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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.

Aim: Cereal/legume intercropping often increases yield, partly because of increased nitrogen (N) and phosphorus (P) acquisition. The aim of this paper was to investigate the role of arbuscular mycorrhizal (AM) fungal common mycorrhizal networks (CMNs) in overyielding by the millet (Setaria italica L.) and chickpea (Cicer arietinum L.) mixture and to find out if the effect of a CMN depends on which of the two species was first colonized by AM fungi (AMF). 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 donor, and the plant in the neighboring, non-inoculated compartment as the 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 (donor) and millet (receiver) leading to overyielding of the mixture, whereas inoculation of millet increased biomass of chickpea (receiver) only, but did not increase N or P acquisition by any of the two species, and there was no overyielding. Chickpea as donor had higher numbers of phosphate-solubilizing bacteria in its rhizosphere compared to chickpea as receiver. The shoot N:P ratio of chickpea as donor was lower than as receiver. Conclusion: Our study demonstrated asymmetry in nutrient gains by a mixture of a cereal and a legume, dependent on which plant species was the 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.

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.

Ecological intensification seeks to achieve crop yield increases by intensifying complementary or facilitative interactions between plant species or varieties. Different species of arbuscular mycorrhizal fungi (AMF) exhibit niche differentiation and show selectivity towards certain plants, which can further enhance complementarity. It is not clear whether in the presence of one AMF species, where mycelial networks connect crop species, opportunities for complementarity effects may be reduced. We grew monocultures and mixtures of maize varieties in a greenhouse with one species of AMF, Funneliformis mosseae, during two consecutive years to investigate whether under such conditions the mycorrhizal symbiosis would affect complementarity and overyielding compared to non-mycorrhizal plants. Variety mixtures showed increased phosphatase activity and mycorrhizal colonization, enhanced phosphorus uptake and overyielding when plants were mycorrhizal. There was no overyielding when plants were non-mycorrhizal. The increase in relative yield total was due to complementarity effects. Synthesis and applications. Our study implies that appropriate agricultural management that enhances mycorrhizal fungal contribution to ecosystem services may result in overyielding in terms of yield or phosphorus uptake through mixing varieties within one crop species.

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.

A farm survey was conducted within a 100 km2 sampling block to collect data on the spatial variation in unfertilized maize biovolume and grain yields in relation to soil organic carbon (SOC), total nitrogen, phosphorus and extractable cations. Key soil factors associated with crop performance were identified using stepwise multiple linear regression modelling. The spatial variation of key soil factors and crop performance indicators (CPIs) was described in terms of spatial dependency. An analysis of variance indicated the variation explained by soil types, sampling units, and administrative units. Soil properties displayed high variability with coefficients of variation of in the range of 50% to 89% for extractable nutrients. Grain yield ranged widely from 0.1 to 11.3 t ha−1, with 31% of the variation being accounted for by measured soil properties. SOC was identified as key soil factor associated with variation in crop performance. SOC displayed moderate spatial dependency with a range of 523 m. Analysis of variation indicate that variation in SOC was sufficiently described by small spatial units (fields). These insights were used to provide a framework for determining an appropriate scale for developing digital soil maps or distance for soil sampling in heterogenous smallholder farming systems. Strategies aimed at refining fertilizer use recommendation can therefore use this guideline.

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.

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.

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.

Aims: The phosphorus (P) resource partitioning hypothesis assumes that dissimilarity in P acquisition traits among plant species leads to enhanced P uptake by crop combinations compared with their sole crops. We developed and implemented a test for this hypothesis. Methods: Two pot experiments were conducted with quartz sands. In Experiment 1, the ability of the crop species to acquire P from sparingly soluble sources (Ca phosphate (CaP), phytate (PhyP) and P-coated Fe (hydr)oxide (FeP)) was tested. In accordance with the species performances in Experiment 1, combinations of millet/chickpea and cabbage/faba bean (which have dissimilar P acquisition traits) and wheat/maize (which have similar traits) were selected for Experiment 2. The biomass production and P uptake were compared between the sole crops and species combinations as well as between the single and mixed P sources. Results: A dissimilarity in P acquisition traits enhanced P uptake by millet/chickpea on CaP/PhyP (as expected) but not by cabbage/faba bean on FeP/PhyP. Despite their similar P acquisition traits, we found enhanced P uptake by wheat/maize on CaP/PhyP. Conclusions: Because of complicating factors such as unstable P acquisition traits and competitive inequality between species, the conditions under which the P resource partitioning hypothesis can be tested are limited. This challenge complicates designing for complementarity in soil P pools by intercrops.

Plant-soil feedback (PSF) describes the process whereby plant species modify the soil environment, which subsequently impacts the growth of the same or another plant species. Our aim was to explore PSF by two maize varieties (a landrace and a hybrid variety) and three arbuscular mycorrhizal fungi (AMF) species (Funneliformis mosseae, Claroideoglomus etunicatum, Gigaspora margarita, and the mixture). We carried out a pot experiment with a conditioning and a feedback phase to determine PSF with different species of AMF and with a non-mycorrhizal control. Sterilized soil was conditioned separately by each variety, with or without AMF; in the feedback phase, each soil community was used to grow each in its "home" soil and in the "away" soil. Plant performance was assessed as shoot biomass, phosphorus (P) concentration and P content, and fungal performance was assessed as mycorrhizal colonization and hyphal length density. Both maize varieties were differentially influenced by AMF in the conditioning phase. In the feedback phase, PSF was generally negative for non-mycorrhizal plants or when plants were colonized by G. margarita, whereas PSF was positive in the other three AMF treatments. When plants were grown on home soil, hyphal length density was larger than on away soil. We conclude that different maize varieties can strengthen positive plant-soil feedback for themselves through beneficial mutualists for themselves, but not across the maize varieties.

Oilseed-derived biochar, a by-product of pyrolysis for biodiesel production, is richer in aliphatic compounds than the commonly studied wood-derived biochar, affecting both its mineralization in soil and its interaction with native soil organic carbon (nSOC). Here, we investigated the soil C sequestration potential of three different oilseed biochars derived from C3 plant material: soyabean, castor bean and jatropha cake. The chemical composition of these biochars was determined by elemental analysis (CHN) and 13C NMR spectroscopy. The cumulative CO2 efflux from 30-day laboratory incubations of biochar mixed with a sandy soil containing nSOC from C4 plants was measured as a proxy for mineralization rate. The relative contribution of each source to CO2 production was calculated based on the 13C-signatures of total CO2 efflux and the source materials (soil and biochars). Our results showed that: (i) castor bean biochar contained relatively large amounts of aliphatic compounds, resulting in a greater mineralization rate than soyabean and jatropha biochars; (ii) CO2 efflux from the soil-biochar mixtures originated mostly from the biochars, suggesting that these biochars contain rapidly decomposable compounds; and (iii) all three oilseed biochars decelerated nSOC mineralization. This negative priming effect appeared to be caused by different factors. We conclude that oilseed biochars have the potential to increase soil C stocks directly and increase soil C sequestration indirectly in the short term through negative priming of nSOC mineralization.

There is increasing evidence that the soil microbial community produces a suite of volatile organic compounds that suppress plant pathogens. However, it remains unknown which soil properties and management practices influence volatile-mediated pathogen suppression. The aim of this study was to relate soil properties to growth suppression of three plant pathogens by soil volatiles. We measured the effect of volatiles emitted from a broad range of agricultural soils on the in vitro growth of the plant pathogenic fungi Rhizoctonia solani and Fusarium oxysporum, and the oomycete Pythium intermedium. Growth suppression of pathogens by soil volatiles could be linked to various soil properties, and some aspects of microbial community composition and field history, using multiple linear regression. Volatile-mediated suppression of mycelial development occurred for each pathogen type, but the magnitude of inhibition differed among soils as well as pathogens. On average R. solani and P. ultimum appeared more sensitive to volatile suppression than F. oxysporum. Suppression of R. solani by volatiles was positively correlated with organic matter content, microbial biomass and proportion of litter saprotrophs in the microbial community, but negatively correlated with pH, microbial diversity (Shannon), and the proportion of Acidobacteria in the community. R. solani, F. oxysporum, and P. intermedium suppression by volatiles was affected by various management practices occurring in the soil's field history, such as reduced tillage, the presence of certain crops in the crop rotation, and the application of solid manure. P. intermedium suppression was also negatively correlated with soil sulphur content. This study identifies pathogen-specific drivers of growth-suppressive volatiles, a critical step in integrating soil volatiles into prediction and management of soil-borne plant diseases.

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.

Intercropping, the simultaneous cultivation of multiple crop species in a single field, increases aboveground productivity due to species complementarity. We hypothesized that intercrops may have greater belowground productivity than sole crops, and sequester more soil carbon over time due to greater input of root litter. Here, we demonstrate a divergence in soil organic carbon (C) and nitrogen (N) content over 7 years in a field experiment that compared rotational strip intercrop systems and ordinary crop rotations. Soil organic C content in the top 20 cm was 4% ± 1% greater in intercrops than in sole crops, indicating a difference in C sequestration rate between intercrop and sole crop systems of 184 ± 86 kg C ha(-1) yr(-1). Soil organic N content in the top 20 cm was 11% ± 1% greater in intercrops than in sole crops, indicating a difference in N sequestration rate between intercrop and sole crop systems of 45 ± 10 kg N ha(-1) yr(-1). Total root biomass in intercrops was on average 23% greater than the average root biomass in sole crops, providing a possible mechanism for the observed divergence in soil C sequestration between sole crop and intercrop systems. A lowering of the soil δ(15) N signature suggested that increased biological N fixation and/or reduced gaseous N losses contributed to the increases in soil N in intercrop rotations with faba bean. Increases in soil N in wheat/maize intercrop pointed to contributions from a broader suite of mechanisms for N retention, e.g., complementary N uptake strategies of the intercropped plant species. Our results indicate that soil C sequestration potential of strip intercropping is similar in magnitude to that of currently recommended management practises to conserve organic matter in soil. Intercropping can contribute to multiple agroecosystem services by increased yield, better soil quality and soil C sequestration.

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.

FeEDDHA (iron (3 +) ethylenediamine-N,N′-bis(hydroxy phenyl acetic acid) products are commonly applied to mend or prevent Fe chlorosis in plants. In soil application, racemic and meso o,o-FeEDDHA are the effective components, while o,p-FeEDDHA tends to adsorb or react to o,p-CuEDDHA. Upon interaction with soil, a plant-independent, gradual decline in soil solution concentration of meso o,o-FeEDDHA and o,p-CuEDDHA has been observed. The aim of this study was to examine to what extent biodegradation contributes to this gradual decline. A 4-week incubation experiment was done with calcareous soil to which FeEDDHA was added. The experiment involved three sterility regimes, two conditioning regimes and three time steps. Soil solution concentrations of meso o,o-FeEDDHA and o,p-CuEDDHA gradually declined in all sterility regimes. Biodegradation did not significantly contribute to the decline in concentration of any EDDHA chelate, except for CoEDDHA, which was formed in small quantities as a result of cation displacement. The rate of the process causing the decline in meso o,o-FeEDDHA and o,p-CuEDDHA concentration was higher at higher temperature and in soil not exposed to gamma irradiation. This study offers no evidence that the effectiveness of soil-applied FeEDDHA fertilizers is compromised by biodegradation.

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.

Distinct extractable organic matter (EOM) fractions have been used to assess the capacity of soils to supply nitrogen (N). However, substantial uncertainty exists on their role in the N cycle and their functional dependency on soil properties. We therefore examined the variation in mineralizable N and its relationship with EOM fractions, soil physical and chemical properties across 98 agricultural soils with contrasting inherent properties and management histories. Mineralizable N was determined by aerobic incubation at 20 °C and optimum moisture content for 20 weeks. We used multivariate statistical modelling to account for multi-collinearity, an issue generally overlooked in studies evaluating the predictive value of EOM fractions. Mineralization of N was primarily related to the size of OM pools and fractions present; they explained 78% of the variation in mineralizable N whereas other soil variables could explain maximally 8%. Both total and extractable OM expressed the same soil characteristic from a mineralization perspective; they were positively related to mineralizable N and explained a similar percentage of the variation in mineralizable N. Inclusion of mineralizable N in fertilizer recommendation systems should be based on at least one OM variable. The most appropriate EOM fraction can only be identified when the underlying mechanisms are known; regression techniques are not suitable for this purpose. Combination of single EOM fractions is not likely to improve the prediction of mineralizable N due to high multi-collinearity. Inclusion of texture-related soil variables or variables reflecting soil organic matter quality may be neglected due to their limited power to improve the prediction of mineralizable N.

The transition from anaerobic to aerobic rice (Oryza sativa L.) cultivation has been reported to decrease Zn bioavailability. To determine and understand the differences in plant Zn uptake between anaerobic and aerobic rice cultivation systems, a field plot experiment was conducted with direct-seeded rice grown on a low-Zn soil under both anaerobic and aerobic conditions. Results showed that the mass fraction of Zn in shoots and Zn uptake of plants in the aerobic field was significantly lower than in the anaerobic field. Visual Zn deficiency symptoms as whitish-brown necrotic patches on leaves were more severe on plants grown in aerobic soils than plants in anaerobic soils, indicating reduced Zn bioavailability. To determine whether the underlying causes could be of a geochemical nature, a controlled soil incubation experiment was done. Using a fully parameterized model, we found that the difference in plant Zn uptake between the two cultivation systems was small compared with that predicted by geochemical modeling, assuming Zn uptake is controlled by the soluble fraction in the soil. Whereas individual effects of soil factors (e.g., pH) were demonstrated to cause large variations in Zn concentration, only about a 15% difference in plant Zn uptake was observed. This large discrepancy reveals that the consideration of separate soil chemical processes, instead of their integrated assessment, may be inappropriate. We speculated that the discrepancy could also be associated with the chemical conditions in the rhizosphere, where the local acidity, dissolved organic C, and redox conditions may deviate significantly from the bulk soil on which the geochemical analysis was based

Received for publication February 2, 2009. Zinc (Zn) uptake by plant roots from soils low in plant-available Zn may be increased by Zn-mobilizing rhizosphere processes, including exudation of low-molecular-weight organic anions. A rhizotron experiment with a low Zn clay soil and a nutrient solution experiment were conducted to test if this occurs in six rice (Oryza sativa L.) genotypes varying in tolerance to low Zn supply. In both experiments, low Zn supply resulted in a marked decrease in biomass production of most genotypes compared with adequate Zn supply. The genotypes showed a significant variation in Zn efficiency. Plants responded to low Zn supply with increased root exudation of malate in both experiments. The malate concentration in the rhizosphere of three genotypes ranged from 0.22 to 0.59 mmol L–1 in rhizotron experiment, and the malate exudation rate of five genotypes ranged from 0.18 to 0.53 nmol g–1 root dw s–1 in the nutrient solution experiment. On average, low Zn supply in the rhizotron experiment increased rhizosphere malate concentration by 64% compared with that at adequate Zn supply. The averaged malate exudation rate at low Zn in the nutrient solution experiment was 40% greater than at adequate Zn supply. The malate exudation of rice genotypes at low Zn was not correlated to Zn efficiency or Zn uptake in either experiment. Based on a soil malate extraction experiment, the observed genotypic difference in rhizosphere malate concentration is expected to have a negligible effect on the concentration of Zn in soil solution. These findings suggest that Zn mobilization by rice genotypes cannot be explained by increased malate exudation alone, indicating that other mechanisms contribute to the variation in plant Zn uptake. The results also emphasize that effects of root exudates on soil properties need to be assessed to evaluate the role of root exudation in nutrient mobilization.

There is considerable concern in Europe that soil organic matter (SOM) contents are declining, which would threaten both agriculture and the environment. We performed a trend analysis of SOM contents in sandy soils, using historic data from routine agricultural soil analyses. Data were selected from grass, grass-maize rotation and maize fields in four adjacent provinces that had been sampled four to five times during the period 1984¿2004. Absolute (at least 1%) and relative changes (SOMt=20/SOMt=0) were calculated and regressed against initial SOM contents. Mean SOM content showed a north-south gradient per cropping system. We found no single uniform trend in SOM contents for any of the three systems. Over the 20-year period, SOM declined in c. 25% of all grasslands, amounting to 185 000 of the 635 000 hectares of land under grass and forage crops in the four provinces, and increased in a total of 267 000 hectares. Carbon accumulation in grassland sandy soils was calculated at 39 g C m¿2 year¿1 (top 5 cm). For the grasslands, initial SOM contents were linearly and negatively related to absolute changes in SOM; the relation with the relative change was best explained by using log-transformed values of SOM. We conclude that in grassland soils in the Netherlands, conservation of SOM requires identification of high-risk fields rather than high-risk areas. For continuous maize on sandy soils, the entire area may be denoted as high-risk, because all fields could reach the critical limit of 3.4% SOM in the near future

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.

Previous studies have shown that soil fungal biomass increases towards more natural, mature systems. Shifts to a fungal-based soil food web have previously been observed with abandonment of agricultural fields and extensification of agriculture. In a previous field experiment we found increased fungal biomass with reduced N fertilisation. Here, we explore relationships between fungi, bacteria, N input and grassland age on real dairy farms in the Netherlands. We hypothesised that also in pastures that are still in production there is a negative relationship between fungal biomass and fertilisation, and that fungal biomass increases with grassland age in pastures that are still in production. We expected the fungal/bacterial biomass ratio to show the same responses, as this ratio has often been used as an indicator for management changes. We sampled 48 pastures from eight organic dairy farms. Sites differed in age and fertilisation rate. We determined fungal and bacterial biomass, as well as ergosterol (a fungal biomarker). Fungal and bacterial biomass and ergosterol, showed a negative relationship with N application rate, and correlated positively with organic matter percentage. In old pastures, fungal biomass and ergosterol were higher than in younger pastures. Because bacterial biomass responded in the same way as fungal biomass, the F/B ratio remained constant, and can therefore¿in our data set¿not be used as an indicator for changing management. We conclude that the changes in fungal and bacterial biomass were driven by changes in organic matter quality and quantity. The negative relationship we found between N application rate and fungal biomass adds to earlier work and confirms the presence of this relationship in pastures with relatively small differences in management intensities. Earlier studies on shifts in fungal biomass focused on ex-agricultural fields or restoration projects. Here we show that fungal biomass is also higher in older agricultural pastures.

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.

Aluminium (Al) toxicity is considered an important factor in forest deterioration caused by soil acidification. A ratio of base cations (BC) to Al in the soil solution lower than 1 is widely used as an indicator for potentially adverse effects on tree health. In our view, the validity of the assumptions underlying the use of the BC:AI ratio as an indicator for Al toxicity in trees has never been evaluated properly. Here, we evaluate the importance of the base cations Ca and Mg in counteracting Al toxicity. Pinus sylvestris and Picea abies seedlings were grown on nutrient solution with a range of Al (0-0.25-0.5-1-2 mM) and base cation (0.25-0.5-2 mM) concentrations, giving BC:Al ratios of 1 at different levels of Al. Increasing concentrations of Al in solution caused growth reductions, which could not be counteracted by increasing concentrations of BC in solution with P. sylvestris and only partly counteracted with P. abies. Increased concentrations of Al in solution decreased the concentrations in shoot and root of both Ca and Mg, while increased concentrations of BC in solution increased tissue concentrations of BC. Growth reductions were, however, not a result of BC deficiencies, as growth reduction already occurred in tree seedlings that maintained adequate concentrations of Ca and Mg. All growth and uptake variables measured showed a higher or equal correlation with the absolute concentrations of Al or Al + BC in solution than with the BC:Al ratio. We conclude that Al toxicity is determined solely by the concentration of Al in solution. Shoot growth decreased significantly as dissolved Al increased at a constant BC:Al ratio of 1. In P. abies, but not in P. sylvestris, dissolved BC can positively affect uptake of BC and growth, which might partly alleviate the toxic effects of Al. Our results show that the mechanistic explanation for the effect of the BC:Al ratio is insufficient to describe Al toxicity. Care should be taken when using models based on the BC:AI ratio to predict the effect of Al on tree growth. (C) 2004 Elsevier B.V. All rights reserved.

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

Increasing plant species richness generally enhances plant biomass production, which may enhance accumulation of carbon (C) in soil. However, the net change in soil C also depends on the effect of plant diversity on C loss through decomposition of organic matter. Plant diversity can affect organic matter decomposition via changes in litter species diversity and composition, and via alteration of abiotic and/or biotic attributes of the soil (soil legacy effect). Previous studies examined the two effects on decomposition rates separately, and do therefore not elucidate the relative importance of the two effects, and their potential interaction. Here we separated the effects of litter mixing and litter identity from the soil legacy effect by conducting a factorial laboratory experiment where two fresh single root litters and their mixture were mixed with soils previously cultivated with single plant species or mixtures of two or four species. We found no evidence for litter-mixing effects. In contrast, root litter-induced CO2 production was greater in soils from high diversity plots than in soils from monocultures, regardless of the type of root litter added. Soil microbial PLFA biomass and composition at the onset of the experiment was unaffected by plant species richness, whereas soil potential nitrogen (N) mineralization rate increased with plant species richness. Our results indicate that the soil legacy effect may be explained by changes in soil N availability. There was no effect of plant species richness on decomposition of a recalcitrant substrate (compost). This suggests that the soil legacy effect predominantly acted on the decomposition of labile organic matter. We thus demonstrated that plant species richness enhances root litter-induced soil respiration via a soil legacy effect but not via a litter-mixing effect. This implies that the positive impacts of species richness on soil C sequestration may be weakened by accelerated organic matter decomposition.

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.

Multi-surface models are widely used to assess the potential ecotoxicological risk in metal-contaminated soils. Their accuracy in predicting metal speciation in soils with low metal levels was not yet tested. Now highly sensitive analytical techniques are available to experimentally validate such models at low concentration levels. The objective of this study was to test the accuracy of a multi-surface model to predict the Zn(2+) concentration and to improve our understanding of Zn bioavailability in low-Zn soils. High-Zn soils were included as controls. Model parameters were determined independently on the basis of earlier peer-reviewed publications. Model output was validated against free Zn(2+) concentrations determined with the soil column Donnan membrane technique in a range of soils varying in potentially available Zn, organic matter, clay silicate, and iron (hydr)oxide contents and pH. Deviations between predicted Zn(2+) concentrations and experimentally determined values over the whole Zn concentration range were less or equal to the experimental standard error, except for one low-Zn soil. The Zn(2+) concentration was mainly controlled by adsorption, where organic matter was predicted to be the dominant soil sorbent. The predicted Zn(2+) concentration depends more sensitively upon changes of the reactive Zn pool (application of 0.6, 1.2, 2.4, and 3.6 mg of Zn kg(-1) of soil) and organic matter content (± 0.2 and 0.4%) than pH changes (± 0.5 and 1 pH unit).

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 work was to study how nitrogen availability affects within-plant allocation to growth and secondary metabolites and how this allocation affects host selection by herbivores. Tomato plants (Lycopersicon esculentum) were grown at six levels of nitrogen availability. When nitrogen availability increased, plant relative growth rate increased, but tissue carbon/nitrogen ratio in the second oldest true leaf and allocation to large glandular trichomes (type VI) as well as to the defense compounds rutin and chlorogenic acid decreased. Leaf protein concentration increased. Two-spotted spider mite (Tetranychus urticae) females responded significantly to these changes: in dual choice tests they preferred leaf disks from plants grown at high nitrogen availability, with a low C/N ratio. This preference persisted in an olfactometer in which the mites were offered only the odors released by leaves with damaged trichomes. We conclude that in a tomato leaf increased nitrogen availability leads to decreased allocation to defenses, and that repellent volatiles released by trichomes play a key role in affecting leaf selection by two-spotted spider mite females.

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.

The quality of dissolved organic carbon (DOC) released from soil organic amendments may influence soil microbial activity and the quality of the soil's DOC pools. Measurements of total DOC are often considered in relation to microbial activity levels but here we propose that quantification of DOC fractions is a more informative alternative. In a laboratory incubation, soil received DOC that was extracted from three organic matter sources: fresh compost, mature compost, and a mixture of the two. Soil microbial respiration (CO2 emission), and concentrations of hydrophobic (humic acids (HA), fulvic acids (FA) and neutrals (HoN)) and hydrophilic (Hi) DOC fractions were measured throughout the 35 d incubation. The A254 specific UV absorption of total and HA DOC were measured at the start and end of the incubation as an indicator of aromaticity. Microbial respiration rates were highest in soils amended with fresh compost DOC, which had a higher proportion of Hi compounds. Concentration of Hi was significantly and positively correlated with soil respiration, explaining 24% more variation than total DOC. Humic acid concentrations significantly decreased over 35 d, including a 33% reduction in HA from an unamended control soil. Compost treated soils' HA pools increased in aromaticity, suggesting preferential mineralization of the least aromatic HA molecules. A decrease in SUVA254 values in other HA pools may be the result of HA degradation in the absence of low-aromatic HA. Our observation of depletion of hydrophobic compounds from the HA fraction provides evidence that humic substances can be a relatively reactive pool, which can provide, together with hydrophilic compounds, a readily available C source to the microbial community.

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

Accurate estimation of mineralizable nitrogen (N) from soil organic matter is essential to improve fertilizer management in agricultural systems. Mineralizable N refers to the amount of N in soil that is released during a certain period (ranging from 1 week to the length of a growing season). It has been estimated from increases in inorganic N during incubation or from N uptake by plants grown in a greenhouse or field. Many chemical soil tests measuring extractable organic N (EON) fractions have been proposed to predict mineralizable N. We evaluated the predictive value of these soil tests, using 2068 observations from 218 papers. Meta-analysis was used to find the best soil test, to analyse differences between field and laboratory experiments, and to determine whether their predictive value is affected by extraction intensity (% of total soil N that is extracted). The concentration of EON was positively related to mineralizable N, explaining on average 47% of the variation. It did not, however, explain more of the variation than total N. Best predictions (57%