Reducing reliance on mineral nitrogen (N) fertilizer is central to improving the sustainability of agricultural grassland management. Increasing sward diversity and the application of organic N fertilizer are potential strategies to achieve this, but their combined effects on soil biodiversity remain less understood. This study examined how different sward types and N fertilization influence soil biodiversity and soil food-web functioning in the short term, using soil nematodes as bioindicator. A field-plot experiment was conducted with four sward types: permanent pasture, perennial ryegrass, six- and twelve-species swards; and three N fertilizer treatments: organic, mineral and a combination of both N fertilizers. Redundancy analysis revealed that nematode community structure differed among the swards but not between N fertilizer treatments. Specifically, the abundance of plant-parasitic nematodes was significantly lower in the sown six- and twelve-species swards compared to perennial ryegrass monoculture and permanent pasture. Plant-parasitic nematode abundance was positively related to the proportion of grasses and negatively to the proportion of legumes and proportion of forbs. The abundance of bacterial feeders in swards fertilized with organic N was comparable to those fertilized with mineral N. Soil food web trophic complexity, as indicated by Structure Index did not vary among the swards nor among N fertilizer treatments. Our findings indicate that increasing sward diversity reduces plant-parasitic nematodes, while organic N fertilizer had limited effect on soil nematode communities in the short term. This suggests that enhancing plant species diversity, specifically the inclusion of legumes and forbs, can contribute to reducing the prevalence of plant-parasitic nematodes, potentially improving sustainability in agricultural grasslands.

The aim of this study was to assess and analyse the effect of species richness on zinc (Zn) and selenium (Se) uptake and concentrations in grasslands while investigating the potential role of grass species in facilitating their uptake in mixtures. We conducted a grassland experiment at both pot and field scales. In the greenhouse pot experiment, three grasses, four legumes and five forb species were grown as monocultures and mixtures along a species richness gradient. In the field experiment, we compared a monoculture of Lolium perenne sward with two multispecies mixtures (comprising grasses, legumes and forbs). At harvest, Zn and Se uptake values were determined for all the monocultures and mixtures in both experiments. Zinc and Se uptake increased with increasing species richness (p < 0.05). Higher Zn and Se uptake in the multispecies mixtures was primarily due to high aboveground biomass production and/or high shoot Zn and Se concentrations of some of the component species in the mixtures. However, there was insufficient evidence supporting the facilitation of Zn and Se uptake by grass species. To optimise Zn and Se uptake by multispecies mixtures, it is suggested that the choice of forage species should be based on biomass production potential as well as Zn and Se concentrations of the component species in the mixture.

Species-rich grasslands are proposed to promote biodiversity and reduce nitrogen (N) fertilization while maintaining productivity. However, the effectiveness of this strategy remains uncertain, particularly since grasslands with varying species richness are managed differently. We compared biomass yield and forage quality of three grassland management types varying in species richness and N fertilization levels in an on-farm context. We compared biomass yield and forage quality of three grassland management types: permanent grassland (Lolium perenne L. dominated) and multispecies grassland (sown with grasses, forbs, and legumes) fertilized with 0, 150, and 300 kg N ha⁻1, and unfertilized semi-natural grassland (highly species-diverse) at twelve farm locations over two years. Multispecies grasslands with 150 kg N ha⁻1 produced yields similar to permanent grasslands fertilized with 300 kg N ha⁻1 due to higher legume cover and biological N2 fixation. Nitrogen fertilization negatively impacted legume cover and likely reduced biological N2 fixation. Multispecies grassland differed less in in terms of yield over two years with varying weather conditions, suggesting enhanced drought tolerance. Semi-natural grasslands had the highest species richness but the lowest biomass yields and forage quality. Transitioning from permanent- to multispecies grasslands presents a promising diversification strategy for livestock farmers aiming to improve sustainability and biodiversity, while reducing N fertilization and maintaining productivity. Yet, species richness remains substantially higher in semi-natural grasslands at the cost of reduced biomass yield and forage quality. Optimization of N fertilization should be considered to improve persistence of legumes and forbs in agricultural multispecies grasslands.

Cover crops are grown between main crops in temperate climates to cover the soil, capture remnant nitrogen (N) from the profile after harvest, and build organic matter in the soil. There is ongoing discussion whether mixtures perform better than monocultures and whether a starter N application should be given to cover crops to enhance their growth and increase the associated ecosystem services, including the capture of remnant N remaining in the soil in autumn. We studied, in a two-year experiment on a sandy soil near Wageningen, the biomass accumulation and N yield of three cover crop species, oilseed radish (Raphanus sativus), black oat (Avena strigosa) and common vetch (Vicia sativa), as well as their 3-species mixture at 5 levels of N input, from 0 to 90 kg N/ha. We also estimated N leaching. We did not identify any effect of increasing N fertilization on the capture of remnant N from the soil profile in any of the cover crops at any N rate. Neither did we identify a decrease in N leaching in any of the cover crops at any N rate. Mineral N concentrations in soil were highest under vetch. We did find that in some combinations of cover crop species and N input, the capture of remnant N from the soil was decreased while N leaching was increased if N fertilizer was given, indicating an increased risk for N leaching. Net N capture of the 3-species mixture was not reduced at high N application rates. Biomass of the 3-species mixture was similar to or higher than the best performing monoculture. Results show that under the conditions of the study (e.g. sandy soil), the ecosystem service of N capture after a main crop was not improved, and in some cases deteriorated, by giving starter N fertilizer. This applied to all cover crop species, legume or non-legume, as well as a species mixture. The mixture was best able to take high N input without a decrease in net N uptake or an increase in N leaching.

Aim Improving nitrogen use efficiency (NUE) is critical for enhancing the sustainability of agricultural grasslands. However, there is limited research assessing NUE of grasslands managed with different nitrogen (N) fertilization inputs. We tested the hypothesis that multispecies swards managed with the combination of chemical and organic fertilizer would perform best with respect to herbage N yield and NUE. Methods A grassland field experiment was conducted over two years with four sward types: permanent pasture (baseline sward), perennial ryegrass monoculture, 6-species and 12-species swards; and three N fertilizer treatments: chemical, organic and a combination of chemical and organic fertilizer. Perennial ryegrass received 170 kg N/ha/yr while the other swards received 70 kg N/ha/yr. Results We found significant main effects of sward types and the N fertilizer treatments on herbage N yield and NUE indices. The multispecies swards (i.e., 6- and 12-species) had higher NUE, more negative partial N balance and higher partial factor productivity than perennial ryegrass sward across both years (P < 0.05). The proportion of legumes in the multispecies swards explained 55% of the herbage N yield. All N fertilizer treatments had NUE > 1 and negative partial N balance with the highest value in chemical fertilizer. NUE > 1 and negative partial N balance indicate greater herbage N yield relative to total N input. Conclusion Multispecies swards consistently outperformed perennial ryegrass across the three NUE indices. This highlights the potential of multispecies swards as a suitable option for improving NUE and minimizing the risk of N surplus without compromising yields.

Agrifood systems emit substantial amounts of greenhouse gases (GHG) into the atmosphere, but simultaneously sequester carbon (C) originating from atmospheric CO2 in soils. Their net effects on the GHG balance are rarely documented. This study aimed to quantify C cycles and GHG emissions in agrifood systems at both village and county levels and explore the potential to transition towards net-zero emissions. We integrated a modified material and nutrient flow model (NUFER) and a soil C cycle model (RothC) to calculate C cycles in the case area of Quzhou, China. Results showed that net photosynthesis predominantly contributed to C input to the agrifood system, while soil respiration and microbial respiration during manure storage accounted for most of the C output at all village types. Net CO2 emissions from the agrifood system in Quzhou were positive because the amount of C sequestration in soils (819 kg C ha-1 yr-1) could not offset CO2 emissions (847 kg C ha-1 yr-1) from fossil fuels used in the agrifood system, despite village-level variations. Scenario analysis demonstrated that the system could achieve net-zero CO2 emissions by adopting good management practices and recycling organic wastes. In addition to these measures, achieving net-zero GHG emissions may necessitate replacing fossil fuels with clean energy. Policies and incentives to promote net-zero emissions and circular agriculture can be mutually reinforcing. This is the first study quantifying C cycles in an entire agrifood system as well as its net CO2 and GHG emissions, facilitating the transition towards net-zero emissions and climate neutrality.

Background and aimsLow boron (B) availability is associated with strongly weathered, coarse-textured, and low organic matter soils, widespread in sub-Saharan Africa (SSA). It is unknown to what extent B fertilization can increase maize yields in SSA. This study aims to understand the soil properties controlling B availability to field-grown maize.MethodsBoron fertilizer omission trials with maize were executed at 15 sites in Kenya, Zambia, and Zimbabwe. Yield, B uptake, and soil parameters potentially relevant for B availability, including extractable soil B (hot water, 0.01 M CaCl2, and 0.43 M HNO3), were determined.ResultsSoil B pools were strongly intercorrelated and were positively correlated with organic carbon, suggesting the relevance of organic matter for soil B availability. Soil parameters described limited variation in B uptake and the yield response to B fertilization. Boron fertilization did not increase yields in any of the 15 sites but increased uptake in 11 sites. Yields were reduced through B fertilization in five sites, likely because B application induced toxicity. No clear critical soil or plant B concentrations indicating deficiency could be derived, but positive yield responses to B fertilization were absent with hot water B levels above 0.69 mg kg-1.ConclusionAssessing B fertilizer needs in maize grown in tropical soils based on soil or plant tissue concentrations remains challenging. Improving soil organic matter status could potentially alleviate B deficiency in crops when present. Recommendations are given to overcome the identified challenges associated with studying B availability in tropical soils.

Abstract Aims To understand the variations among grassland species in their ability to take up zinc (Zn) and selenium (Se) when cultivated as monocultures and to assess the effect of species richness on Zn and Se uptake. Additionally, to examine whether interspecific facilitation or species composition of the mixtures contributes to the potential increase in Zn and Se uptake in mixtures. Methods We conducted a grassland diversity experiment at both pot and field scales. In the greenhouse pot experiment, three grasses, four legumes and five forb species were grown as monocultures and mixtures along a species richness gradient. In the field experiment, we compared a monoculture of Lolium perenne sward with two multispecies mixtures (comprising grasses, legumes, and forbs). At harvest, Zn and Se uptake values were determined for all the monocultures and mixtures in both experiments. Results The capacity of the monoculture species to take up Zn and Se varied significantly in the pot experiment. Zinc and Se uptake increased with increasing species richness (P < 0.05). Higher Zn and Se uptake in the multispecies mixtures was primarily due to the large aboveground biomass production and/or high tissue Zn and Se concentration of some of the component species in the mixtures. However, there was insufficient evidence supporting interspecific facilitation. Conclusion To optimize Zn and Se uptake by multispecies mixtures, it is suggested that the choice of forage species should be based on biomass production potential as well as Zn and Se concentrations of the component species in the mixture.

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 contained 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.

AimsFertilisating crops with zinc (Zn) is considered important to enhance agricultural productivity and combat human Zn 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.MethodsZinc 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.ResultsA positive maize yield response to soil Zn fertilisation was found at only two out of nineteen locations, despite soil Zn levels being below suggested critical concentrations at most locations. Soil properties nor plant concentrations were able to explain maize yield response to Zn fertilisation. However, positive responses in Zn uptake and grain Zn concentrations to Zn fertilisation were found at the majority of sites, especially in soils with low pH and organic carbon contents. Labile soil Zn measurements related more with Zn uptake (R-2 = 0.35) and grain Zn concentrations (R-2 = 0.26) than actual available Zn measurements.ConclusionsWe conclude that soil Zn fertilisation did not increase maize yields, but 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.

Context: Increased nitrogen (N) losses from linear agri-food systems result in severe environmental issues. These problems can be mitigated by circular N use. Yet, circularity is a scale-specific problem, and the feasibility of pathways towards N circularity at different scales is unclear.Objective: This study aimed to 1) evaluate N circularity of a complete agri-food system, including households, at both the village and county scale; and 2) assess potential pathways towards N circularity Methods: We used the county of Quzhou (China) with 342 villages as a case study and applied a modified version of the NUFER model to calculate N flows, using national statistical data and own survey data collected from farms and households. To evaluate N circularity, we selected four key performance indicators: N import, N loss, N use efficiency and N recycling rate. Results and conclusions: Our analysis showed significant variation at the village scale, depending on local production and consumption patterns. At county level, total N import was 546 kg ha-1 yr-1, of which 54% was lost to the environment. The N use efficiency of the agri-food system and N recycling rate of excreta were both <30%. We investigated scenarios to increase N circularity, including application of good management practices in crop and animal production; household dietary change to more plant protein; recycling of organic waste; and growing legumes instead of cotton. All measures combined increased the system’s N use efficiency by 140% and N recycling rate by 77%, while reducing N import by 61% and N loss by 69%. Recycling of organic waste was the most effective and most feasible strategy to promote circularity. Significance: Our study bridges the gap between village and county level N cycles, illustrates possibilities to transition towards circular N use, and can help policymakers worldwide to achieve more sustainable agri-food systems.

The nitrogen fertilizer replacement value (NFRV) quantifies the value of organic amendments as a nitrogen (N) fertilizer, and is commonly defined as the extent to which organic fertilizer N can replace mineral fertilizer N. NFRVs can be calculated by comparing the crop N uptake from equal N application rates of mineral and organic fertilizer, or by comparing the N rates of both fertilizers needed to obtain equal crop N uptake. Currently, NFRVs are mainly known for animal manure, whereas other organic waste products may become available as fertilizer products in the future. In this study, a pot experiment with spring wheat was performed to (1) assess NFRVs of a range of organic amendments; (2) compare NFRVs based on equal N application with NFRVs based on equal N uptake; and (3) assess which product characteristics explain observed variation. Observed NFRVs varied between 6.2 and 78.8%, with the lowest value for raw food waste and the highest for fishmeal. NFRVs were overestimated when calculated based on equal N application rate (with on average 6.9% point), and more so at high N application rate (9.0% point). NFRVs should therefore be calculated based on equal N uptake from organic and mineral fertilizers. Nitrogen concentration of the organic fertilizer provided the best explanation of variation observed in NFRVs (R 2 = 0.86). These findings give valuable insights into the large variation in value of organic waste streams as organic fertilizer and can support decisions on sustainable N application rates, to increase crop N uptake and reduce N losses to the environment.

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.

Trace elements such as zinc (Zn), copper (Cu) and boron (B) are important micronutrients for crop production. Their bioavailability is essential to crops yield quantity and quality in many tropical soils. Nutrient bioavailability depends partly on the soil nutrient status, and in particular on the reactive and soluble fractions. Adsorption/desorption and precipitation/dissolution processes control the partitioning of the reactive pool over the solid and solution phase. However, so far the solid-solution partitioning of trace elements has mostly been studied in temperate and often contaminated soils. We studied, therefore, the solid-solution partitioning of Zn, Cu and B for 172 soils from Burundi, Rwanda and Kenya, using extensive soil characterization in combination with multi-surface modelling and two types of empirical Freundlich type partition relations. Our aim was to enhance the understanding of the soil chemical processes that control the solid-solution partitioning of the three micronutrients in these soils from the tropics with a multi-surface model, and to use this knowledge as benchmark to develop partition relations that require less input data and are more convenient tools for predicting the concentration in solution based on existing soil data. We show that the generic multi-surface model applied to these tropical soils performs similarly for Zn and Cu as in previous studies on temperate and contaminated soils. The Zn and Cu speciation was dominated by adsorption to soil organic matter, with an increased importance of metal (hydr)oxides with increasing pH. Given its generally low concentrations in these soils, dissolved organic matter was found to be important only for the solution speciation of Cu. The adsorption of B was mainly to metal (hydr)oxides at low pH, and with increasing pH soil organic matter became more important. The multi-surface model overestimated the dissolved B concentration for most soil samples, which we attributed to an inaccurate estimation of reactive B. Interestingly, the variation in observed and modeled solid-solution partitioning expressed as Kd of Cu and B among the soils was relatively small (~1 log L kg- 1), and the concentration in solution was consistently mainly controlled by the reactive concentration. Generally, the optimized partition relations resulted in a smaller prediction error compared to the multi-surface models. The partition relations in which the concentration in solution was optimized, resulted generally in an overestimation for the lowest observed concentrations, and an underestimation for highest concentrations of all three elements. Partition relations with optimized Freundlich parameters Kf and n resulted in more robust predictions since the prediction error was not related to the actual measured concentration. The partition relations from this study are easy-to-use tools for predicting the dissolved concentrations of Zn, Cu and B in soils from the tropics with low contents of these micronutrients and can therefore enhance the use of current existing soil information data for Sub-Saharan Africa.

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.

Using fertilisers is indispensable for closing yield gaps in Sub Saharan Africa. Current fertiliser recommendations, however, are often blanket recommendations which do not take spatial variation in soil conditions within a region or country into account. Soil maps can potentially support fertiliser recommendations at a higher spatial resolution. The QUantitative Evaluation of the Fertility of Tropical Soils (QUEFTS) model is a decision support tool that predicts crop yields as an indicator of soil fertility and can be used to evaluate yield responses to fertilisers. It was designed for field level output and runs on field-specific soil information. The aim of this study was to compare two methods for developing maps of QUEFTS output, i.e. maize yield and the yield-limiting nutrient, with Rwanda as a case study. We used a database containing soil analysis results of 999 samples collected across Rwanda. Transfer functions were applied to predict the required P-Olsen and Exchangeable K input for QUEFTS based on the soil data. For the "Calculate-then-Interpolate " (CI) method, transfer functions and QUEFTS were applied to point data, and the final output was then interpolated using random forest modelling. For the "Interpolate-then-Calculate " (IC) method, maps of the soil parameters were developed first, before applying calculations. Implications of the chosen method (i.e. CI or IC) on QUEFTS predictions on a national scale were evaluated using set-aside locations. Results showed low precision and accuracy of QUEFTS maize yield predictions across Rwanda. The CI method performed better in predicting QUEFTS yield and yield-limiting nutrient than the IC method. Correlations between mapped yield predictions and predictions on set-aside evaluation locations were similar for the CI (r = 0.444) and IC (r = 0.439) methods. The poorer performance of the IC method was mostly due to overestimation of yields, which was most likely caused by the effect of smoothing on the soil maps used as input for QUEFTS. We conclude that the CI method is the preferred method for spatial application of QUEFTS.

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.

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 (R 2 = 0.35) and grain Zn concentrations (R 2 =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.

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). 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. 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. 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.