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Sub-Saharan Africa (SSA) faces significant food security risks, primarily due to low soil fertility leading to low crop yields. Climate change is expected to worsen food security issues in SSA due to a combined negative impact on crop yield and soil fertility. A common omission from climate change impact studies in SSA is the interaction between change in soil fertility and crop yield. Integrated soil fertility management (ISFM), which includes the combined use of mineral and organic fertilizers, is expected to increase crop yield but it is uncertain how this advantage is maintained with climate change. We explored the impact of scenarios of change in soil fertility and climate variables (temperature, rainfall, and CO2) on rainfed maize yield in four representative sites in SSA with no input and ISFM management. To do so, we used an ensemble of 15 calibrated soil-crop models. Reset and continuous simulations were performed to assess the impact of soil fertility vs climate change on crop yield. In reset simulations, SOC, soil N and soil water were reinitialized each year with the same initial conditions. In continuous simulations, SOC, soil N and soil water values of a given year were obtained from the simulation of the previous year, allowing cumulative effects on SOC and crop yields. Most models agreed that with current baseline (no input) management, yield changed by a much larger order of magnitude when considering declining soil fertility with baseline climate (-39%), compared with considering constant soil fertility but changes in temperature, rainfall and CO2 (from -12% to +5% depending on the climate variable considered). The interaction between change in soil fertility and climate variables only marginally influenced maize yield (high agreement between models). The model ensemble indicated that when accounting for soil fertility change, the benefits of ISFM systems over no-input systems increased over time (+190%). This increase in ISFM benefits was greater

A diversity of agricultural practices and systems enable the accrual of soil organic C (SOC) stocks, with variable efficiencies. These C-storing practices increase SOC stocks, either by increasing the inputs of plant biomass or exogenous organic matter, or by decreasing the outputs of SOC reducing SOC mineralisation rates, or both. In the perspective of contributing to climate change mitigation, the temporal stability of the additional SOC stored is critical. Different approaches can be used to assess the stability of soil organic matter, such as physical fractionation of soil organic matter, chemical extractions, long term incubations and analysis of the thermal behaviour of the organic matter using RockEval© pyrolysis. These address contrasting residence times, such as of months to years (long term incubations), to several decades and centuries (particle size fractionation, RockEval© pyrolysis coupled with PARTYSOC model) We used the literature and long-term agricultural experiments in which management options (application of exogenous organic matter, conservation agriculture, organic agriculture, agroforestry) result in increased SOC stocks. We investigated the stability of the additional SOC stored, compared to the reference management option. Methods currently used in the literature to assess the temporal stability of soil organic matter do not address the same SOC kinetic pools. Care must be taken to specify which range of residence times is considered when using any method intending to evaluate the biogeochemical stability of soil organic matter, as well as when using the terms stable or labile. Management options result in slightly contrasted stability of the additional organic carbon, the application of exogenous organic matter resulting in the most stable additional carbon, compared to management options that increase belowground plant biomass inputs to soil. Carbon storing agricultural management options mobilize different stabilization processes of soil

Conservation agriculture (CA), combining reduced or no tillage, permanent soil cover, and improved rotations, is often promoted as a climate-smart practice. However, our understanding of the impact of CA and its respective three principles on top- and subsoil organic carbon stocks in the low-input cropping systems of sub-Saharan Africa is rather limited. This study was conducted at two long-term experimental sites established in Zimbabwe in 2013. The soil types were abruptic Lixisols at Domboshava Training Centre (DTC) and xanthic Ferralsol at the University of Zimbabwe farm (UZF). The following six treatments, which were replicated four times, were investigated: conventional tillage (CT), conventional tillage with rotation (CTR), no tillage (NT), no tillage with mulch (NTM), no tillage with rotation (NTR), and no tillage with mulch and rotation (NTMR). Maize (Zea mays L.) was the main crop, and treatments with rotation included cowpea (Vigna unguiculata L. Walp.). The soil organic carbon (SOC) concentration and soil bulk density were determined for samples taken from depths of 0–5, 5–10, 10–15, 15–20, 20–30, 30–40, 40–50, 50–75 and 75–100 cm. Cumulative organic inputs to the soil were also estimated for all treatments. SOC stocks at equivalent soil mass were significantly (p<0.05) higher in the NTM, NTR and NTMR treatments compared with the NT and CT treatments in the top 5 cm and top 10 cm layers at UZF, while SOC stocks were only significantly higher in the NTM and NTMR treatments compared with the NT and CT treatments in the top 5 cm at DTC. NT alone had a slightly negative impact on the top SOC stocks. Cumulative SOC stocks were not significantly different between treatments when considering the whole 100 cm soil profile. Our results show the overarching role of crop residue mulching in CA cropping systems with respect to enhancing SOC stocks but also that this effect is limited to the topsoil. The highest cumulative organic carbon inputs to the soil were obser

The biogeochemical effects of conservation agriculture (CA), such as soil organic carbon storage and greenhouse gas emissions, have been extensively studied. However, recent research has shown that management practices also have biogeophysical effects on both local and global climates by altering surface albedo and energy partitioning. We assessed the biogeophysical impacts of CA in maize fields during two successive seasons (2021/22-2022/23) at two long-term experimental sites in Zimbabwe with contrasting soil properties: Domboshawa Training Center (DTC) with a light abruptic Lixisol (sandy soil) and the University of Zimbabwe Farm (UZF) with a dark xanthic Ferralsol (clayey soil). We monitored surface albedo, longwave radiation, leaf area index (LAI), and soil moisture/temperature under three treatments: conventional tillage (CT), notillage (NT), and no-tillage with mulch (NTM). Our findings reveal that, across all treatments during the two monitored seasons, the average surface albedo of the xanthic Ferralsol at UZF was consistently lower than that of the abruptic Lixisol at DTC. It results a cooling effect in both NT and NTM treatments compared to CT in the clayey soil at UZF. During the 2021/22 season, the mean annual radiative forcing (RF) of NT and NTM were -0.83 W.m-² and -0.43 W.m-2 respectively, while during the second season (2022/23) the annual mean RF was -1.43 W.m-2 for NT and -1.03 W.m-2 for NTM.On the sandy soil at DTC, a warming effect was observed due to soil darkening induced by mulching. The mean annual RF of NT in this site was -3.34 W.m-2 during the first season and -2.78 W.m-2 during the second. In contrast, NTM showed a warming effect with an RF of 1.2 W.m-2 in 2021/22, and 2.77 W.m-2 during the 2022/23 season. The RF induced by albedo change were converted into CO2-equivalents in order to compare it with biogeochemical effects of CA through changes in soil N2O emissions and SOC storage. The results demonstrated an opposite effect on RF and o

Introduction - Climate impacts of agricultural management practices such as mulching and no-tillage are usually evaluated with regard to soil organic carbon stock changes or greenhouse emissions. However, albedo effects of these practices that have been found to be of similar importance for climate change mitigation are usually ignored in this kind of assessment. This is even true for agricultural systems in Africa, which are less studied than temperate systems. In this study, we aim to assess the effect of land management on albedo dynamics and Radiative Forcing (i.e. climatic impact) in two long-term experiments established in Zimbabwe in 2013 with contrasting soil types. The Radiative Forcing (RF), is a metric used to quantify the change in Earth energy budget (radiation absorbed and emitted by the Earth) relative to an assumed default state (Betts 2000; Forster et al. 2007). The albedo of a cropland depends on soil properties, surface rugosity, soil moisture and coverage by plant litter, but also on plant density, phenology, architecture and spectral properties. Note that the latter may change for instance with phenology. The RFs resulting from land management changes are determined by their effects on the surface albedo dynamics but also by the solar radiation and atmospheric transmittance dynamics. Using a soil-crop model such as STICS coupled with a spatialization method that allows to represent changes in vegetation and soil properties is a promising solution to upscale RF related to albedo effects associated to land management changes at regional to global scales. 1. Modelling albedo using STICS Surface albedo is the fraction of solar radiation reflected by Earth surface back to the space. Currently, STICS estimates surface albedo as a function of soil and vegetation albedo (Brisson et al., 2008). Soil albedo is a function of soil colour and moisture and also depends on the presence of mulch at the surface. Current formalism considers total leaf area index

The year 2023 will likely be the hottest ever recorded on our planet. Adapting to climate change and climate extremes is increasingly becoming a day-to-day concern for African farmers, along with food security and income issues. Agricultural adaptations like varietal choice and fertilizer doses have deserved great attention from the crop modeling community, and are overall well accounted for by crop models. Agroecological practices, for example residue mulching, rotation and intercropping with legumes and application of organic amendments offer great potential to adapt to climate change. Yet, they have deserved less attention when it comes to the modeling of their performance in tropical context. In this abstract, we describe a collective research effort to update and test the Stics soil-crop model to account for the impact of agroecological practices on cropping system performance in the tropics. We built on multiple years of measurements in contrasting experimental sites from cool to warm, semi-arid to sub-humid subtropical environments, in Senegal, Zimbabwe, Mali, Burkina Faso, Kenya, Brazil and Madagascar. We assessed the skills and pitfalls of the model to simulate i) new cereal and legume crops ii) cereal-legume intercropping, iii) crop residue decomposition and feedbacks on crop growth and iv) crop residue mulching. We calibrated a new set of parameters for tropical maize (Falconnier et al., 2020), sorghum (Traoré et al., 2022, Ganeme et al., in revision), millet (Sow et al., forthcoming), rice (Ranaivoson et al., 2022), and legumes like cowpea (Traoré et al., 2022, Ganeme et al., in revision) and groundnut (Civil, 2022). Model accuracy (rRMSE) for end-of-season variables like aboveground biomass and grain yield was in the range of 20 to 50%. The scrutiny of in-season soil water and plant leaf area index (LAI) indicated that water stress was often underestimated, possibly because of underestimation of soil evaporation, and underestimation of the impact of wat

Can farmers in sub-Saharan Africa (SSA) boost crop yields and improve food availability without using more mineral fertilizer? This question has been at the center of lively debates among the civil society, policy-makers, and in academic editorials. Proponents of the “yes” answer have put forward the “input reduction” principle of agroecology, i.e. by relying on agrobiodiversity, recycling and better efficiency, agroecological practices such as the use of legumes and manure can increase crop productivity without the need for more mineral fertilizer. We reviewed decades of scientific literature on nutrient balances in SSA, biological nitrogen fixation of tropical legumes, manure production and use in smallholder farming systems, and the environmental impact of mineral fertilizer. Our analyses show that more mineral fertilizer is needed in SSA for five reasons: (i) the starting point in SSA is that agricultural production is “agroecological” by default, that is, very low mineral fertilizer use, widespread mixed crop-livestock systems and large crop diversity including legumes, but leading to poor soil fertility as a result of widespread soil nutrient mining, (ii) the nitrogen needs of crops cannot be adequately met solely through biological nitrogen fixation by legumes and recycling of animal manure, (iii) other nutrients like phosphorus and potassium need to be replaced continuously, (iv) mineral fertilizers, if used appropriately, cause little harm to the environment, and (v) reducing the use of mineral fertilizers would hamper productivity gains and contribute indirectly to agricultural expansion and to deforestation. Yet, the agroecological principles directly related to soil fertility—recycling, efficiency, diversity—remain key in improving soil health and nutrient-use efficiency, and are critical to sustaining crop productivity in the long run. We argue for a nuanced position that acknowledges the critical need for more mineral fertilizers in SSA, in combination

Can farmers in sub-Saharan Africa (SSA) boost crop yields and improve food availability without using more mineral fertilizer? This question has been at the center of lively debates among the civil society, policy-makers, and in academic editorials. Proponents of the “yes” answer have put forward the “input reduction” principle of agroecology, i.e. by relying on agrobiodiversity, recycling and better efficiency, agroecological practices such as the use of legumes and manure can increase crop productivity without the need for more mineral fertilizer. We reviewed decades of scientific literature on nutrient balances in SSA, biological nitrogen fixation of tropical legumes, manure production and use in smallholder farming systems, and the environmental impact of mineral fertilizer. Our analyses show that more mineral fertilizer is needed in SSA for five reasons: (i) the starting point in SSA is that agricultural production is “agroecological” by default, that is, very low mineral fertilizer use, widespread mixed crop-livestock systems and large crop diversity including legumes, but leading to poor soil fertility as a result of widespread soil nutrient mining, (ii) the nitrogen needs of crops cannot be adequately met solely through biological nitrogen fixation by legumes and recycling of animal manure, (iii) other nutrients like phosphorus and potassium need to be replaced continuously, (iv) mineral fertilizers, if used appropriately, cause little harm to the environment, and (v) reducing the use of mineral fertilizers would hamper productivity gains and contribute indirectly to agricultural expansion and to deforestation. Yet, the agroecological principles directly related to soil fertility—recycling, efficiency, diversity—remain key in improving soil health and nutrient-use efficiency, and are critical to sustaining crop productivity in the long run. We argue for a nuanced position that acknowledges the critical need for more mineral fertilizers in SSA, in combination

Numerical models are crucial to understand and/or predict past and future soil organic carbon dynamics. For those models aiming at prediction, validation is a critical step to gain confidence in projections. With a comprehensive review of ~250 models, we assess how models are validated depending on their objectives and features, discuss how validation of predictive models can be improved. We find a critical lack of independent validation using observed time series. Conducting such validations should be a priority to improve the model reliability. Approximately 60% of the models we analysed are not designed for predictions, but rather for conceptual understanding of soil processes. These models provide important insights by identifying key processes and alternative formalisms that can be relevant for predictive models. We argue that combining independent validation based on observed time series and improved information flow between predictive and conceptual models will increase reliability in predictions.

Identifying the drivers of soil organic carbon (SOC) stock changes is of the utmost importance to contribute to global challenges like climate change, land degradation, biodiversity loss, or food security. Evaluating the impacts of land use and management practices in agriculture and forestry on SOC is still challenging. Merging datasets or making databases interoperable is a promising way, but still has several semantic challenges. So far, a comprehensive thesaurus and classification of management practices in agriculture and forestry has been lacking, especially while focusing on SOC storage. Therefore, the aim of this paper is to present a first comprehensive thesaurus for management practices driving SOC storage (DATA4C+). The DATA4C+ thesaurus contains 224 classified and defined terms related to land management practices in agriculture and forestry. It is organized as a hierarchical tree reflecting the drivers of SOC storage. It is oriented to be used by scientists in agronomy, forestry, and soil sciences with the aim of uniformizing the description of practices influencing SOC in their original research. It is accessible in Agroportal (http://agroportal.lirmm.fr/ontologies/DATA4CPLUS, last access: 24 March 2022) to enhance its findability, accessibility, interoperability, and reuse by scientists and others such as laboratories or land managers. Future uses of the DATA4C+ thesaurus will be crucial to improve and enrich it, but also to raise the quality of meta-analyses on SOC, and ultimately help policymakers to identify efficient agricultural and forest management practices to enhance SOC storage.