Silvopasture systems could be effective carbon sinks but their mitigation potential across geo-climatic gradients and management regimes remains understudied. To address this, we compared the simulated establishment of high-density hybrid walnut (Juglans regia x nigra) silvopasture across Europe over 30 years using DayCent with different scenarios and assumptions: regional-scale simulations with i) country-specific nitrogen (N) input rates, and ii) similar medium N input (∼100 kg N ha-1 yr-1), contrasted with iii) country-scale scenarios, co-designed with stakeholders (for the Netherlands, Switzerland, and Spain). Simulated regional greenhouse gas (GHG) mitigation potential (from CO2 plus N2O) was largely driven by net primary productivity and N availability. Mitigation potentials of the similar N input rates scenario were around 1 t CO2-Ceq ha-1 yr-1 between 43 and 52° N latitude, up to 2 t CO2-Ceq ha-1 yr-1 in the most suitable area. For the high Belgian and Dutch N input rates (250-400 kg N ha-1 yr-1), the country-specific N input scenario suggested even higher mitigation potentials, but it is possible that this was an artifact of oversensitivity of DayCent to N, and requires further field investigation. The mitigation potentials at lower latitudes in both regional scenarios ranged from 0.7 t CO2-Ceq ha-1 yr-1 to no mitigation. Except for Switzerland, mitigation potentials of country-scale scenarios correlated strongly with those of regional simulations (R2 of 0.97 across all three countries; 0.88, 0.77, and 0.43 for the Netherlands, Spain and Switzerland, respectively), despite varying N input rates within the Netherlands and replacing walnut by Quercus ilex/suber trees in Spain. Thus, geo-climatic productivity constraints dominated the simulated GHG mitigation potential while management choices influenced the degree of its realization, especially in mountainous Switzerland.

Abstract. Nitrogen (N) is essential for agricultural productivity, but excessive N inputs result in substantial losses to the environment. Conducting N assessments at national scales is challenging because observational data are limited, especially over long time periods. Here we compiled detailed datasets and performed high-resolution biogeochemical modelling to quantify N budgets for Switzerland's diverse agricultural ecosystems over four decades. Between the 1980s and the 2010s, N use efficiency improved from 47 % to 57 % in croplands and from 63 % to 71 % in grasslands, while losses through leaching and gas emissions decreased by 24 % in croplands and 4 % in grasslands. These improvements are closely linked to the implementation of national-scale agri-environmental policies that reduced fertilizer use in the 1990s. However, despite increased efficiency, cropland soils experienced substantial N depletion between 1995 and 2011 (−23 kg N ha-1 yr-1) in croplands. Our results demonstrate that policy reforms have improved agricultural system functioning and reduced losses, but also reveal risks associated with unbalanced soil N, underscoring the need for integrated N management for sustainable agriculture.

Abstract. Agriculture is the dominant source of anthropogenic nitrous oxide (N2O), a potent greenhouse gas with a high global warming potential. In Switzerland, substantial changes in fertiliser use alongside climatic conditions have occurred over the past four decades, yet the relative contributions of management practices versus environmental change to long-term N2O emission trends remain incompletely understood. Here, we applied the biogeochemical model DayCent at 1 km resolution across Switzerland, integrating spatially explicit datasets on climate, soil properties and agricultural management, to quantify N2O emissions for the period 1981–2020 from croplands and grasslands and attribute emission changes to management versus climate drivers. Simulated national N2O emissions from agricultural soils declined by roughly 5 % from 4.0 to 3.8 kt N yr⁻¹ between the 1980s and 2010s, primarily due to a 25 % reduction in N fertiliser use. Our attribution simulations suggested that under real climate conditions, such a decrease in fertiliser N inputs (–25 %) over the period studied lowered emissions by 15.2 % in croplands and by 12.0 % in grasslands (including permanent meadows and pastures, and high alpine summer pastures). However, compared to a control scenario (in the absence of climate change), rising temperatures over the past 40 years offset these gains, increasing emissions by 5.7 % in croplands and 13.6 % in grasslands. These results show that warming-induced N2O emissions partially negate mitigation from improved fertiliser management, highlighting the need for integrated agricultural N2O mitigation strategies that are resilient to future warming.

Context: To avoid soil fertility decline and increased greenhouse gas (GHG) emissions, it has been recommended to improve maize yields in sub-Saharan Africa with sustainable practices such as Integrated Soil Fertility Management (ISFM), instead of relying solely on mineral fertilizer. However, the yield responses and environmental trade-offs of ISFM likely depend on soil and climatic conditions. Objective: To explore this, we used the DayCent model to simulate 30-year average yields of maize monoculture across Kenya under 17 different ISFM scenarios, co-created with Kenyan smallholder farmers. We compared yields, changes in SOC stocks and N2O emissions against current baseline conditions (monocropping with minimal nutrient inputs). Methods: The scenario that best represented a 'feasible-input' level consisted of 2 t C ha-1 yr-1 of farmyard manure and 60 kg N ha-1 season-1 of mineral fertilizer. Other scenarios included different amounts (0, 1, and 2 t C ha-1 yr-1) and types of organic inputs in combination with four rates of mineral N fertilizer (0, 30, 60, and 90 kg N ha-1 season-1). The uncertainty of model predictions was quantified through Monte Carlo simulations. Results and Conclusions: The model results indicate a significant potential for yield improvements in the humid regions of western Kenya (from 3.7 to 8.1 t ha-1 yr-1) with the 'feasible-input' compared to the baseline scenario; GHG emissions per kg of yield were generally lower (the median value reduced from 0.9 to 0.5 kg CO2-eq kg-1 yield). However, in the semi-arid regions of eastern Kenya, maximum yields under any scenario were 1.1 t ha-1 yr-1, reached at inputs of 1 t C farmyard manure ha-1 yr-1 or 60 kg mineral N ha-1 season-1. The uncertainty analysis showed a high confidence in the 'feasible-input' scenario's ability to increase yields and reduce SOC losses compared to the baseline, but a high uncertainty regarding its impact on GHG emissions. Specifically, the 95% credibility intervals for the combined CO2 and N2O emissions ranged from reductions of up to 1000 kg CO2-eq ha-1 yr-1 to increases of up to 200 kg CO2-eq ha-1 yr-1. Significance: These results strongly support the use of ISFM practices to enhance maize yields and mitigate soil fertility losses, particularly in areas with sufficient rainfall. However, due to the high uncertainty surrounding simulated N2O emissions, we cannot establish with certainty whether ISFM reduces GHG emissions on a per hectare basis or increases them.

Measurement of changes in soil organic carbon (SOC) under various management practices at the field scale poses significant challenges due to inherent spatial and temporal variability. In comparison ecosystem biogeochemical models offer a robust framework for simulating nutrient cycling, SOC, and greenhouse gas emissions that can be used to identify and evaluate long-term effects and strengths of climate change mitigation strategies. DayCent is a coupled soil-plant dynamic model that has been widely used to simulate long-term ecosystem responses to changes in soil management and climate in the US. Its application to agricultural systems in Ireland requires a calibration and evaluation for common management practices across a range of pedo-climatic conditions. The objective of this study was therefore a) to calibrate the DayCent model with several types of field data and to evaluate its performance in simulating SOC and soil N2O emissions and b) to explore the sensitivity of model parameters to different types of field data. Our aim was to simulate the effects of a long-term application of dairy, pig, and mineral fertilizers on grass yields, SOC and soil organic nitrogen (N) stocks, and soil N2O fluxes in a long-term permanent grassland experiment. To calibrate the model, the data from control and high pig slurry application treatments from 1970 to 2022 were used. The calibration was separated into two steps: a) the first step was a manual calibration for SOC and soil organic N, volumetric soil water content, and grass yield; b) the second step was an automatic calibration for soil temperature, daily N₂O emission, soil NO₃⁻ and NH₄⁺ concentrations with the PEST parameter estimation software. All remaining treatments, that varied in the rate and type of animal slurry application, were used in the independent model evaluation. Using this information the performance of the calibrated model was substantially improved for SOC stock (rRMSE=0.17, r2=0.54, d=0.78, n=102) compared to the default model (rRMSE=0.25, r2=0.29, d=0.45, n=102) across all validation treatments. Similarly, an improvement was found for soil organic N stock in the validation treatments (rRMSE=0.19, r2=0.70, d=0.78, n=102) compared to the default model (rRMSE=0.30, r2=0.64, d=0.53, n=102). Improvements in simulating daily N2O emissions (calibrated model: rRMSE=5.30, r²=0.08, d=0.44, n=186; default model: rRMSE=2.97, r²=0.02, d=0.22, n=186), and soil NO₃⁻ and NH₄⁺ concentrations were still quite uncertain across validation treatments. In conclusion, the calibrated DayCent successfully simulated the long-term dynamics of SOC and soil organic N stocks, grass yields, soil water content, and soil temperature across varying nutrient application rates, although there were some limitations in simulating daily and annual N₂O emissions, and mineral N concentrations. While further testing under various pedo-climatic conditions is necessary, DayCent has the potential to be used as a tool for optimizing nutrient management strategies under Irish conditions.

Managed grasslands influence global warming by the exchange of the greenhouse gases (GHG) like carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4). Application of animal waste, such as slurry, rich in inorganic nitrogen (N), may escalate soil processes and thus soil GHG emissions, particularly in organic systems that rely on input of animal manures without chemical inputs.The objective of this study was to evaluate GHG mitigation potential of biological amendments that might be relevant to organic systems and their effects on soil N and N leaching over a 2-month period. To achieve this a plot-scale field experiment on a grassland site in Rosemount (Dublin, Ireland) was conducted over a period from May to July 2023. Closed static chamber technique was used to measure soil emissions of N2O, CH4 and CO2 with an increased sampling frequency after the slurry application. The dynamics of soil ammonium, nitrate and dissolved organic N were evaluated weekly in soil surface samples from 0-15 cm and in a 10-day interval in the leachate collected at a 50 cm depth. The grass yield was assessed twice during the course of the experiment. The plots were equally irrigated to stimulate soil processes during the dry periods. The treatments assigned to the plots in a randomised complete block design with 5 replicates included control (CON), cattle slurry (SLU) and slurry mixed with biochar (BIO; added at 2 kg/m2), neem oil high with slurry (NEEM H; added at 100% of N applied) and neem oil low with slurry (NEEM L; added at 20% of N applied). The slurry was applied at 50 kg N ha-1 to all plots apart from CON.The application of neem oil at both levels of input consistently reduced soil N2O and CH4 daily emissions (p

The agricultural sector is responsible for substantial amounts of greenhouse gas emissions that exacerbate climate change. Such greenhouse gas emissions from upland crops are difficult to abate because they are dominated by nitrous oxide (N2O) production from soil processes. Strategies to reduce these emissions focus on N fertilizer management, and there is a widespread assumption that legume crops, which do not receive N fertilizer, emit little N2O. Here we show that this assumption is incorrect; approximately 40% of N2O emissions from the most extensive cropping system in North America-the maize-soybean rotation-occur during the soybean phase. Yet, due to the lack of N fertilizer input, opportunities for emissions abatement from the soybean phase are unclear. Using models of cropping systems, we developed a strategy that combines cover-crop management and earlier planting of extended growth soybean varieties to reduce emissions from soybean production by 33%. These practices, which complement N fertilizer management in maize, are widely accessible and represent an immediate, climate-smart strategy to reduce nitrous oxide emissions from soybean production, thus not only contributing to climate-change mitigation but also maintaining productivity while adapting to changing weather patterns. Soil processes involved in agricultural practices emit considerable levels of nitrous oxide, which detrimentally contribute to climate change. This study explores strategies to reduce nitrous oxide emissions while maintaining crop productivity in the US maize-soybean rotational cropping system.

Sustainable intensification schemes such as integrated soil fertility management (ISFM) are a proposed strategy to close yield gaps, increase soil fertility, and achieve food security in sub-Saharan Africa. Biogeochemical models such as DayCent can assess their potential at larger scales, but these models need to be calibrated to new environments and rigorously tested for accuracy. Here, we present a Bayesian calibration of DayCent, using data from four long-term field experiments in Kenya in a leave-one-site-out cross-validation approach. The experimental treatments consisted of the addition of low- to high-quality organic resources, with and without mineral nitrogen fertilizer. We assessed the potential of DayCent to accurately simulate the key elements of sustainable intensification, including (1) yield, (2) the changes in soil organic carbon (SOC), and (3) the greenhouse gas (GHG) balance of CO2 and N2O combined. Compared to the initial parameters, the cross-validation showed improved DayCent simulations of maize grain yield (with the Nash–Sutcliffe model efficiency (EF) increasing from 0.36 to 0.50) and of SOC stock changes (with EF increasing from 0.36 to 0.55). The simulations of maize yield and those of SOC stock changes also improved by site (with site-specific EF ranging between 0.15 and 0.38 for maize yield and between −0.9 and 0.58 for SOC stock changes). The four cross-validation-derived posterior parameter distributions (leaving out one site each) were similar in all but one parameter. Together with the model performance for the different sites in cross-validation, this indicated the robustness of the DayCent model parameterization and its reliability for the conditions in Kenya. While DayCent poorly reproduced daily N2O emissions (with EF ranging between −0.44 and −0.03 by site), cumulative seasonal N2O emissions were simulated more accurately (EF ranging between 0.06 and 0.69 by site). The simulated yield-scaled GHG balance was highest in control treatments without N addition (between 0.8 and 1.8 kg CO2 equivalent per kg grain yield across sites) and was about 30 % to 40 % lower in the treatment that combined the application of mineral N and of manure at a rate of 1.2 t C ha−1 yr−1. In conclusion, our results indicate that DayCent is well suited for estimating the impact of ISFM on maize yield and SOC changes. They also indicate that the trade-off between maize yield and GHG balance is stronger in low-fertility sites and that preventing SOC losses, while difficult to achieve through the addition of external organic resources, is a priority for the sustainable intensification of maize production in Kenya.

Abstract. Sustainable intensification schemes that increase crop production and soil fertility, such as integrated soil fertility management (ISFM), are a proposed strategy to close yield gaps and achieve food security in sub-Saharan Africa while maintaining soil fertility. However, field trials are insufficient to estimate the potential impact of such technologies at the regional or national scale. Upscaling via biogeochemical models, such as DayCent, from the field-scale to a larger region can be a suitable and powerful way to assess the potential of such agricultural management practices at scale, but they need to be calibrated to new environments and their reliability needs to be assured. Here, we present a robust calibration of DayCent to simulate maize productivity under ISFM, using data from four long-term field experiments. The experimental treatments consisted of the addition of low- to high-quality organic resources to the soil, with and without mineral N fertilizer. We assess the potential of DayCent to represent the key aspects of sustainable intensification, including 1) yield, 2) changes in soil carbon, and 3) global warming potential. The model was calibrated and cross-evaluated with the probabilistic Bayesian calibration technique. The standard parameters of DayCent led to poor simulations of maize yield (Nash-Sutcliffe modeling efficiency; EF 0.33) and changes in SOC (EF -1.3) for different ISFM treatments. After calibration of the model, both the simulation of maize yield (EF 0.51) and the change in SOC (EF 0.54) improved significantly compared to the model with the standard parameter values. A leave-one-site-out cross-evaluation indicated the robustness of the approach for spatial upscaling (i.e., the significant improvement, described before, was achieved by calibrating with data from 3 sites and evaluating with the remaining site). The SOC decomposition parameters were altered most severely by the calibration. They were an order of magnitude higher compared to the default parameter set. This confirms that the decomposition of SOC in tropical maize cropping systems is much faster than in temperate systems and that the DayCent temperature function is not suitable to capture this with a single parameter set. Finally, the global warming potential simulated by DayCent was highest in control -N treatments (0.5–2.5 kg CO2 equivalent per kg grain yield, depending on the site) and could be reduced by 14 to 72 % by combined application of mineral N and manure at a medium rate. In three of the four sites, the global warming potential was largely (> 75 %) dominated by SOC losses. In summary, our results indicate that DayCent is suitable for estimating the impact of ISFM from the site to the regional level, that trade-offs between yields and global warming potential are stronger in low-fertility sites, and that the reduction of SOC losses is a priority for the sustainable intensification of maize production in Kenya.

In sub-Saharan Africa, maize is one of the most important staple crops, but long-term maize cropping with low external inputs has been associated with the loss of soil fertility. While adding high-quality organic resources combined with mineral fertilizer has been proposed to counteract this fertility loss, the long-term effectiveness and interactions with site properties still require more understanding. This study used repeated measurements over time to assess the effect of different quantities and qualities of organic resource addition combined with mineral nitrogen (N) on the change of soil organic carbon (SOC) contents over time (and SOC stocks in the year 2021) in four ongoing long-term experiments in Kenya. These experiments were established with identical treatments in moist to dry climates, on coarse to clayey soil textures, and have been conducted for at least 16 years. They received organic resources in quantities equivalent to 1.2 and 4 t C ha(-1) yr(-1) in the form of Tithonia diversifolia (high quality, fast turnover), Calliandra calothyrsus (high quality, intermediate turnover), Zea mays stover (low quality, fast turnover), sawdust (low quality, slow turnover) and local farmyard manure (variable quality, intermediate turnover). Furthermore, the addition of 240 kg N ha(-1) yr(-1) as mineral N fertilizer or no fertilizer was the split-plot treatment. At all four sites, a loss of SOC was predominantly observed, likely because the sites had been converted to cropland only a few decades before the start of the experiments. Across sites, the average decline of SOC content over 19 years in the 0 to 15 cm topsoil layer ranged from 42 % to 13 % of the initial SOC content for the control and the farmyard manure treatments at 4 t C ha(-1) yr(-1), respectively. Adding Calliandra or Tithonia at 4 t C ha(-1) yr(-1) limited the loss of SOC contents to about 24 % of initial SOC, while the addition of sawdust, maize stover (in three of the four sites) and sole mineral N addition showed no significant reduction of SOC loss over the control. Site-specific analyses, however, did show that at the site with the lowest initial SOC content (about 6 g kg(-1)), the addition of 4 t C ha(-1) yr(-1) farmyard manure or Calliandra with mineral N led to a gain in SOC contents. The other sites lost SOC in all treatments, albeit at site-specific rates. While subsoil SOC stocks in 2021 were little affected by organic resource additions (no difference in three of the four sites), the topsoil SOC stocks corroborated the results obtained from the SOC content measurements (0-15 cm) over time. The relative annual change of SOC contents showed a higher site specificity in farmyard manure, Calliandra and Tithonia treatments than in the control treatment, suggesting that the drivers of site specificity in SOC buildup (soil mineralogy, soil texture, climate) need to be better understood for effective targeting management of organic resources. Farmyard manure showed the highest potential for reducing SOC losses, but the necessary quantities to build SOC are often not realistic for smallholder farmers in Africa. Therefore, additional agronomic interventions such as intercropping, crop rotations or the cultivation of crops with extended root systems are necessary to maintain or increase SOC.

Over one-third of rivers and a quarter of lakes in Ireland are failing to meet their environmental quality standards for nutrients. This is impacting Ireland’s goal to improve water quality and achieve its EU Water Framework Directive targets. Nitrogen (N) and phosphorus (P) leached from different forms of fertilizer application may contribute to groundwater and surface water contamination causing numerous environmental issues in the agroecosystem and health problems in human beings. There is a lack of knowledge on the magnitude of the effects of the nutrient management strategies and their long-term implementation on N and P nutrient leaching from the soil to the groundwater and surface waters in Ireland. The accurate optimization of the nutrient management strategies requires monitoring of soil water concentrations under the range of management strategies and quantification of the N and P leaching from the soil profile.The objective is to study the N and P leaching in organic and inorganic forms from a long-term application of cattle and pig slurry and mineral NPK fertilizer over the drainage seasons 2021/22 in Hillsborough (54°28′ 0′′ N, 6°6′ 0′′ W), Northern Ireland, UK. The long-term experiment in Hillsborough was set up in 1970 to measure the effects of frequent applications of organic and inorganic nutrients on plant productivity and soil biogeochemistry. It was set up in a split-plot design with nutrient management as a main factor and grass-species biodiversity (rye-grass vs multi-species sward treatment) as a subplot factor replicated three times. The multispecies sward was reseeded (05/08) after ploughing (31/07) in spring 1969. Leaching was measured in six nutrient management (control, synthetic fertilizer, pig and cattle slurry with high and low rates) and both biodiversity treatments. Soil solution from an 80 cm dept was extracted using suction cups every ten days over a 5-month drainage period. N and P leaching were quantified from nutrient concentrations of various species multiplied by an effective rainfall obtained through Schulte’s Soil Moisture Deficit hybrid model. The total N, total oxidized N, ammonium, total P, and dissolved reactive P have been analyzed, the N and P losses have been quantified for the above species. Nutrient concentrations and losses were analyzed as repeated measures with a linear mixed effects model in R.Most of the concentrations of various nutrient species were affected by an interactive effect between nutrient management and biodiversity level. The influence of permanent grassland renovation and reseeding is also evident. However, the concentrations are low, below the water quality thresholds defined by water quality legislation and national background levels in groundwater or surface waters despite the high rates of continuous nutrient applications for over 50 years. This is likely associated with the high natural attenuation capacity of the soil driven by clay-loam texture. The data will be further used in the optimization of the nutrient management strategies using an ecosystem process-based model DayCent. We believe that the results of this study will have direct implications for agri-environmental policies in the Rep. of Ireland and in the UK.

<p>Sustainable intensification practices, such as integrated soil fertility management (ISFM), form a strategy to close yield gaps while maintaining soil fertility and, typically, are locally tested in field trials. However, to estimate the potential impact of ISFM on a regional scale, field trials are insufficient and biogeochemical models are required. These models need to be calibrated and evaluated when applied to new environments. Here, we present a robust calibration of the DayCent agroecosystem model to simulate the impact of ISFM practices on maize productivity in Kenya, using a probabilistic Bayesian calibration technique with data from long-term field trials at four sites in central and western Kenya. We assessed the efficiency of DayCent in simulating: 1) maize grain yield, 2) changes in soil organic carbon (SOC), and 3) nutrient use efficiency of applied nitrogen (N) fertilizer under different ISFM treatments, which consisted of different organic resources combined with the addition or absence of mineral N fertilizer. After model calibration, both the simulations of maize yield (Nash Sutcliffe Efficiency, NSE 0.51) and change in SOC (NSE 0.54) improved significantly compared to runs using the standard DayCent parameters (NSE of 0.33 and -1.3 for yield and SOC change, respectively). A leave-one-site-out cross evaluation indicated the robustness of the approach for spatial extrapolation, i.e., the significant improvement of model simulations was achieved by calibrating the model with data from three sites and then evaluating it with data from the remaining site. The values of model parameters related to SOC decomposition were most altered  by the calibration, i.e., they were an order of magnitude higher compared to the default parameter values (derived for temperate climates). This suggests that the DayCent temperature function is not suitable to capture SOC decomposition across climates with a single set of parameter values. Further, similar maize yields were simulated for all treatments that received mineral N fertilizer and DayCent underestimated the yield increase observed in the field trials of the combined application of organic resources and mineral N compared sole mineral N application. In contrast, at low levels of nutrient inputs DayCent proved sufficiently sensitive to capture differences in maize yield levels. Finally, while mean yields by treatment were simulated well, year-to-year yield variation was not captured well by DayCent. In summary, our results indicate that DayCent is capable to estimate the mean impact that ISFM practices at typical rates of mineral fertilizer and organic resource applications have on yield and SOC, but may not be capable to estimate the differences in yield potential at very high inputs. While the cross evaluation indicated a robustness for upscaling, the suboptimal representation of year-to-year yield variabilities shows that future projections under a changing climate may be biased by the DayCent model. Consequently, improved model structures, such as improved soil moisture representation, are needed to reduce uncertainty.</p>

Context: Crop productivity in sub-Saharan Africa cannot be substantially improved without simultaneously addressing short-term crop nutrient demand and long-term soil fertility. Integrated soil fertility management tackles both by the combined application of mineral fertilizers and organic resource inputs but few studies examined its' long-term effectiveness.Objective: To address this knowledge gap, this study analysed maize yield trends in four long-term (31-37 cropping seasons) field experiments in Kenya with contrasting soil textures and under different climates. Methods: All sites had two maize cropping seasons per year, received a base P and K fertilization and tested combinations of organic resource addition (1.2 and 4 t C ha-1 yr-1 ranging from farmyard manure, to high-quality Tithonia diversifolia and Calliandra calothyrsus material to low-quality saw dust), combined with (+N) and without (-N) mineral N fertilizer (120 kg N ha-1 season-1). General maize yield trends across sites and site specific trends were analyzed.Results: Across sites, the no-input control experienced significant average maize yield reductions of 50 kg ha-1 yr-1 over the study period. In contrast, the treatment with farmyard manure +N maintained yields at both 1.2 and 4 t C ha-1 yr-1. High initial yields following additions of Tithonia and Calliandra, reduced over time. Assessment by site showed site specificity of maize yields and yield trends. For example, the two climatically favorable sites in western Kenya experienced yield gains with high quality organic resources at 4 t C ha-1 yr-1, leading to yields of up to 8 t ha-1 per season, while sites in central Kenya experienced yield losses, leading to 3.5 t ha-1 per season. Yield site specificity for +/- mineral N treatments was stonger than for organic resource treatments, e.g. the clayey site in central Kenya in the end showed no yield differences between +/- N, except for the 1.2 t C ha-1 yr-1 farmyard manure treatment. Yet, farmyard manure plus mineral N consistently achieved highest yields of all organic resource treatments at all sites and farmyard manure addition at 1.2 t C ha-1 yr-1 (about 5 t dry matter) was the most N-efficient treatment.Conclusions: At realistic application rates, maize yield in integrated soil fertility management is best sustained by a combined application of farmyard manure and mineral N. Implications: Mixed crop-livestock systems and a combined manure and mineral N application are key ingredients for sustained productivity of smallholder systems in sub-Saharan Africa.

Efficient utilization of incident solar radiation and rainwater conservation in rain-fed smallholder cropping systems require the development and adoption of cropping systems with high resource use efficiency. Due to the popularity of cassava-maize intercropping and the food security and economic importance of both crops in Nigeria, we investigated options to improve interception of photosynthetically active radiation (IPAR), radiation use efficiency (RUE), soil moisture retention, and yields of cassava and maize in cassava-maize intercropping systems in 8 on-farm researcher-managed multi-location trials between 2017 and 2019 in different agroecologies of southern Nigeria. Treatments were a combination of (1) maize planting density (low density at 20,000 maize plants ha(-1) versus high density at 40,000 maize plants ha(-1), intercropped with 12,500 cassava plants ha(-1)); (2) fertilizer application and management targeting either the maize crop (90 kg N, 20 kg P and 37 kg K ha(-1)) or the cassava crop (75 kg N, 20 kg P and 90 kg K ha(-1)), compared with control without fertilizer application. Cassava and maize development parameters were highest in the maize fertilizer regime, resulting in the highest IPAR at high maize density. The combined intercrop biomass yield was highest at high maize density in the maize fertilizer regime. Without fertilizer application, RUE was highest at low maize density. However, the application of the maize fertilizer regime at high maize density resulted in the highest RUE, soil moisture content, and maize grain yield. Cassava storage root yield was higher in the cassava fertilizer regime than in the maize fertilizer regime. We conclude that improved IPAR, RUE, soil moisture retention, and grain yield on nutrient-limited soils of southern Nigeria, or in similar environments, can be achieved by intercropping 40,000 maize plants ha(-1) with 12,500 cassava plants ha(-1) and managing the system with the maize fertilizer regime. However, for higher cassava storage root yield, the system should be managed with the cassava fertilizer regime.

Under the United Nations Framework Convention on Climate Change (UNFCCC), industrialized countries and countries with economies in transition (so called Annex 1 countries) are encouraged to move towards more sophisticated approaches for national greenhouse gas reporting. To develop a model-based approach for estimating nitrous oxide (N2O) emissions from agricultural soils, model calibration is one of the first important steps. Extensive multisite field observations are necessary for this purpose, as agricultural management in Western Europe is complex (e.g., diverse crop rotations, different types of fertilizer and soil tillage). In the present study, we used ca. 24,000 daily N2O flux observations from six cropland sites, two in France and four in Switzerland, to conduct an automatic data-driven calibration of the biogeochemical model DayCent. This model is planned to be used for greenhouse gas reporting in the entire European Union as well as in Switzerland. After a site-specific calibration, a leave-one-out (LOO) cross-evaluation was conducted to assess the model's ability to predict N2O emissions for sites it was not calibrated for. Mean observed N2O fluxes for 54 interactions of crop cycles, field studies and treatments were used to evaluate the model. The LOO cross-evaluation resulted in a R2 of 0.63 for the prediction of mean N2O fluxes per crop cycle, compared to an R2 of 0.51 obtained with default parameterization. Our results showed that the improvement in N2O predictions was associated with the adjustment of only seven parameters controlling the N cycle in soil (e.g., the maximum daily nitrification amount and the inflection point for the effect of water-filled pore space on denitrification) out of several hundred parameters. These parameters showed a wide range of values between sites, revealing an important challenge for calibration-based improvement of N2O simulations. Despite the remaining uncertainty, our model-based estimates of N2O emission per crop cycle (2.64 kg N ha-1) were clearly closer to measurements (2.67 kg N ha-1) than commonly used emission factor approaches (1.60-1.71 kg N ha-1). Based on extensive field observations, our results suggest that, after data-driven calibration of only few N cycle parameters, DayCent simulations are useful for reporting N2O emissions of complex cropland management. These model based-estimates were more accurate, because they consider key drivers that are disregarded by simpler approaches. Moving towards more complex methods of N2O reporting, is therefore expected to improve the accuracy and additionally allows to assess mitigation options.

Abstract. In sub-Saharan Africa, long-term maize cropping with low external inputs has been associated with the loss of soil fertility. While adding high-quality organic resources combined with mineral fertilizer has been proposed to counteract this fertility loss, the long-term effectiveness and interactions with site properties still require more understanding. This study used repeated measurements over time to assess the effect of different quantities and qualities of organic resource addition combined with mineral N on the change of soil organic carbon concentrations (SOC) over time (and SOC stocks in the year 2021) in four ongoing long-term trials in Kenya. These trials were established with identical treatments in moist to dry climates, on coarse to clayey soil textures, and have been managed for at least 16 years. They received organic resources in quantities equivalent to 1.2 and 4 t C ha−1 per year in the form of Tithonia diversifolia (high quality, fast turnover), Calliandra calothyrsus (high quality, intermediate turnover), Zea mays stover (low quality, fast turnover), sawdust (low quality, slow turnover) and local farmyard manure (variable quality, intermediate turnover). Furthermore, the addition or absence of 240 kg N ha−1 per year as mineral N fertilizer was the split-plot treatment. At all sites, a loss of SOC, rather than gain, was predominantly observed due to a recent conversion from permanent vegetation to agriculture. The average reduction of SOC concentration over 19 years in the 0 to 15 cm depth ranged from 42 % to 13 % of the initial SOC concentration for the control and the farmyard manure treatments at 4 t C ha−1 yr−1, respectively. Adding Calliandra or Tithonia at 4 t C ha−1 yr−1 limited the loss of SOC concentrations to about 24 % of initial SOC, while the addition of saw dust, maize stover (in 3 of 4 sites) and sole mineral N addition, showed no significant reduction in SOC loss over the control. Site specific analyses, however, did show, that at the site with the lowest initial SOC concentration (about 6 g kg−1), the addition of 4 t C ha−1 yr−1 farmyard manure or Calliandra plus mineral N led to a gain in SOC concentrations. All other sites lost SOC in all treatments, albeit at site specific rates. While subsoil SOC stocks in 2021 were little affected by organic resource additions (no difference in 3 of 4 sites), the topsoil SOC stocks corroborated the results for SOC concentrations. The relative annual change of SOC concentrations showed a higher site specificity in high-quality organic resource treatments than in the control, suggesting that the drivers of site specificity in SOC buildup (mineralogy, climate) need to be better understood for effective targeting of organic resources. Even though farmyard manure showed the most potential for reducing SOC loss, our results clearly show that maintaining SOC with external inputs only is not possible at organic resource rates that are realistic for small scale farmers. Thus, additional agronomic interventions such as intercropping, crop rotations or strong rooting crops may be necessary to maintain or increase SOC.

Ecological intensification (EI) could help return agriculture into a 'safe operating space' for humanity. Using a novel application of meta-analysis to data from 30 long-term experiments from Europe and Africa (comprising 25,565 yield records), we investigated how field-scale EI practices interact with each other, and with N fertilizer and tillage, in their effects on long-term crop yields. Here we confirmed that EI practices (specifically, increasing crop diversity and adding fertility crops and organic matter) have generally positive effects on the yield of staple crops. However, we show that EI practices have a largely substitutive interaction with N fertilizer, so that EI practices substantially increase yield at low N fertilizer doses but have minimal or no effect on yield at high N fertilizer doses. EI practices had comparable effects across different tillage intensities, and reducing tillage did not strongly affect yields. Intensifying food production sustainably is critical given growing demand and agriculture's environmental footprint. This meta-analysis finds that practices such as adding organic matter and increasing crop diversity can partly substitute for nitrogen fertilizer to sustain or increase yields.

Maize monoculture with low external inputs, as frequently practiced in sub-Saharan Africa, usually leads to the long-term loss of soil fertility. This threatens the already poor yields in the region. Practices that add organic and mineral resources to the soil therefore promise to counteract soil fertility loss by providing the potential feed-stock for microbes to build new soil organic matter. We studied the effect of organic and mineral resource addition from five organic amendment types of different quality (relative N, lignin and polyphenol contents) and quantity and from chemical nitrogen fertilizer, on soil organic carbon (SOC) and soil nitrogen in the 15 cm topsoil of four long-term trials in contrasted sites in Kenya. They had different climate and soil texture and lasted between 16 and 19 years. Treatments were identical among sites, the organic resources were Tithonia diversifolia (high quality and fast turnover) and Calliandra calothyrsus (high quality and slow turnover), stover of Zea mays (low quality and fast turnover), sawdust from Grevillea robusta trees (low quality and slow turnover) and locally available farmyard manure (undefined quality and slow turnover). The organic resources were added in the quantities of 1.2 and 4 t C ha-1 yr-1 and the experiments included a split-plot treatment of ±N addition (120 kg ha-1 in each of the two growing seasons per year). Despite site-specific differences, the general trend across sites indicated that SOC is usually lost with all treatments. Typical losses ranged from 1.9% to 0.6% loss of initial SOC yr-1 for the control and the farmyard manure (at 4t C ha-1 year-1) respectively. Adding Calliandra or Tithonia at 4t C ha-1 yr-1 also enable to slow the loss (about 1.1% of initial SOC yr-1 lost). Nevertheless, the addition of 4t C ha-1 yr-1 farmyard manure and Calliandra calothyrsus, together with mineral N addition, achieved a gain in SOC over time only in the site which had lowest initial SOC contents (about 6 g C kg-1), a sand of 31% content and a climate that was suitable for maize growth. In contrast, another site with low initial SOC content, high sand content, but a less suitable climate, with frequent failures of the maize crop, lost SOC in all treatments. In the site with initially 25 g C kg-1, the farmyard manure treatment at 4t C ha-1 yr-1 with N addition was the only treatment that could maintain SOC, while in the site with initially highest SOC (about 30 g C kg-1), all treatments lost SOC. The mineral N addition, with the exception of two treatments in the lowest fertility site, had no significant effect on the response of SOC to the different organic resource treatments. Our results indicate that farmyard manure may be the most suitable resource to reduce losses of SOC, but increases may only be possible in sites with initially low SOC contents, e.g. where, because of sufficiently long cultivation activities, a new steady state with low SOC contents has already been attained.

Cassava is growing in importance in Nigeria as a food and industrial crop. Current yields are low due to poor soil fertility and because farmers do not use improved germplasm, clean planting material, or improved crop management in Nigeria. To provide feasible agronomic recommendations targeting increased root yield, the effects of tillage intensity, fertilizer application, plant density and weed control were tested in 230 farmers' fields in southwestern Nigeria over two years. In 2016, tillage treatments were zero, single and double passage with a disc plough, followed by ridging (soil shaping) versus leaving the soil flat. Fertilizer application at 75:20:90 kg ha(-1) NPK was tested against a control and two plant densities (10,000 versus 12,500 ha(-1)) were compared. In 2017, plant density at 10,000 ha(-1) and double plough were excluded, while preand post-emergence herbicide application versus farmer's choice of weed control (i.e. manual weeding using hand hoe) was introduced. Cassava was harvested at 12 months after planting, and yields were recorded as fresh root mass. In 2016, double plough (15.9 Mg ha(-1)) had a minor advantage over single plough (14.3 Mg ha(-1)), while zero plough produced 12.9 Mg ha(-1) (P < 0.001). Ridging increased yield significantly (P < 0.01) by 2.3 Mg ha(-1) after single and zero plough, but not after double plough. Across tillage treatments, planting at 12,500 plants ha(-1) and fertilizer application increased yields by 1.5 and 4.2 Mg ha(-1), respectively. In 2017, ridging resulted in a yield increase of 1.7 Mg ha(-1) after single plough and 5.6 Mg ha(-1) after zero plough. Fertilizer application increased root yield by 2.9 Mg ha(-1) across tillage treatments. The use of herbicides negatively affected cassava yields in zero plough fields, compared with manual weeding. After ploughing, yield in herbicide based and manual weed control were not different. Cassava root yield response to tillage intensity strongly varied across fields, with low-yielding fields commonly responding less frequently to tillage. We conclude that unresponsive fields require measures other than increased tillage intensity to increase cassava root yields and that cost-intensive tillage operations must be targeted to responsive fields together with fertilizer application and improved weed control.