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Cover crop integration in agriculture rotation is associated to multiple agronomic and environmental benefits. However, the effect of cover crop identity on the following cash crop productivity and nutrient uptake is still uncertain, particularly in relation to soil types. We set up an experiment to test the effects of four different cover crop species (Indian mustard, lupine, field pea, and oat) on maize above- and belowground biomass as well as on nitrogen and phosphorus nutrition after incorporation of the cover crop litter in two soil types (clay and sandy soil). We observed that aboveground and belowground biomass of maize was always higher in sandy than clay soil likely due to better soil physical properties. On general, in clay soil, the presence of a preceding cover crop promoted or did not modify the aboveground and the belowground maize productivity compared to bare soil. On the other hand, in sandy soil, the decomposing litter of non-leguminous cover crops decreased maize aboveground productivity whereas any preceding cover crop decreased maize root biomass. The burial of leguminous litter significantly increased the N uptake by maize in both soil types. For what concerns the phosphorus uptake by maize, it appears that due to high phosphorus soil availability, the selected cover crops did not play a major role in improving P uptake, with the only exception of field pea. Our data show that leguminous cover crops improved the N status of maize particularly under conditions of low N fertilization rates.

This study takes advantage of Swiss long-term field experiments (> 30 yrs) with different K fertilization rates at three sites to (i) test the possibility to generalize linear relationships between K extracts (ammonium acetate, K-AA; ammonium acetate EDTA, K-AAE; water, K-H2O; and water saturated with CO2, K-CO2), (ii) determine the K fertilization effect on soil exchangeable K, (iii) determine the K fertilization effect on shoot biomass and grain yield of winter wheat, (iv) analyze the possibility to derive a soil K critical value, and (v) determine a critical plant K (Kc) dilution curve as a function of shoot biomass (SB) using a “classical” and a Bayesian method. Shoot biomass during the growing season, grain yield, and four soil extracts were measured in 2018 after more than 30 years with four to five rates of K fertilization. Unpublished data of soil K-AAE concentrations, and grain yield and K concentrations since the start of the experiments were also used to analyze the relationship between soil K-AAE and the cumulative K budget. The K-AA and K-AAE concentrations can be converted from one to the other [K-AAE = 26.8 + (1.11 × K − AA)], while the relationship between K-H2O and K-CO2 depends on soil pH. The K-AAE concentrations were positively related to the cumulative K budget for K-AAE ranges from a minimal K concentration up to a K holding capacity that were specific to each site. The lack of K fertilization during several decades decreased shoot biomass in 2018 and grain yield over the course of the experiments at only one of the three sites. The K-AAE values corresponding to non-limiting soil K conditions at this site (50—75 mg K kg−1) were close to the critical values previously reported but the large range suggests that more soil parameters should be taken into account to improve the accuracy of the fertilization guidelines. The Bayesian and “classical” methods used for estimating the Kc curve yielded similar results (Kc = 58.21 × SB−0.45) that should be confirmed in future studies under a range of pedoclimatic conditions along with the effect of other nutrients and wheat cultivars.

Aims The theory of ecological stoichiometry mostly builds on studies of natural terrestrial ecosystems, whereas only limited stoichiometry information is available in response to agronomic practices. Methods We designed a greenhouse experiment in order to disentangle the specific role of cover crop identity and soil characteristic in affecting nutrient stoichiometry of a plant-microbe-soil system. Results Nutrient ratios of cover crop biomass were species-specific and the growth rate explained, for most species considered, the stoichiometric differences in response to soil type. In contrast, the nutrient stoichiometry of soil microbes was more homeostatic and did not respond to either cover crop identity or soil type. Compared to bare soil, the presence of cover crop enhanced microbial phosphorus immobilization in the clay-rich soil, whereas it promoted microbial carbon biomass and microbial nitrogen immobilization in the sandy-rich soil. A greater microbial cumulative respiration in clay soils, where a higher microbial biomass C at the beginning of the incubation was observed, suggested a major role of soil type, compared to cover crop identity, in affecting microbial metabolism. Conclusions By understanding the stoichiometric constraints in the plant-microbe-soil system, our findings can help to implement agro-ecological practices by selecting appropriate cover crop species in relation to soil type in order, for example, to avoid nutrient limitation due to microbial nutrient immobilization.

Soil organic carbon (SOC) accumulation in agroecosystems is a promising solution to simultaneously improve food security and mitigate climate change. Indeed, because of their large carbon deficit, cropland soils can potentially sequester a substantial amount of atmospheric carbon (C). To estimate the soil C-sequestration potential, it is critical to derive reliable estimations of the current soil C-saturation level. This step is essential to obtain an accurate quantification of C-deficits in cultivated soils. In addition, it is important to identify agricultural practices that favor SOC accumulation in order to reduce the soil C-deficit. Based on a 30-year old soil monitoring network of multiple cropland (CR) and permanent grassland (PG) sites established in western Switzerland, we (i) quantified the C-deficit in croplands, (ii) identified the factors driving the C-deficit and (iii) evaluated the assumption that grasslands can be used as C-saturated reference sites. We demonstrated that SOC in CR were depleted by a third compared to PG. The main factor affecting C-deficit in CR was the proportion of temporary grasslands (TG) within the crop rotation. We also showed that PG have not reached their C-saturation level in the study area and that additional C could be stored in PG soil under optimal management. When accounting for pedo-climatic differences, the C-deficit of CR that do not include TG in the rotation was equivalent to 3 kg C m-2 down to 50 cm depth. The relationship between the proportion of TG in the rotation and SOC stocks in the topsoil (0-20 cm) and subsoil (20-50 cm) was linear and similar at both depths, revealing the strong potential of the subsoil to sequester C.

International audience; Reliable estimations of soil organic carbon (SOC) deficits in agroecosystems are crucial in evaluating the atmospheric C sequestration potential of agricultural soils and supporting management decisions. Nonetheless, the co-benefit on soil quality resulting from SOC accrual is rarely considered. Here, we assessed SOC saturation and soil physical quality in permanent grasslands (PG) and croplands (CR) by applying the C-saturation concept and the SOC:clay ratio as an indicator of soil physical quality to a set of long-term monitoring sites in western Switzerland. For this goal, we produced a new relationship between the silt + clay (SC) particles and the C stored in the mineral-associated fraction (MAOM(C)) and we assessed the assumption that grasslands can be used as C-saturated reference sites. The saturation in PG was not coincidental as it depended on the C accrual history. Hence, PG with the lowest MAOM(C) have not reached their C-saturation level and present a potential SOC storage under optimal management. The MAOM(C) saturation in CR was low (62 +/- 4%) and corresponded to a deficit of -8.8 +/- 1.2 mg C g(-1 )soil as compared to the current level in PG. The saturation was mainly affected by the proportion of temporary grassland in the crop rotation. The relative distribution of C between MAOM (similar to 80%) and the fine and coarse particulate organic matter (POM) was not affected by land-use types. The MAOM(C) saturation in this study (MAOM(C) = 0.372 x SC + 4.23) was similar to that reported in the litterature, but discrepancies appeared when the silt and clay contents were considered separately. SC was by far the main factor explaining MAOM(C) amount in PG (semi-partial R-2 : 0.66). In contrast to other studies, the C content of MAOM in PG (43 mg C g(-1) SC) was not related to the SC content, suggesting a fixed maximal value in C-saturated soils. Nonetheless, MAOM(C) saturation may be underestimated as the least saturated PG might still accumulate MAOM(C). Finally, the SOC:clay ratio was correlated with MAOM(C) saturation level in CR, but not in PG suggesting that targeting SOC accrual in CR optimizes the benefits between soil C storage and soil quality.

International audience; Developing more sustainable forage systems requires efficient decision support tools for fertilization management. Soil phosphorus (P) tests have long been used as decision support tools for fertilization management but, more recently, plant nutrition indices using the P concentration of shoot biomass were developed to assess the P nutrition status of grasslands. The objectives of this study were to (i) evaluate the relationship between the phosphorus nutrition index (PNI) and the yield response to P fertilization (ii) analyze relationships between PNI and soil plant-available P (SPAP) indicators, and (iii) evaluate PNI assets for P diagnosis in forage system. Five long-term (≥ 9 years) grassland P fertilization experiments under different soil and climate environments in Canada, Switzerland, France and Romania were used. Three SPAP indicators were tested: CP, the soil solution orthophosphate ions (oPion) concentration (mg P L−1), Olsen P (mg P kg−1), and, a process-based assessment (Qw + Pr) from the sum of oPion in the soil solution (Qw, mg P kg−1) and diffusive oPion with time and CP (Pr, mg P kg−1). PNI was calculated as sward P concentration divided by the critical P concentration. The cumulative effect of P fertilization resulted in a wide range of SPAP values. Overall, CP varied from 0.03 to 3.6 mg P L−1, (Qw + Pr60 min) from 6−52 mg kg−1, and Olsen P from 4−40 mg kg−1. The PNI varied from 48–94% in plots with no applied P, and from 83–121% in P-fertilized plots. A generally positive relationship between relative forage dry matter yield and PNI was established, with a critical PNI value of 92% that distinguishes P-limited and non-P-limited grassland nutrition. Positive relationships between PNI and the three SPAP indicators confirmed that the soil P status influenced the grassland P nutrition status. Critical values on a stock basis for a target PNI value of 92% were similar for Olsen P (12.9 kg P ha−1) and (Qw + Pr60 min) (13.5 kg P ha−1). This study opens perspectives for P diagnosis improvement in forage systems.

Increasing soil organic carbon (SOC) in agro-ecosystems enables to address simultaneously food security as well as climate change adaptation and mitigation. Croplands represent a great potential to sequester atmospheric C because they are depleted in SOC. Hence, reliable estimations of SOC deficits in agro-ecosystems are crucial to evaluate the C sequestration potential of agricultural soils and support management practices. Using a 30-year old soil monitoring networks with 250 sites established in western Switzerland, we identified factors driving the long-term SOC dynamics in croplands (CR) and permanent grasslands (PG) and quantified SOC deficit. A new relationship between the silt + clay (SC) soil particles and the C stored in the mineral-associated fraction (MAOMC) was established. We also tested the assumption about whether or not PG can be used as carbon-saturated reference sites. The C-deficit in CR constituted about a third of their potential SOC content and was mainly affected by the proportion of temporary grassland in the crop rotation. SOC accrual or loss were the highest in sites that experienced land-use change. The MAOMC level in PG depended on the C accrual history, indicating that C-saturation level was not coincidental. Accordingly, the relationship between MAOMC and SC to determine soil C-saturation should be estimated by boundary line analysis instead of least squares regressions. In conclusion, PG do provide an additional SOC storage capacity under optimal management, though the storage capacity is greater for CR.

Recycling phosphorus (P) is crucial to meet future P demand for crop production. We investigated the possibility to use calcium phosphite (Ca-Phi) waste, an industrial by-product, as P fertilizer following the oxidation of phosphite (Phi) to phosphate (Pi) during green manure (GM) cropping in order to target P nutrition of subsequent maize crop. In a greenhouse experiment, four GM crops were fertilized (38 kg P ha−1) with Ca-Phi, triple super phosphate (TSP) or without P (Control) in sandy and clay soils. The harvested GM biomass (containing Phi after Ca-Phi fertilization) was incorporated into the soil before maize sowing. Incorporation of GM residues containing Phi slowed down organic carbon mineralization in clay soil and mass loss of GM residues in sandy soil. Microbial enzymatic activities were affected by Ca-Phi and TSP fertilization at the end of maize crop whereas microbial biomass was similarly influenced by TSP and Ca-Phi in both soils. Compared to Control, Ca-Phi and TSP increased similarly the available P (up to 5 mg P kg−1) in sandy soil, whereas in clay soil available P increased only with Ca-Phi (up to 6 mg P kg−1), indicating that Phi oxidation occurred during GM crops. Accordingly, no Phi was found in maize biomass. However, P fertilization did not enhance aboveground maize productivity and P export, likely because soil available P was not limiting. Overall, our results indicate that Ca-Phi might be used as P source for a subsequent crop since Phi undergoes oxidation during the preliminary GM growth.

Increasing soil organic carbon (SOC) in agroecosystems is a promising solution to simultaneously address climate change mitigation, adaptation, and food security. Yet, the best management practices that could achieve these goals remain to be identified. Here, we analyze the long-term effects of application of green manure, cereal straw, farmyard manure, and cattle slurry on SOC in a 37 year long field experiment in Switzerland. The treatment effects were compared against control conditions that received only optimal mineral fertilization. More specifically, this study aimed at evaluating the effect of organic amendments on SOC accumulation and distribution in different soil particle-size fractions by means of a set of indicators about organic matter quality (biological reactivity, humification index) and microbial activity (extracellular enzyme activities). In the absence of organic matter input, application of mineral fertilizers alone resulted in the lowest SOC content and the highest humification index of the bulk soil organic matter. Among the organic amendments, cereal straw, farmyard manure, and cattle slurry promoted a higher SOC content and a lower humification index due to an increase of SOC in the clay-size fraction. The annual C accrual reached 4.4&permil, per year over 37 years with farmyard manure. The higher biological reactivity measured for the green manure and cereal straw amendments was associated with higher soil enzymatic activities, while C retention coefficients decreased by at least 2.5 times compared to animal-derived amendments. The low availability of nutrients in green manure and straw amendments as suggested by the high phosphatase and N-acetylglucosaminidase activities may indicate a reduction in C retention of organic matter inputs due to nutrient microbial mining with plant-derived amendments.

Potatoes require high nitrogen (N) fertilizer rates because of their poor N efficiency. Better understanding of N dynamic in potato crops could improve N efficiency and thus enhance crop profitability and reduce N losses. A similar field experiment was conducted in Switzerland in 3 years, from 2009 to 2011, to investigate the yield and quality response to N fertilization of two commercial potato cultivars with different tuber qualities, Bintje and Laura. Five doses of ammonium nitrate were tested: 0, 80, 120, 160 and 200 kg N ha−1. Aboveground and belowground biomass evolution, total yield, starch concentration and tuber sizes were measured annually. In 2011, the total N uptake and the soil mineral N content were also measured during the growing season and at harvest. The study showed that N fertilization had a positive effect on yield and the percentage of large tubers (> 70 mm) and a negative effect on starch concentration. Both cultivars presented the same potential yield, although cv. Laura’s yield was more affected by N fertilization deficiency and more responsive to the late N fertilizer application. At harvest, both cultivars had a similar N uptake efficiency and N utilization efficiency. However, they differed with respect to N uptake dynamics. Nitrogen uptake was slower for cv. Laura than for cv. Bintje due to a longer period required for the development of the belowground biomass. The results provide useful recommendations for improvement of N fertilization practices (e.g. rate and time of application) of these two cultivars in Swiss conditions.

International audience; Climate change is increasing crop losses and yield variability with impacts for global food security. In this context, conservation agriculture appears as a potential solution to maintain crop productivity, soil fertility and environmental sustainability. Therefore, understanding the combined effects of soil tillage and crop succession over a long period is of primary interest. In this study, we analyzed data from a 50 year long-term field experiment to assess (i) the change of climatic parameters, wheat yield and soil organic carbon (SOC) content; (ii) the combined effects of crop succession (monoculture vs. crop rotation) and soil tillage system (minimum tillage vs. plough) on wheat yield, SOC content and other soil properties at three soil depths (0-10, 10-20 and 20-50 cm); and (iii) the relative contributions of climatic parameters, wheat phenology and agricultural practices on wheat yield variability. Wheat yield was 16% higher in crop rotation compared to monoculture, while soil tillage system had no significant effect on wheat yield during the period 1977-2016. Despite a SOC content decline over time, which was especially marked during the first ten years of the study, SOC content was 7% higher in the minimum tillage treatment compared to the plough treatment, while crop rotation had no significant effect. In 2016, after 50 years of experimentation, both crop succession and soil tillage systems influenced soil properties. Over the 50-year period, the climatic conditions around the heading phase explained 22% of yield variability, while 18% of this variability was explained by crop succession and 6% by the growing degree days until heading stage. In a context of conservation agriculture promotion, our long-term field experiment provides key evidence that the combination of both minimum soil tillage and crop rotation improves soil fertility and crop productivity.

The potential to use calcium phosphite (Ca-Phi) as phosphorus (P) fertilizer may represent an effective recycling of P-containing by-products. A greenhouse experiment was conducted to investigate the effect of Ca-Phi (38 kg P ha−1) on soil properties and the growth parameters of four green manure species in clay and sandy soils using Ca-Phi, TSP (triple superphosphate) and control (no fertilization) as treatments. Eight weeks after sowing, we measured aboveground biomass yield, phosphite (Phi) concentration in plant biomass, different soil P pools as well as microbial biomass nutrients. Compared to control, the addition of Ca-Phi did not negatively affect green manure yield, except for lupine (Lupinus albus L.) in clay soil. The Phi concentration in plant biomass varied across species and soil type with a maximum concentration of about 400 mg Phi kg−1 for mustard (Brassica juncea L.) in clay soil. Compared to control, TSP and Ca-Phi fertilization had a similar effect on different P pools and microbial biomass nutrients (C, N and P) although the response was soil–type dependent. In the sandy soil, after Ca-Phi addition the amount of available P (PNHCO3) increased to the same extent as in the TSP treatment (i.e. around 6 mg P kg−1) suggesting that Ca-Phi was, at least partly, oxidized. In the clay soil with high P fixing capacity, Ca-Phi promoted higher PNaHCO3 than TSP likely due to different solubility of chemical P forms. Additional studies are however required to better understand soil microbial responses and to quantify the P agronomical efficiency for the following crop under Ca-Phi fertilization.

Optimizing the use of phosphorus (P) fertilizers is essential considering the environmental issues linked to over fertilization and the limited rock phosphate reserve. Diagnosing the crop P status by calculating a phosphorus nutrition index (PNI) based on the plant critical P concentration (Pc) could help determine the minimum amount of P fertilizer to ensure maximum crop yield. This study investigated (i) the effect of P fertilization on winter wheat (Triticum hybernum L.) shoot biomass and grain yield after several decades of different annual P fertilization rates, (ii) the stability over several site-years of the Pc expressed either as a function of shoot biomass or shoot nitrogen (N) concentration, and (iii) the possibility of using the PNI or the shoot N-to-Pc ratio as nutritional indicators to diagnose P deficiency and predict the expected response to P fertilization. Shoot biomass and its P and N concentrations were measured weekly at five site-years along with grain yield after several decades of P fertilization treatments that ranged between 0 and 5/3 of the theoretical P crop uptake. The P fertilization did not affect the grain yield, but it generally increased shoot biomass especially at the CD 27- -37 developmental stages. The Pc expressed either as a function of shoot biomass or shoot N concentration differed among site-years and this was attributed to differences in crop N status. When N was not deficient, we developed a Pc dilution curve as a function of shoot biomass (SB) (4.56 × SB−0.279) along with a linear relationship (Pc = 1.10 + 0.061 × N) and a power function (Pc = 0.34 × N0.632) between Pc and shoot N concentration. The N-to-Pc ratio was related to shoot biomass accumulation according to a power function, but the relationship also differed among site-years. The relative shoot biomass responded positively to the PNI up to different thresholds for limiting and non-limiting N conditions. The relative grain yield, however, was not related to the PNI. The PNI, based on the Pc dilution curve, has potential as a nutritional indicator to diagnose P deficiency and the expected response to P fertilization, but more research is needed to clarify the effect of N deficiencies on the Pc dilution curve.

There is an increasing awareness about the need for improving nitrogen use efficiency in crop production in order to meet economic and ecological standards. The present work provides insight into the varietal factors of wheat that determine nitrogen use in the plant. The performance of eleven winter wheat varieties in terms of grain yield and nitrogen uptake and use efficiency was tested within the framework of a 40-year long term field experiment of organic enrichment and mineral nitrogen fertilisation treatments. Globally, organic enrichment had a beneficial effect on the yield and grain nitrogen concentration and showed a strong interaction with the amount of applied mineral nitrogen fertiliser. Manure application generally had positive effects on varietal performances mainly due to indirect long term effects on the soil properties. Varieties showed a broad range of responses to the treatments of the long term experiment, revealing significant genotype × environment interactions. Nevertheless, the varieties which performed well at high input levels were also the best at low input levels, suggesting that the genotype × environment interactions were not strong enough to inverse the performance ranking. Similarly, the varietal traits associated with high yielding or grain nitrogen concentration in high input conditions were the same as those identified under low input conditions. To conclude, these results suggest that the selection of wheat for nitrogen efficiency is possible under any nitrogen fertilisation regime. However, to be adapted to low input or organic agriculture, varieties also need traits other than nutrient use efficiency, for example, disease resistance, resilience to abiotic stresses and competitiveness against weeds.

International audience; Phosphorus (P) crop fertilization requires optimal management to avoid the waste of a non-renewable resource and water pollution, but current methods for assessing soil phyto-available P and plant P requirements are not sufficiently precise to meet this goal. The objectives of the present study were to (1) evaluate the effect of long-term P fertilization on the grain yield of winter wheat, maize, and rapeseed, (2) validate or establish models of critical shoot P concentration (P-C) based on relationships of shoot P concentration with either shoot biomass or shoot nitrogen (N) concentration, and (3) assess both plant-based and soil-based diagnostic tools for managing P fertilization. A long-term field experiment with contrasted P fertilizer treatments, established in 1971 by Agroscope in Changins (Switzerland), was used to measure the shoot biomass and P concentration of winter wheat in 2011, maize in 2012, and rapeseed in 2014 weekly during the growing period and the grain yield at harvest. Soil available P in the 0-0.20m soil layer was assessed by three chemical extractions: ammonium acetate EDTA (P-AAE), sodium bicarbonate (P-NaHCO3), and CO2-saturated water (P-CO2). Long-term P fertilization increased soil available P extracted by P-CO2 (+24%), P-AAE (+200%), and P-NaHCO3 (+155%), shoot growth and grain yield by 8.4% and 26.2% for winter wheat and rapeseed respectively but had no effect on maize. The relationships between P-C and shoot biomass or N concentration were described respectively by allometric and linear models (R-2>0.85, n=21, 28 and 32 for winter wheat, maize and rapeseed respectively; slope P values for linear models

Aims Biological nitrogen fixation by legumes is expected to play a greater role in future cropping systems. Our study evaluated 19 legume species grown as cover crops in Swiss agroecosystems. Methods Two field experiments were set up to monitor the biomass production and nitrogen content of 19 legumes and two non-legumes. The proportion of nitrogen derived from atmospheric N2 (%Ndfa) was assessed using the 15N natural abundance method. In parallel, a pot experiment was set up to determine the species-specific B values necessary to apply this method. Results Some species produced an important amount of biomass in 3 months, up to 6.86 t/ha for Vicia faba. Five species, Lathyrus sativus, Pisum sativum, Vicia sativa, Vicia villosa, and V. faba, acquired more than 100 kg/ha of N through biological fixation. Important amounts of nitrogen were also derived from the soil. %Ndfa values showed high variability between and within species, ranging from 0 % to almost 100 %. Conclusions Some legumes showed high N accumulation even in a short growing period, and could play an important role in fixing renewable nitrogen in crop rotation.

To address the influence of soil tillage reduction on crop yield and soil properties, an experiment was set up in 1969 in the western part of Switzerland. A conventional tillage treatment with plough was compared to a minimum tillage treatment and a deep non inversion tillage treatment, converted to no till in 2007. Evolution of crop yield through time was investigated, as well as the soil properties in 2013. Mean soil properties and their stratification with depth were assessed. The results showed that, after 44 years, globally, all tillage treatments allowed to maintain similar yields in the long term. However, during the same time, soil properties have changed deeply. Soil organic carbon has decreased compared to the initial situation, in all treatments except in the minimum tillage. This treatment also allowed to reach high clay to carbon ratio in the upper layer, suggesting good soil structural quality compared to the other treatments. In contrast, this did not result in significant differences in carbon stocks between tillage treatments, probably due to low carbon inputs in all treatments. In addition, a strong stratification pattern with depth was observed for most of the nutrients in the minimum tillage treatment, while the situation was more homogeneous in the plough treatment. The adoption of no till also modified soil properties and lead to clear stratification patterns after only six years. These results showed that crop yield could globally be maintained in reduced tillage systems, while insuring high soil fertility and structural quality. The important decrease in the number of tillage interventions and intensity of disturbance induced an improvement of soil properties. Reduced tillage practices could thus be advantageously adopted to insure crop production together with soil fertility improvement in rather short time period.

The sustainable P management of grassland ecosystems require a better understanding of the effect of P fertilization on the growth of grassland swards and the relationship between shoot P and N concentrations with the overall goal of developing improved methods of predicting P requirements of grassland swards. Our objectives were to determine the importance of P supply during the primary growth in spring, to analyze the optimal relationship between shoot P and N concentrations for timothy (Phleum pratense L.) and multi-species swards under a wide range of P nutrition, and conditions of soil and climate, and to confirm for grassland swards the decrease in the N:P ratio during crop growth previously reported for annual crops. Experiments with varying rates of P fertilization were conducted for two to five consecutive years at sites with timothy swards in Canada (Levis, Normandin, Charlottetown, and Quebec) and Finland (Maaninka), and at sites with multi-species swards from long-term P fertilization experiments in Switzerland (Les Verrieres) and France (Erce). Dry matter (DM) yield, and N and P concentrations were measured on four dates with one-week intervals from the vegetative to late heading stages of development during primary growth. Our results (i) indicate that a P deficiency mostly affects the early season growth of grassland swards corresponding to shoot biomass less than approximatively 1 Mg DM ha−1, (ii) confirm the optimal relationship between shoot P and N concentrations for timothy and multi-species swards but with variations of the relationship with the level of N nutrition and between timothy and multi-species swards, and (iii) confirm for grassland swards the decrease in the N:P ratio during the primary growth and the close relationship of the N:P ratio to shoot biomass.

Potassium (K) is a crucial element for plant nutrition and its availability and spatial distribution in agricultural soils is influenced by many agro-environmental factors. In Switzerland, a soil monitoring network (FRIBO) was established in 1987 with 250 sites distributed over the whole of the canton of Fribourg (representing 4% of the surface area of Switzerland), whose territory is shared between the Swiss Midlands and the Western Alp foothills. In this study area, diverse geological deposits (sandstone, marlstone, silts and calcareous rocks), soil types (Cambisols, Gleysols, Rendzinas, Luvisols and Fluvisols) and land uses (cropland, permanent grassland and mountain pasture) are present, making the network interesting for assessing the relative contribution of environmental variables and land use management on soil properties. The aims of the present study were to (i) characterize the soil K status in the Fribourg canton according to four different extraction methods; (ii) analyse the spatial variability of soil K in relation to land use, soil type, soil parent material and topography; (iii) evaluate the spatial predictability of K at the canton level; and (iv) analyse the implications for K fertilization management. The overall amount of soil total K averaged 13.6 g·kg − 1 with significant variations across the sites (5.1–22.1 g·kg − 1 ). The spatial distribution of total K content was influenced by relatively extended soil forming processes, as suggested by (i) a significant global spatial autocorrelation measure at the 10 km scale (Moran's I = 0.43); (ii) significant differences observed among soil types and soil parent materials and (iii) significant correlations with land attributes such as elevation ( r = − 0.51). On the other hand, available mean K forms were significantly different among land uses, with the highest mean values of available K encountered in permanent grasslands, from 46.3 mg·kg − 1 (water extraction) to 198 mg·kg − 1 (acetate ammonium + EDTA extraction). All K forms (total and available) showed similar spatial regional patterns for all spatial interpolation methods, with areas dominated by permanent grassland and crops presenting higher values. However, these trends were less pronounced for the available K forms due to the prevalence of on-farm management practices for these K forms ( e.g. fertilization), likely inducing high spatial and temporal variability. This hypothesis was supported by spatial clustering of low and/or high K fertility status that could be related to local particular farming practices. Grasslands require particular attention with regard to overall high K fertility status.

Controlling phosphate (P) release from agricultural soils to water while maintaining optimal plant growth conditions remain a major challenge for the development of sustainable agricultural systems. To achieve this, it is important to have a proper knowledge of the amount of soil P that can be mobilized by water and of the kinetics of P release. We evaluated the ability of a flow-through reactor in which 33 P labeled soils can be inserted and leached continuously with deionized water, to assess P release. The experiment was conducted on five grassland soils presenting a large range in P availability. The availability of P in these soils was further modified by submitting them to 0 to 3 plant growth cycles with Italian ryegrass ( Lolium multiflorum ) with three levels of P added (0, 20 and 40 mg P kg soil −1 ). The P input–output balance, water and oxalate extractable P, the degree of P saturation of the soil and the amount of isotopically exchangeable P ( E value) were assessed in all samples. A subset of these soil samples was labeled with 33 P, introduced in a flow-through reactor and the release of P and 33 P measured over 14 days. The cumulated amount of P released after 14 days was strongly correlated to the amount of oxalate extractable P, isotopically exchangeable P ( E value), and water extractable P. The P release kinetics was modeled with a 2 pools model with each pool following first order kinetics. Plants were able to take up P from both pools. Assuming that the leached P had the same isotopic composition as the pool of soil P it came from it became possible to quantify the amount of isotopically exchangeable remaining in the soil which was called the D value. D decreased during the three first days of the flow-through experiment and then increased linearly with time reaching a maximum after 14 days. This maximum remained lower than the oxalate extractable P. Processes contributing to this increase were isotopic exchange and possibly also some organic P mineralization. The D value was strongly linearly correlated to E values measured after different exchange times, but for a given exchange time, the D value was lower than the E value, whereas equality could have been expected. This difference was related to the high rate of 33 P export from the soil at the beginning of the flow-through experiment. The D value was also strongly correlated to the oxalate and water extractable P. In conclusion, we suggest that the use of the flow-through reactor yields relevant information on the amount of P that can be leached from a given soil, and that the D value delivers information on the amount of isotopically exchangeable P remaining in the soil and therefore which could still be leached if sufficient time would be given.

Phosphorus (P) is the second essential nutrient for plant growth but can become an ecological and economical concern in case of over-fertilization. Soil P dynamic is influenced by many parameters like soil physical-chemical properties and farming practices. A better understanding of the factors controlling its distribution is required to achieve best P crops management. In Switzerland, the FRIBO network was launched in 1987 and comprises of 250 sites covering a wide diversity of soils and three different land uses (croplands, grasslands and mountain pastures) across the Fribourg canton. A spatial investigation of the different P forms for the FRIBO network led to the following main conclusions: i) The P status in agricultural soils was significantly different among the three land uses encountered, with the highest mean values of available P found in croplands (from 2.12 to 81.3 mg.kg-1 according to the indicator used), whereas total P was more abundant in permanent grasslands (1186.2 mg.kg-1), followed by mountain pastures (1039.0 mg.kg-1) and croplands (935.0 mg.kg-1). A full characterization of the soil P status provides necessary data on P distribution related to soil properties and land use, and should help to develop more accurate estimation procedures and fertilization strategies in a near future; ii) Environmental variables derived from digital elevation model (DEM) only explained a small part of the spatial variation of the different P forms (20 to 25%). Thus, the geostatistic analyses revealed that land use play a major role in soil P distribution. However, this pattern was less visible for total P than for available P. Future studies should include more data points as well as additional variables such as parent material and soil type to accurately estimate the role of soil parameters on the distribution of P-related forms.

Soil carbon sequestration (SCS) is one of the cheapest and technically least demanding carbon dioxide (CO2) removal (CDR) or negative CO2 emission technologies. For a realistic assessment of SCS, it is critical to evaluate how much carbon (C) can be stored in soil organic matter under actual agricultural practices. This includes typical crop rotations and fertilization strategies, depends on resources that are available (e.g. farmyard manure (FYM)) and are affordable for farmers. Furthermore, it is important to assess SCS based on given climatic and soil conditions. Here, we evaluate changes in soil C storage for Switzerland using data from eleven long-term field experiments on cropland and permanent grassland that include common local practices. At all sites, changes in soil organic carbon (SOC) stocks were measured in topsoil (∼0-0.2 m) in response to a total of 80 different treatments including different types of mineral or organic fertilization (e.g. FYM, slurry, peat, compost) or soil management (tillage vs. no-till). The treatments were applied to different, diverse crop rotations or grass mixtures that are representative for Switzerland. We found that topsoils lost C at an average rate of 0.29 Mg C ha−1 yr−1, although many of the investigated treatments were expected to lead to SOC increases. Based on a linear mixed effects model we showed that SOC change rates (ΔSOC) were driven by C inputs to soil (harvest residues and organic fertilizer), soil cover and initial SOC stocks. The type of land use or soil tillage had no significant effect. Our analysis suggests that current efforts to manage soils sustainably need to be intensified and complemented with further techniques if Switzerland wants to achieve the goal of the 4 per 1000 initiative.

International audience; Though the potassium (K) status of the soil often relies only on its exchangeable-K content, release from nonexchangeable K also contributes to plant nutrition. This process, as well as the opposite one, K fixation, are diffusion controlled and thus depend on time. This study was conducted to (i) assess the influence of time (2 h, 1 d, and 8 d) on soil K dynamics; and (ii) compare the soil ability for K release and fixation (β) estimated from sorption–desorption experiments with the soil fixation capacity (FC) estimated from fixation experiments at two different K doses. The studied soil samples came from a long-term fertilization experiment established in 1976 by the Swiss Research Station Agroscope ACW in a Gleyic Cambisol (FAO classification system). They correspond to two extreme K treatments: (i) no K fertilizer applied but with P fertilizer applied in quantities equivalent to the uptake by the crops and (ii) K and phosphorus (P) fertilizers applied in quantities equivalent to the uptake by the crops with an additional 166 kg K ha−1 y−1 and 26.2 kg P ha−1 y−1. Estimated equilibrium K concentrations decreased and, conversely, the soils’ K buffer power (i.e., the soil capacity to resist a change in soil solution K concentration following removal or input of K to the soil–plant system), β, and FC increased when the soil K content decreased. Time significantly influenced only the sorption–desorption curve and the release–fixation of the low-K-status soil. Here, K buffer power, β, and FC increased with time. A first-order model described the time series of exchangeable-K levels during the K-fixation-capacity experiments. However, a parabolic diffusion model was superior at explaining the variance. FC and β were linearly related (r2 = 0.92; P < 0.01), but FC systematically overestimated β. This shows that FC did not appear as a perfect surrogate of β.

The combined effects of the nature of fertilizers (chemical and/or organic), splitting of manure inputs and tillage intensity (reduced or conventional) on soil properties, crop production and crop response to nitrogen (N) fertilization were studied in Changins, Switzerland between 1997 and 2009. Five main-treatments were tested in a split-plot design: (i) mineral fertilizer with reduced-tillage (MinRT), (ii) manure every year plus mineral fertilizer with reduced-tillage (Ma1RT), (iii) manure every year plus mineral fertilizer with conventional-tillage (Ma1CT), (iv) manure every three years plus mineral fertilizer with reduced-tillage (Ma3RT) and (v) slurry every year plus mineral fertilizer with reduced-tillage (Slu1RT). Sub-treatments included two levels of N-fertilization: an optimal dose (according to the Swiss fertilization guidelines) and a sub-fertilization (60% of the optimal dose). The soil was a Calcaric Cambisol with, in 1997, 20.5 g kg −1 of soil organic matter (SOM) in the first twenty centimeters. After twelve years of experimentation, SOM contents were 19.8, 20.3, 21.3, 21.5, and 22.8 g kg −1 under respectively Ma1CT, MinRT, Ma1RT, Slu1RT and Ma3RT treatments. The main-treatments do not have a significant effect on SOM contents and chemical soil properties. When N-fertilization was non-limited (optimal dose) and manure was applied, tillage intensity had not significant effect on grain yield. When N-fertilization was non-limited with reduced tillage (RT), the crops in the treatments with organic fertilizers yielded 2–13% more grains (0.2, 0.3, 0.4 and 0.5 t ha −1 more for respectively rapeseed, spring cereal, maize and winter wheat) than those in treatments with mineral fertilizers only. The sub-fertilization (60% of the optimal dose) decreased the grain yields by 9, 13, 15, 7 and 16%, respectively, in MinRT, Ma3RT, Ma1RT, Ma1CT, Slu1RT. In conclusion, organic fertilizers and reduced tillage provide effective means to conserve soil fertility and crop production in the studied soil, although both enhance N fertilizer needs. Splitting manure applications into lower amounts annually did not bring any benefits to soil properties or crop production.

Isotopically exchangeable phosphate (P) is the main source of P for most crops. The amount of P that is located on the solid phase of a soil and that can exchange with P in the soil solution can be calculated knowing the concentration of water-extractable P (CP ) and two kinetic parameters: (i) the ratio between the total introduced radioactivity added as 33P in a soil/water system and the radioactivity remaining after 1 min in this solution (R/r1minute ) and (ii) the rate of disappearance of 33P from the solution with time (n). The aim of this article was to propose statistical models describing R/r1minute and n as a function of CP and other soil properties. By doing so we made the hypothesis that it is possible, knowing CP and selected soil properties, to calculate the amount of P located on the solid phase of the soil that remains isotopically exchangeable after 1 min and 60 min (Pr1minute , Pr60minutes ). This work was done in two steps. First, isotopic exchange kinetic experiments were carried on nin...

In agro-ecosystems, fertilization practices are crucial for sustaining crop productivity. Here, based on a 50-year long-term experiment, we studied the influence of fertilization practices (inorganic and/or organic) and nitrogen (N) application rates on (i) soil physicochemical properties, (ii) microbial and earthworm communities and (iii) crop production. Our results showed that soil organic carbon content was increased by incorporation of crop residues (+2.45%) and farmyard manure application (+6.40%) in comparison to the use of mineral fertilizer alone. In contrast, soil carbon stock was not significantly affected by these fertilization practices. Overall, only farmyard manure application improved soil physicochemical properties compared to mineral fertilization alone. Soil microbial population was enhanced by the application of organic amendments as indicated by microbial biomass and phospholipid-derived fatty acids contents. The fertilization practices and the N application rates affected significantly both the biomass and composition of earthworm populations, especially the epigeic and endogeic species. Finally, farmyard manure application significantly increased crop yield (+3.5%) in comparison to mineral fertilization alone. Crop residue incorporation rendered variable but similar crop yields over the 50-year period. The results of this long-term experiment indicate that the use of organic amendments not only reduces the need for higher amount of mineral N fertilizer but also improves the soil biological properties with direct effects on crop yield. (C) 2016 Agroscope. Published by Elsevier B.V.

Phosphorus (P) losses from many agricultural soils are driven by fast transport processes, such as surface runoff, shallow interflow and macropore flow. Close interaction of rain with topsoil layers (0–5 cm), rich in available P, results in high P concentrations in these rapid runoff pathways. This is particularly pronounced in permanent grassland soils typically characterized by a P enrichment of the topsoil. One option to mitigate P losses from grassland is therefore to reduce soil P content, particularly available P, in this critical surface layer. Omitting application of P fertilizer will reduce soil P in the long term but more drastic methods may be necessary to achieve P loss reductions in shorter timescales. In this study, three management options to reduce P in runoff from grassland soils were evaluated in a field experiment lasting 2 years on two grassland sites: (i) no P inputs, (ii) no P inputs plus soil tillage and (iii) no P inputs plus tillage combined with application of a P sorbing soil amendment (iron oxide). Withholding P inputs did not decrease either the available P in the soil or the P concentrations in runoff from sprinkler experiments over a 2-year period. Both the treatments which included tillage caused immediate reductions in both soil available P and runoff P, with Fe ⁄ OH amendment being more effective. However, in the second year the effects of both tillage treatments were less apparent and only the Fe ⁄ OH treatment significantly decreased runoff P. The results indicate that the long-term enrichment of soils due to P application in excess of plant needs is unlikely to be remedied by short-term treatments. However, further work is required to assess the applicability of our conclusion to other sites.

National audience

International audience; We analyzed the linearity of relationships between soil test P (STP) and cumulative phosphorus (P) budget using data from six long-term fertilized grassland sites in four countries: France (Erce´ and Gramond), Switzerland (Les Verrie`res), Canada (Le´vis), and Finland (Maaninka and Siikajoki). STP was determined according to existing national guidelines. A linear-plateau model was used to determine the presence of deflection points in the relationships. Deflection points with (x, y) coordinates were observed everywhere but Maaninka. Above the deflection point, a significant linear relationship was obtained (0.33\r 2 \0.72) at four sites, while below the deflection point, the relationship was not significant, with a negligible rate of STP decrease. The relationship was not linear over the range of STP encountered at most sites, suggesting a need for caution when using the P budget approach to predict STP changes in grasslands, particularly in situations of very low P fertilization. Our study provides insights and description of a tool to improve global P strategies aimed at maintaining STP at levels adequate for grassland production while reducing the risk of P pollution of water.

Accurate measurement and characterization of phosphate rock dissolution are important for a better understanding of phosphorus (P) availability in soils. An incubation study was carried out on two New Zealand topsoils (0–15 cm; high P buffering capacity Craigieburn and low P buffering capacity Templeton) amended with North Carolina phosphate rock (NCPR) and water‐soluble phosphate (WSP) at 218 mg P kg−1 (equivalent to 60 kg P ha−1). Isotopic exchange kinetics was carried out after 12 h and 28 days of incubation to characterize P availability. This study showed that sensitivity of capacity factors (r1/R, n) to explain changes in E1min values was affected by the P buffering capacity of the soils. The recovery of applied P in the E pool (RecinE%) with extended incubation time was similar from the NCPR and WSP treatments (3.1–3.3%) in the Craigieburn soil compared with the Templeton soil in which RecinE% values were greater in WSP (9%) than NCPR (1.3%) treatment. The higher values of P derived from t...

The mineralization of organic forms of phosphorus (P) in soil contributes significantly to plant P uptake, although quantification of organic P mineralisation has been impeded by methodological difficulties. An isotopic dilution method, based on tracer kinetic theory, was used to study the organic P flux rates for unamended and green manure amended soil in an incubation experiment carried out over 35 days. Firstly, the specific activity (SA) between two points of time as affected by dominant biological processes (under conditions of constant soil respiration rates) was determined in a series of successive labelling incubation experiments. Secondly, the instantaneous specific activity (instSA) between two points of time was also determined by shaking an unlabelled soil with 33P, i.e. soil samples that were not labelled with 33P, but kept under conditions identical to the labelled soil samples. This gives the net size of the exchangeable inorganic P pool in the labelling experiments at two points of time during incubation. The SA dilution in the labelling experiments at a constant exchangeable inorganic P pool between two times was attributed to the release of non-labelled inorganic P from the organic P pool by mineralisation. The daily gross organic P mineralisation rates for the 21 days of incubation were 0.06 and 0.27 mg P kg−1 day−1 for unamended and green manure amended soils, respectively. The study demonstrated that green manure amendment contributed to an overall increase in soil P availability through enhanced organic P mineralisation.

Summary Many irrigation experiments determine phosphorus (P) losses from soil. Often, these studies cannot be compared, because the irrigation water was not characterized. We used calcium-rich tap water and deionized water to investigate the influence of water composition on P concentrations in induced runoff. We irrigated two grassland sites: one acid and one calcareous. Less P was measured in runoff from tap water irrigation than from deionized water, especially for the acid soil. Batch experiments confirmed the findings of the field experiments. Tap water decreased water-soluble phosphate and increased calcium in the solid phase. This interaction increased with decreasing soil:water ratio. Water of low ionic strength gave results comparable to rainwater. Our findings demonstrate that solution chemistry and the soil:water ratio can strongly influence the availability of P for transport. We recommend that P tests or irrigation experiments should use water resembling that of the system of interest. Irrigation experiments aiming to simulate P losses by surface runoff should be carried out with water having a composition comparable to rainwater.

Isotopically exchangeable phosphate (P) is a major source of P for plants. In practice, however, plant-available P is assessed by chemical extractions solubilizing a mixture of P forms the availability of which is ill defined. We undertook an isotopic approach to assess the exchangeability of P extracted by (1) CO2-saturated water (P-CO2), (2) ammonium acetate EDTA (P-AAEDTA), and (3) sodium bicarbonate (P-NaHCO3) compared to the exchangeability of P extracted by water. Five topsoils with similar P-fertilization histories but different soil properties were studied. Phosphorus was extracted from soils labeled with carrier-free 33P after 1 week of incubation, and the specific activity (SA = 33P / 31P) of the extracts was compared with the SA of P extracted by water to calculate the amount of P isotopically exchangeable that had been solubilized during the extraction. P-CO2 extracted between 20 and 100 times less P than P-AAEDTA and P-NaHCO3. The SA of P-CO2 was not different from the SA of water-extractable P, showing that P-CO2 solubilized similar forms of P as water and that these forms can be considered as available. The SA of P extracted by the two other methods ranged between 25% and 63% for P-AAEDTA and 66% and 92% for P-NaHCO3 of the SA of water-extractable P. The fraction of exchangeable P extracted by AAEDTA decreased linearly with increasing soil pH, suggesting that this method dissolves slowly or non-exchangeable P from calcium phosphates. Phosphataustauschbarkeit, bestimmt mittels vier chemischer Extraktionsmethoden Isotopisch austauschbares Phosphat (P) ist eine der Haupt-P-Quellen fur Pflanzen. In der Praxis wird pflanzenverfugbares P mittels chemischer Extraktionen ermittelt. Dabei werden unterschiedliche P-Formen extrahiert, deren Verfugbarkeit nur ungenugend definiert ist. Mittels der Isotopentechnik verglichen wir die Austauschbarkeit von P, das extrahiert wurde durch 1) CO2-gesattigtes Wasser (P-CO2), 2) Ammoniumacetat EDTA (P-AAEDTA) und 3) Natrium-Bicarbonat (P-NaHCO3), mit der Austauschbarkeit von mit Wasser extrahiertem P. Dabei wurden funf Oberboden mit gleicher P-Dungung, aber unterschiedlichen Bodeneigenschaften untersucht. Die Boden wurden wahrend einer Inkubationszeit von einer Woche mit 31P-tragerfreiem 33P markiert. Um die Menge an isotopisch austauschbarem P, welches wahrend der Extraktion aus dem Boden gelost wurde, zu bestimmen, wurde die spezifische Aktivitat (SA = 33P / 31P) der Extrakte mit der SA von P, das mit Wasser extrahiert wurde, verglichen. Die Extraktion mit CO2-gesattigtem Wasser setzte 20- bis 100mal weniger P frei als diejenige mit AAEDTA oder NaHCO3. Die SA von P-CO2 unterschied sich nicht von derjenigen von mit Wasser extrahierbarem P. Das weist was darauf hin, dass mit P-CO2 dieselben P-Formen gelost werden wie mit Wasser; diese P-Formen konnen als pflanzenverfugbar angesehen werden. Die SA von P, das mit AAEDTA bzw. NaHCO3 extrahiert wurde, machte 25–64% bzw. 66–92% der SA von mit Wasser extrahierbarem P aus. Die mit AAEDTA extrahierte austauschbare P-Fraktion nahm linear mit ansteigendem Boden-pH ab, was darauf hindeutet, dass mit dieser Methode langsam oder nicht austauschbares P aus Calcium-Phosphaten gelost werden kann.

To evaluate the effect of three phosphorus (P) fertilization regimes (no P, P input equivalent to P off-take by crops, P input higher than P off-take) on crop yield, P uptake, and soil P availability, seven field experiments (six in crop rotations, one under permanent grassland) were conducted in Switzerland during nine years (six trials) or 27 years (one trial). Soil total P (Pt), inorganic P (Pi), organic P (Po), and the amount of isotopically exchangeable soil P were measured in the 0–20 cm and 30–50 cm layers of the arable soils and in the 0–10 cm layer of the permanent grassland soil. Omitting P fertilization resulted in significant yield decreases only in one field crop trial as the amount of P isotopically exchangeable within one minute (E1min) reached values lower than 5 mg P (kg soil)–1. In the absence of P fertilization Pi decreased on average from 470 to 410 mg P (kg soil)–1 in the upper horizon of 6 sites while Po decreased only at two sites (from 510 to 466 mg P (kg soil)–1 on average). In all the treatments of the trials started in 1989 the E1min values of the upper horizon decreased on average from 15.6 to 7.4 mg P (kg soil)–1 between 1989 and 1998. These decreases were also observed when P inputs were higher than crops needs, showing that in these soils the highest P inputs were not sufficient to maintain the high initial available P levels. Finally for the six arable trials the values of the isotopic exchange kinetics parameters (R/r1, n, CP) and P exchangeable within 1 minute (E1min) at the end of the experiment could be estimated from the values measured at the beginning of trial and the cumulated P balance. Der Einfluss der Phosphordungung auf den Pflanzenertrag und den Phosphorstatus des Bodens Die Begrenzung der Dungung mit Phosphat (P) bei Boden, die bereits hohe Gehalte an verfugbarem P aufweisen, kann zur Verringerung der P-Verluste in Oberflachen- und Grundwasser beitragen. Sieben Feldversuche (6 Versuche mit Ackerkulturen, 1 Versuch mit Dauerwiese) wurden wahrend 9 Jahren (6 Versuche) resp. 27 Jahren (1 Versuch) durchgefuhrt mit dem Ziel, den Einfluss von drei Dungungsraten (keine P-Gabe, P-Gabe entsprechend dem P-Entzug durch die Pflanzen und P-Gabe hoher als der P-Entzug durch die Pflanzen) auf Pflanzenertrag, P-Aufnahme und P-Verfugbarkeit zu studieren. Die Gehalte an anorganischem P (Pi) und organischem P (Po), sowie deren Summe (Pt) und die Gehalte an isotopisch austauschbarem P wurden in den Bodentiefen von 0–20 cm und 30–50 cm der ackerbaulichen Versuche und in einer Bodentiefe von 0–10 cm in der Dauerwiese gemessen. Der Verzicht auf P-Dungung fuhrte nur in einem der ackerbaulichen Versuche zu einer signifikanten Abnahme des Pflanzenertrags, wobei der Gehalt an isotopisch austauschbarem P (E1min) unter 5 mg P (kg Boden)–1 fiel. Pi sank bei Verzicht auf P-Dungung im oberen Horizont von 6 Boden durchschnittlich von 470 auf 410 mg P (kg Boden)–1, wahrend Po nur in 2 Boden absank (durchschnittlich von 510 auf 466 mg P (kg Boden)–1). Ueber alle Dungungsverfahren jener Versuche, die 1989 begonnen hatten, sank E1min im oberen Horizont von 1989 bis 1998 durchschnittlich von 15.6 auf 7.4 mg P (kg Boden)–1. Die Abnahme wurde auch beobachtet, wenn die P-Gabe hoher war als der P-Entzug durch die Pflanzen, was zeigt, dass in diesen Boden selbst die hochste P-Gabe nicht genugte, um den ursprunglich hohen Gehalt an verfugbarem P aufrecht zu erhalten. Schlieslich wurden in den 6 ackerbaulichen Versuchen die Parameter der Isotopenaustauschkinetik (R/r1, n, CP) und E1min am Ende des Versuches mit den ursprunglichen Werten zu Beginn des Versuchs verglichen. Es ergaben sich signifikante Beziehungen zwischen den Werten am Ende des Versuchs einerseits und den ursprunglichen Werten und der P-Bilanz andererseits.

Residual fertilizer phosphorus (residual P) may significantly contribute to crop P nutrition. To test this hypothesis, a pot experiment was conducted with ryegrass (Lolium perenne) and clover (Trifolium repens) grown separately on three different soils which either had not received P fertilizer for at least nine years (0F) or had received P fertilizer equivalent to crop P off-take (F). Soils in the pot experiment were given either none (0F, F) or a single rate of 15 mg P (kg soil)–1 as diammonium phosphate (0F+DAP, F+DAP). In the treatments 0F+DAP and F+DAP DAP had been labeled with 33PO4 while in the treatments 0F and F the pool of available soil P had been labeled with carrier-free 33PO4. This allowed estimating the quantities of P in plant dry matter that derived from native soil P, residual fertilizer P or fresh fertilizer P. Fourteen to 62 % of the P in the above ground biomass of white clover or perennial ryegrass were derived from residual P whereas 7 to 28 % were derived from freshly applied DAP. The proportion of P derived from residual P was correlated to the total amount of P fertilizer added to the soils, while the proportion of P derived from DAP was correlated to the concentration of P in the soil solution of the 0F and F soils. Aufnahme von Phosphat aus Ruckstanden von fruherer Dungung und aus frisch ausgebrachtem Diammoniumphosphat durch Lolium perenneund Trifolium repens In Dungerruckstanden enthaltener Phosphor (P) kann moglicherweise signifikant zur P-Ernahrung von Kulturpflanzen beitragen. Um diese Hypothese zu prufen, wurde im Topfversuch die P-Aufnahme durch Lolium perenne und Trifolium repens untersucht. Als Substrat dienten drei Boden, die entweder seit mindestens 1989 keine P-Gabe mehr erhalten hatten (0F) oder die jahrlich P-Gaben im Umfang der P-Entzuge durch die vorgangigen Kulturen erhalten hatten und deshalb P-Ruckstande enthielten (F). Es ergaben sich folgende Verfahren: 0F: ohne P aus Dungerruckstanden und frischer Dungung; 0F+DAP: mit P aus frischer Dungung (Diammoniumphosphat, 15 mg P (kg Boden)–1); F: mit P aus Dungerruckstanden; F+DAP: mit P aus Dungerruckstanden und frischer Dungung. In den Verfahren 0F + DAP und F + DAP war DAP mit 33PO4 markiert. In den Verfahren 0F und F war der verfugbare P des Bodens mit carrier-freiem 33PO4 markiert. Dies ermoglichte die Erfassung der Aufnahme von P durch die Pflanzen differenziert nach Herkunft aus Boden, Dungerruckstanden und frischem Dunger. Von Dungerruckstanden stammten 14–62 %, von frischem Dunger 7–28 % des in den Sprossen der Pflanzen gefundenen P. Der von Dungerruckstanden stammende Anteil P in den Pflanzensprossen war korreliert mit dem gesamten Gehalt an P das dem Boden als Dunger zugefuhrt worden war. Der von frischem DAP stammende Anteil war korreliert mit der P-Konzentration in der Bodenlosung.

Vegetative conversion from grass to forest may influence soil nutrient dynamics and availability. A short-term (40 weeks) glasshouse experiment was carried out to investigate the impacts of ryegrass (Lolium perenne) and radiata pine (Pinus radiata) on soil phosphorus (P) availability in 15 grassland soils collected across New Zealand using 33P isotopic exchange kinetics (IEK) and chemical extraction methods. Results from this study showed that radiata pine took up more P (4.5–33.5 mg P pot−1) than ryegrass (1.1–15.6 mg pot−1) from the soil except in the Temuka soil in which the level of available P (e.g., E1min Pi, bicarbonate extractable Pi) was very high. Radiata pine tended to be better able to access different forms of soil P, compared with ryegrass. There were no significant differences in the level of water soluble P (Cp, intensity factor) between soils under ryegrass and radiata pine, but the levels of Cp were generally lower compared with original soils due to plant uptake. The growth of both ryegrass and radiata pine resulted in the redistribution of soil P from the slowly exchangeable Pi pool (E> 10m Pi, reduced by 31.8% on the average) to the rapidly exchangeable Pi (E1min-1d Pi, E1d-10m Pi) pools in most soils. The values of R/r1 (the capacity factor) were also generally greater in most soils under radiata pine compared with ryegrass. Specific P mineralisation rates were significantly greater for soils under radiata pine (8.4–21.9%) compared with ryegrass (0.5–10.8%), indicating that the growth of radiata pine enhanced mineralisation of soil organic P. This may partly be ascribed to greater root phosphatase activity for radiata pine than for ryegrass. Plant species × soil type interactions for most soil variables measured indicate that the impacts of plant species on soil P dynamics was strongly influenced by soil properties.

Use of isotope methods to measure the availability of phosphorus (P) in soils that are well supplied with P is well established. We have evaluated such methods for acid tropical soils with very small P contents, which are less well studied. The isotopically exchangeable P in soil suspensions (E value) and that in plant growth experiments (L value) were measured in soils that had received varying amounts of P fertilizer in two field experiments in Colombia. We determined the E values over 4-5 weeks of equilibration allowing for the kinetics of isotope exchange. The decrease in radioactivity in the soil solution at a particular time, t, divided by that at the start (r t /R) was described by three parameters (r 1 /R, r∞/R, and a coefficient n) derived from the time course of isotopic exchange over 100 minutes. Values of E t were calculated either from measured values of r t /R or those extrapolated until 12 weeks. Agrostis capillaris was grown on the same soils labelled with carrier-free 33 P-orthophosphate ions to obtain L values. Agreement between E values derived from measured and extrapolated values of r t /R was satisfactory, but errors in n and r∞/R limited the precision with which we could estimate E values. For most soils, the P concentrations in the soil solution were greater than the detection limit of the malachite green method (0.9 μg l -1 ) but smaller than its quantification limit (3.6 μg l -1 ). In the soils with the least available P, the P content of the seed limited the determination of the L value. The E values were strongly correlated, but not identical, with the L values measured for the same time of isotopic exchange. We conclude that these approaches are not precise enough to detect in these soils the ability of a plant to access slowly exchangeable forms of P or to quantify the mineralization of organic P. However, these isotope techniques can be used to estimate the total fraction of added fertilizer P that remains available to the plants.

Appropriate evaluation of phosphorus (P) availability in soil is aprerequisite for ensuring the productivity and long-term sustainable managementof agroecosystems. Fifteen soils presently under grassland were collected fromdifferent areas of New Zealand and soil P availability was assessed by isotopicexchange kinetics (IEK) and related to P forms obtained by chemicalfractionation (sequential extraction). Concentrations of total P determined inthe 15 soils ranged from 375 to 2607 mg kg−1(mean1104 mg kg−1). Mean concentrations of inorganic P(Pi) extracted by sequential extraction with ammonium chloride, sodiumbicarbonate, sodium hydroxide (first), hydrochloric acid and sodium hydroxide(second) were 1.2, 41, 205, 113 and 23 mg kg−1,respectively. Mean concentrations of organic P (Po) extracted by sodiumbicarbonate, sodium hydroxide (first) and sodium hydroxide (second) were 133,417 and 105 mg kg−1, respectively. Similarly,results from IEK analysis showed that the intensity (water soluble Pi (Cp)),capacity (R/r1 and n), and quantity (E value,isotopically exchangeable P pools (E1 min,E1 min–24 h,E24 h–3 m,E > 3 m)) factors varied markedlyamongst soils. Thus Cp concentrations ranged from 0.02–1.90 mgL−1, while concentrations of Pi determined in theE1 min, E1 min–24,E24 h–3 m,E>3 m pools were 2–29 (mean 10), 10–321(76), 11–745 (152), and 8–498 (177) mgkg−1, respectively. The corresponding values forR/r1 and n were 1.0–17.7 (mean 4.5) and0.10–0.50 (mean 0.37), respectively. Regression analysis revealed that Cpconcentrations were exponentially and inversely proportional toR/r1,n and P sorption index (PSI)(R2 = 0.806(P

Aqueous extraction contributes to the formation and weathering of planetary materials and renders electrolytes such as phosphate available for biology. In this context, the solubility of phosphate is measured in planetary materials, represented by the Mars meteorites Nakhla, Dar al Gani 476 (DaG 476), Elephant Morraine 79001 (EETA 79001), and terrestrial analogs, and in the Murchison CM2 and Allende CV3 carbonaceous chondrites. The Mars meteorites contain high levels of phosphate that is readily extracted by water, up to 15 mg kg −1 in Nakhla and DaG 476 and 38 mg kg −1 in EETA 79001, while the terrestrial analogs and the carbonaceous chondrites contain 0.5 to 6 mg kg −1 . Correspondingly, high phosphate concentrations of 4 to >28 mg L −1 are obtained in extracts of the Mars meteorites at high solid/solution ratios, exceeding the concentrations of 0.4 to 2.0 mg L −1 in the extracts of the terrestrial analogs. A wide range of planetary conditions, including N 2 and CO 2 atmospheres, solid/solution ratios of 0.01 to 1.0 kg L −1 , extraction times of 1 to 21 d, and temperatures of 20 to 121°C affect the amounts of extractable phosphate by factors of only 2 to 5 in most materials. Phosphate-fixing capacity and exchangeable phosphate are assessed by the isotopic exchange kinetics (IEK) method, which quantifies the amount of P isotopically exchangeable within 1 min (E 1min ) and between 1 min and 3 months (E 1min-3m ) and the amount of P that cannot be exchanged within 3 months (E >3m ). The IEK results show that the DaG 476 Mars meteorite and terrestrial analogs have low P-fixing capacities, while the carbonaceous chondrites have high P-fixing capacities. Aqueous processing under early planetary CO 2 atmospheres has large effects on the available phosphate. For example, the fraction of total P that is exchangeable in 3 months increases from 1.6 to 11%, 13 to 51.6%, and 43.9 to 90.4% in the DaG 476 Mars meteorite, Allende, and Murchison, respectively. The results show that solutions with high phosphate concentrations can form in the pores of planetary lava ash and basalts and in carbonaceous asteroids and meteorites. These solutions can help prebiotic synthesis and early microbial nutrition. The Martian and carbonaceous chondrite materials contain sufficient phosphate for space-based agriculture.

Switzerland yearly produces more than 260,000 Mg of compost, two thirds of which is recycled in agriculture and horticulture. This research was undertaken to examine the forms and availability of inorganic P (Pi) in Swiss composts made from solid kitchen and garden wastes using the isotopic exchange kinetic technique, a sequential Pi extraction and magic angle spinning (MAS) solid-state 31P nuclear magnetic resonance (NMR) spectroscopy. The different approaches described in this paper demonstrate the presence of a complex mixture of Pi species in the studied composts. Isotopic exchange experiments and sequential extraction showed that these composts contained relatively large concentrations of rapidly available Pi. Significant correlations were observed between the concentration of water-soluble Pi (Cp), and the total N, C and P content of composts suggesting that organic substances partly controlled the amount of rapidly available Pi. Significant correlations were observed in alkaline composts between the amount of Pi which can not be exchanged within 3 months and the total P and Ca content. In alkaline composts solid-state MAS 31P NMR results suggested the presence of a range of slightly soluble and poorly crystallized Ca-P compounds such as apatites or octacalcium phosphates and of organic P compounds. The slowly or non-exchangeable Pi present in these composts could therefore be bound to Ca in the form of apatites or octacalcium phosphates.

Wide variation in results exists in the literature on the effectiveness of composts to sustain the phosphorus (P) nutrition of crops. The aim of this work was to assess the importance of some soil and composts properties on the utilization of compost-P by white clover (Trifolium repens L.). This study was carried out with samples collected from four composts made from solid kitchen and garden wastes, and with two soil samples taken from the A horizon of a P-rich sandy acidic Dystrochrept and of a P-limited clayey calcareous Eutrochrept. Changes in the amount of inorganic P (Pi) isotopically exchangeable within 1 min (E1min) were measured during 32 weeks in incubated soil-composts or soil-KH2PO4mixtures where P sources had been added at the rate of 50 mg P kg−1 soil. Uptake of compost-P or KH2PO4-P by white clover was measured on the same amended soils during 16 weeks. In both soils, the application of composts resulted after 32 weeks of incubation in E1min values ranging between those observed in the control without P and those observed in the KH2PO4treatment, i.e., in values ranging between 4.2 and 5.9 mg P kg−1 in the sandy acidic soil and between from 1.6 to 4.3 mg P kg−1 in the clayey calcareous soil. The total coefficient of utilization of compost-P (CU-P) by white clover reached values in both soils for the four composts ranging between 6.5% and 11.6% of the added P while in the presence of KH2PO4 the CU-P reached values ranging between 14.5% in the clayey calcareous soil and 18.5% in the sandy acidic soil. Results obtained in the sandy acidic soil suggest, that white clover initially used a fraction of the rapidly exchangeable compost P, while at a latter stage plant roots enhanced the mineralisation of compost organic P and took up a fraction of the mineralized P. These relations were not observed in the clayey calcareous soil probably because of its high sorbing capacity for P. In the sandy acidic soil, composts application increased the uptake of soil P by the plant from 31.4 mg P kg−1 soil in the control without P to values ranging between 37.9 to 42.7 mg P kg−1 soil in the presence of composts. This indirect effect was related to a general improvement of plant growth conditions in this soil induced by compost addition (from 9.9 g DM kg−1 soil in the control without P to values ranging between 14.0 to 16.1 g DM kg−1 soil in the presence of composts) and/or to the release of Al- or Fe bound soil P to the solution due to soil pH increase following compost application. Finally the total coefficient of utilization of P (CU-P) derived from KH2PO4 and composts was related to the total amount of N exported by white clover in the P-limited clayey calcareous soil but not in the P-rich sandy acidic soil. This suggests that in a soil where N2 biological fixation is limited by low P availability, the CU-P of a compost by white clover is not only related to the forms of P present in the compost but also to its effect on N nutrition. However, it is not clear whether this improved N nutrition was due to compost mineralisation, or to an indirect compost effect on the N2 biological fixation.

The loss of phosphorus (P) on overland flow and subsurface drainage from soils receiving long-term applications of fertilizer and manure has been linked to the accelerated eutrophication of fresh waters. This loss is initiated by the release of P from soil to solution, which for overland flow can be estimated by water extraction and for subsurface drainage waters by 0.01 M CaCl 2 extraction. Although this release is rapid, the information available on the kinetics of P loss is insufficient to support improved soil P management. In this study, an isotopic exchange kinetics (IEK) approach was used to assess the effect of two solutions (water and 0.01 M CaCl 2 ) and different soil-to-solution ratios on soil isotopically exchangeable P (Et). Results are described by a compartmental analysis that quantified the amount of P isotopically exchangeable within 1 min (E 1min ), 30 min (E 30min ), 24 hr (E 24hr ), and between 24 hr and 3 months (E 24hr-3 o ). The quantity of P in each compartment was then compared with the concentration of P in overland flow and subsurface drainage waters. Isotopically exchangeable soil P within 1 min (1:5 soil to solution ratio) was correlated most closely with the concentration of P in overland flow (r2 = 0.84 with water) and subsurface drainage waters (r2 = 0.93 with 0.01 M CaCl 2 ). For overland flow, a significant correlation was maintained at a soil-to-solution ratio of 1:100 for more exchangeable P pools than at other ratios (1:5 or 1:10). Similarly, the relationship between isotopically exchangeable P in 0.01 M CaCl 2 (but not Et in water) and subsurface drainage waters P was maintained for exchange times up to 24 hr, reflecting the greater contact period of subsurface drainage water with soil compared with overland flow. The results suggest that the concentration of P in overland flow and subsurface drainage waters is dependent on the rapid or short-term (1 min and

Soil organic P (P o ) mineralization is an important process in P cycling. No accurate method for its quantification is available because any mineralized inorganic P (P l ) may be rapidly sorbed onto the soil solid phase where it cannot be separated from already present P i , A method for measuring soil P o mineralization is explored using isotopic dilution techniques under conditions of constant soil respiration rates. First, the specific activity (SA) as affected by physicochemical processes was extrapolated from an isotopic exchange kinetics batch experiment. Second, the SA was assessed during incubation after labeling soil with 33 PO 4 , Lower SA measured during incubation than extrapolated from the batch experiment was attributed to the release of nonlabeled P 1 due to mineralization of nonlabeled P o , In order to separate biological from biochemical mineralization processes, one set of samples was γ-irradiated to stop the microbial activity while maintaining phosphatase activity. The γ-irradiated soil revealed higher mineralization rates than the corresponding nonirradiated soil. This was explained by an increase of the amount of easily mineralizable P o derived from killed microbial cells by γ-irradiation. Consequently, a gross, but overestimated, biochemical P mineralization can be assessed. In the nonirradiated soil, mineralization not only of nonlabeled, but also of recently synthesized labeled P. resulting from microbial turnover, may occur. Thus, in the nonirradiated soil, after several days a gross, biologically and biochemically mediated mineralization is increasingly underestimated. During the first 7 d, the mineralization rate in the nonirradiated soil was 1.7 mg P kg -1 d -1 , which is an amount approximately equivalent to soil solution P in this soil, indicating that soil P mineralization is a significant process in delivering available P i .

Low phosphorus (P) in acid sandy soils of the West African Sudano-Sahelian zone is a major limitation to crop growth. To compare treatment effects on total dry matter (TDM) of crops and plant available P (P-Bray and isotopically exchangeable P), field experiments were carried out for 2 years at four sites where annual rainfall ranged from 560 to 850 mm and topsoil pH varied between 4.2 and 5.6. Main treatments were: (i) crop residue (CR) mulch at 500 and 2000 kg ha−1, (ii) eight different rates and sources of P and (iii) cereal/legume rotations including millet (Pennisetum glaucum L.), sorghum [Sorghum bicolor (L.) Moench], cowpea (Vigna unguiculata Walp.) and groundnut (Arachis hypogaea L.). For the two Sahelian sites with large CR-induced differences in TDM, mulching did not modify significantly the soils' buffering capacity for phosphate ions but led to large increases in the intensity factor (CP) and quantity of directly available soil P (E 1min). In the wetter Sudanian zone lacking effects of CR mulching on TDM mirrored a decline of E 1min with CR. Broadcast application of soluble single superphosphate (SSP) at 13 kg P ha−1 led to large increases in C P and quantity of E 1min at all sites which translated in respective TDM increases. The high agronomic efficiency of SSP placement (4 kg P ha−1) across sites could be explained by consistent increases in the quantity factor which confirms the power of the isotopic exchange method in explaining management effects on crop growth across the region.

In this study, an isotope exchange kinetics (IEK) approach was used to study soil Zn exchangeability in five polluted and six nonpolluted soils. Results were used in a compartmental analysis to quantify the amount of Zn isotopically exchangeable within 1 min, the amount of Zn exchangeable between 1 min and 15 d, and the amount of Zn that could not be exchanged within 15 d. Results derived from short-term IEK experiments (100 min) allowed for a precise prediction of the increase of exchangeable Zn throughout 14 d in most of the soils studied. The compartmental analysis conducted on the 11 samples allowed clear separation of the polluted from the nonpolluted soils and showed that most of the soil Zn is either very slowly or not at all exchangeable. The proposed approach yielded simultaneously the intensity factor, the quantity factor, and a parameter related to soil properties governing Zn sorption onto soil [r(1)/R]. The amount of Zn exchangeable within 15 d was found to be very close to Zn extractable by 0.005 M diethylene triamine pentaacetic acid (DTPA) + 0.01 M CaCl{sub 2} + 0.1 M triethanolamine (TEA) (Zn{sub DTPA}) in all soils, suggesting that both methods were characterizing the samemore » pool of soil Zn.« less