaneously obtained (C sequestration and increased crop yield), they cannot be simultaneously maximized. This means that the carbon dilemma can only be partially solved by adding PyOM to the soil.

Door menselijk handelen (verandering in van natuurlijke vegetatie naar landbouw) is de organische C voorraad in de bodem met 10-59% afgenomen, afhankelijk van de landgebruiksverandering. Dit heeft geleid tot een toename van de CO2 concentratie in de atmosfeer. Deze toename heeft bijgedragen aan opwarming van de aarde en de daaraan gerelateerde milieuproblemen. Een ander risico van de vermindering van de organische C voorraad in de bodem is vermindering van de bodemvruchtbaarheid. Mede hierdoor wordt de opgave om de voedselproductie de komende 50 jaar te verdubbelen, een grote uitdaging. Vastlegging van C in de bodem kan een goedkope en milieuvriendelijke maatregel zijn die bijdraagt aan vermindering van effecten van klimaatverandering op een termijn van 20 – 50 jaar. Deze maatregel kan bovendien bijdragen aan voedselzekerheid doordat ze de bodemkwaliteit in landbouwsystemen verhoogt.

Predicting the potential of soils to supply N is of considerable importance to maximize agricultural N use efficiency and to minimize environmental losses. This research examines and evaluates the current soil testing approach, which uses extractable organic N (EON) fractions to predict soil N supply, using isotopic 15N tracing, multivariate statistics and meta-analytical techniques. Almost all 20 EON fractions that have been developed during recent decades significantly reflect the potential of soils to supply N, in spite of the strong differences in size and composition of EON due to extraction methodology. The EON fractions have therefore been considered as highly bio-available N pools in soil. However, most of them performed either worse than or similarly to total N as predictor of soil N supply, and the uncertainty of the predicted soil N supply (even under controlled environmental conditions) is still too big for serious improvement of fertilizer management. A micro-diffusion method is developed to estimate gross EON fluxes in order to investigate the biochemical basis for observed relationships between EON and soil N supply. The fate of EON fractions in N mineralization, in particular those fractions that are obtained with weak hydrolyzing salt solutions, is comparable to that of dissolved organic N (DON). Both DON and EON can be considered as (intermediate) decomposition waste products in an abiotic and biotic controlled equilibrium with total N. Therefore, their relationship with soil N supply likely reflect that both DON, EON, and soil N supply are mutually dependent on total N. The dependency of soil N supply on methodological and environmental issues strongly encourages more effort to be put into validation and up-scaling, particularly regarding the quantification of the differences between laboratory and field experiments. A combination of soil testing with simulation modeling is necessary to account for the numerous environmental factors controlling soil N supply. The exact EON fraction that can be used in such an approach is less important and practical considerations may be decisive to select one for routine application in soil analysis. In conclusion, a holistic approach, which considers spatial and temporal variability of both soil N supply and crop N demand, may provide a successful approach to improving fertilizer management at the farm-scale.