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Sustainable alternatives to slash-and-burn shifting cultivation in the (sub)humid tropics often rely on the use of external nutrient inputs to address soil fertility decline. The objective of this study was to investigate whether maize productivity and SOC can be sustained under permanent cropping with sole and combined use of compost and mineral nitrogen (N) fertilizer. Here, we report results from a long-term experiment carried out in Gagnoa, Ivory Coast, from 1971 to 1990. The experiment followed a randomized block design comprising eight replicates of 12 treatments. The two studied factors were compost (0 or 10 t DM ha−1 yr−1) and mineral N (0, 40, 80, 120, 160 or 200 kg N ha−1 yr−1) additions. Average maize grain yields of the first cropping cycles were significantly lower without compost (5.05 ± 1.57 t ha−1) than with compost addition (6.07 ± 1.31 t ha−1). The annual yield variability as shown by the standard deviation of the mean was reduced by 20% with compost addition. Without compost, 53% of the initial SOC stock in the 0–20 cm soil layer was lost, resulting in a SOC loss rate of − 0.62 t C ha−1 yr−1 compared to 21% with compost (−0.27 t C ha−1 yr−1). Compost addition therefore reduced SOC loss with an apparent SOC storage rate of 0.35 t C ha−1 yr−1. The conversion rate of organic carbon (OC) inputs to SOC was about 12%. The Introductory Carbon Balance Model (ICBM) reproduced well SOC dynamics, especially without compost. Without mineral N and without compost, maize grain yield decreased with decreasing SOC concentration until the introduction of leguminous crops in the second cropping cycle. We conclude that combined application of compost with mineral N fertilizers was effective at maintaining maize productivity but inadequate to prevent the decline of SOC stocks, despite large additions.

Climate variability is predicted to increase in sub-Saharan Africa. This will impact the food security of the populations who rely on rainfed cereal production. Sustainable intensification of cropping systems aims at increasing food production while adapting to future climate and mitigating environmental impacts. The opportunity to diversify cereal-based cropping systems with legumes to increase resilience has not been quantitively assessed so far. In this study, we explored the impact of interannual climate variability for a set of cropland allocations, going from maize only to various share and number of legume crops. We explored the hypotheses that maize and legumes inter-annual performances are not correlated, so that mixing the two crops will increase the resilience of the cropland allocation. We calibrated the STICS soil-crop model for maize, cowpea, groundnut and pigeon pea, with data from on-farm trials in Murehwa district in sub-humid Zimbabwe, conducted in the 2021- 2022 and 2022-2023 growing seasons. Crop performance in terms of average yield and its variability was simulated with the calibrated crop model for the historical climate (1996-2016), on red-clay and granitic sandy soils, for more fertile homefields and less fertile outfields. Two planting dates (early-late) and three levels of fertilization (0-80-160 kg N/ha), brought by mineral fertiliser only, manure only, or by a combination of 30% manure and 70% mineral fertiliser, were investigated. The simulated crop performance was used to compute for each land allocation the agricultural productivity, the economic profitability and the environmental performance. From the on-going simulation results, we will draw critical insights on the complementarities between maize and legumes. More specifically, we expect smaller but more stable groundnut yield compared with fertilized maize, so that diversification with groundnut will increase production stability and food security in terms of proteins. With regard to crop management, we foresee that a mix of mineral and organic fertilization will limit nitrogen leaching compared with mineral fertilization only, the first strategy giving similar energy production but greater environmental performance and gross margin. Under favourable rainy seasons, early planting is expected to increase productivity of both cereals and legumes and thus land allocation resilience, while late planting might be more detrimental to the cereal. These results can help inform the current research agenda around the benefits of cereal-based systems integrating legumes and trade-offs between intensification and diversification of rainfed cropping systems in sub-Saharan Africa.

Biogeochemical effects of conservation agriculture (CA), i.e. soil organic carbon storage and greenhouse gas emissions have been investigated in numerous studies. Recent ones suggest that management practices also have significant biogeophysical impacts on local and global climate through changes in surface albedo and energy partitioning. For Africa, relevant studies are scarce. In this study, we assessed the biogeophysical effects of CA in maize fields over the 2021/22 and 2022/23 seasons at two sites established in 2013 in Zimbabwe on an abruptic Lixisol at Domboshawa Training Center (DTC) and on a xanthic Ferralsol at the University of Zimbabwe Farm (UZF). We hypothesised that CA would have different effects on albedo dynamics and radiative forcing (RF) depending on 1) the presence/absence of crop residues at soil surface, 2) soil type/colour. We monitored dynamics of albedo, longwave radiation, LAI, soil moisture/temperature under three different treatments: conventional tillage (CT), no-tillage (NT) and no-tillage with mulch (NTM). Our results showed that NT and NTM induced a RF cooling effect (higher albedo compared to CT) in the clayey soil at UZF but the mean annual RF of NT was higher than that under NTM with -0.83 W.m-2 and -0.43 W.m-2 respectively. In sandy soil (at DTC), we observed a warming effect due to the soil darkening effect induced by mulching. The mean annual RF of NT was -3.4 W.m-2 (cooling effect compare to CT treatment) against 1.2 W.m-2 for NTM (warming effect). Next step is to compare the albedo induced RF induced by NT and NTM with the other biogeophysical effects and the biogeochemical effects on climate of those treatments. These results suggest that application of mulch or crop residues on sandy soils might not be relevant for a climate change mitigation perspective, even if it provides benefits for soil health and adaptation to climate change.

Agroforestry is a land-use system where woody perennials are deliberately combined with agricultural crops and/or livestock on the same land-management units in some form of spatial arrangement or temporal sequence. Agroforestry has the potential to respond to multiple challenges related to soil carbon sequestration including soil fertility improvement, land restoration, food security and adaptation and resilience to climate change. In this chapter, we show how agroforestry systems (AFSs) address several of the above-mentioned challenges, and play a key role in boosting soil carbon sequestration (SCS) and improving soil functions. Other co-benefits are also considered, i.e., soil and ecosystem services, and the wellbeing of rural populations (increase in income, access to non-timber products, etc.), mainly in the less developed countries. Attention is also paid to the main barriers, which may lessen or halt SCS in AFSs. We make recommendations for implementing and improving AFSs to foster SCS and to meet the challenges faced by human societies in the 21st century.

Food insecurity in sub-Saharan Africa (SSA) is partly due to low staple crop yields, resulting from low soil fertility and low nutrient inputs. Integrated soil fertility management (ISFM), that includes the combined use of mineral and organic fertilizers, can contribute to increasing yields and sustaining soil organic carbon (SOC) in the long term. Soil-crop simulation models help assess the performance of ISFM under current and future climate. Yet, uncertainty in model predictions can be high, resulting from poor model calibration and/or inadequate model structure. Multi-model assessments help understand model uncertainty and reduce it. We compared the performance of 16 soil-crop models using data from four long-term experiments at sites in SSA with contrasting climates and soils. Each experiment had four experimental treatments: i) no exogenous inputs, ii) addition of mineral nitrogen (N), iii) organic amendments, and iv) combined mineral and organic inputs. We assessed model performance after partial and full calibration. Model ensemble accuracy was greater with full calibration than with partial calibration, and improved more for crop yields (rRMSE 53 vs 17%) than for SOC (RMSE 21 vs 12%). Uncertainty of model simulations increased over the course of the long-term experiments. Uncertainty with SOC simulations increased when organic amendments were added to the soil, whilst uncertainty with yield predictions was largest when no inputs were applied. Differences between individual models were linked to uncertainties in simulating roots and soil N dynamics. The discrepancies between models increased when organic amendments were applied. Our study is the first multi-model comparison for long-term simulations of crop yield and SOC and their feedbacks in SSA. The results highlight the need for long-term experiments that monitor roots and N dynamics and provide the corresponding data required for soil-crop model improvements and calibration.

This study quantified soil organic carbon (SOC) and soil total nitrogen (STN) changes in three long-term (14 years) experiments comparing conventional tillage (CT) and different no-till (NT) cropping systems using a diachronic approach and an equivalent soil mass approach. The three experiments were established in 2009 in a randomized complete block design with three replicates and comprise maize (MaiEx)-, soybean (SoyEx)-, and cassava (CasEx)-based cropping systems. Soil samples were collected in 2021, 10 years after the first sampling (2011), at 7 depths: 0–5, 5–10, 10–20, 20–40, 40–60, 60–80, and 80–100 cm. At 0–10 cm depth, NTs significantly increased SOC stock at a rate of 0.35, 0.51 and 0.73 Mg C ha-1 y-1 for CasEx, SoyEx and MaiEx, respectively. CT exhibited a positive trend only on CasEx with an accumulation rate of 0.14 Mg C ha-1 y-1 (0-10 cm). A significant SOC accumulation was observed under NT cropping systems for CasEX and MaiEx up to 60-80 and 80-100 cm depth, respectively. By contrast, STN stock significantly increased under NT cropping systems only at 0–5 cm depth with an accumulation rate of 13.9, 22.9 and 34.0 kg N ha-1 y-1 under CasEx, SoyEx and MaiEx, respectively. A depletion of N stocks was observed under CT and NT cropping systems at deeper soil layers. Over 10 years of time, SOC accumulation rate of NT, at 0-100 cm depth, reached 1.08 and 1.73 Mg C ha-1 yr-1 for CasEx and MaiEx, respectively while non-significant change was observed under NT SoyEx and CT of the three experiments. The findings suggest that long-term NT cropping systems with high amount of biomass-C inputs and crop rotation enhance SOC sequestration even at deeper soil layers. However, these results raise questions about the dynamics of N under NT cropping systems.

Major drivers of gains or losses in soil organic carbon (SOC) include land management, land-use change, and climate change. Thousands of original studies have focused on these drivers of SOC change and are now compiled in a growing number of meta-analyses. To critically assess the research efforts in this domain, we retrieved and characterized 192 meta-analyses of SOC stocks or concentrations. These meta-analyses comprise more than 13,200 original studies conducted from 1910 to 2020 in 150 countries. First, we show that, despite a growing number of studies over time, the geographical coverage of studies is limited. For example, the effect of land management, land-use change, and climate change on SOC has been only occasionally studied in North and Central Africa, and in the Middle East and Central Asia. Second, the meta-analyses investigated a limited number of land management practices, mostly mineral fertilization, organic amendments, and tillage. Third, the meta-analyses demonstrated relatively low quality and transparency. Lastly, we discuss the mismatch between the increasing number of studies and the need for more local, reusable, and diversified knowledge on how to preserve high SOC stocks or restore depleted SOC stocks.

Global estimates indicate that agroforestry provides the largest opportunity for agricultural carbon sequestration, while simultaneously delivering diverse benefits to farmers and society, placing it among the most promising natural climate solutions (NCS). Policy and financial actions to support a transition to agroforestry are predicated on the ability to consistently identify agroforestry systems, estimate their carbon benefits, and monitor them over time. However, our review finds critical gaps in each of these areas: climate-positive agroforestry is poorly defined, mitigation estimates are imprecise and use only a fraction of available carbon data, and detection and monitoring are hindered by uncertainty in field and remote-sensing data. To pave the path from theory to practice, we refine standard agroforestry definitions to the subset of agroforestry NCS practices, highlight underappreciated opportunities for agroforestry expansion, and identify avenues to leverage existing data and emerging techniques to predict, map, measure, and track agroforestry's contribution to climate progress.

There is growing interest in agroforestry as a climate solution, given its potential to store additional carbon in agricultural landscapes, while also enhancing livelihoods and biodiversity. However, substantial uncertainty remains around how much carbon can be captured, and how that varies by location and by practice. One of the central challenges is the sheer diversity of agroforestry practices employed across the globe. Species identity, planting density, and management practices, as well as many other factors, will influence the overall climate mitigation potential of an individual agroforestry system. Although recent reviews have begun to compile carbon sequestration rates and stocks within agroforestry systems, the current evidence base is not fully comprehensive. Individual reviews have examined only a subset of the existing literature and typically partition agroforestry systems into coarse categories that do not reflect the diversity of actual on-the-ground practices. As individuals, corporations, and governments decide whether and how to deploy agroforestry as a climate solution during this climate critical decade, there is a strong need for a readily available and comprehensive dataset to better predict climate outcomes across diverse agroforestry systems. We have therefore conducted a systematic review of published studies to find empirical estimates of carbon sequestration rates and stocks in agroforestry systems. After reviewing over 18,000 papers, we have identified 800 or more papers that appear to have the necessary information. We are compiling this information into a consistent data structure to create a publicly available dataset that can help to accelerate our scientific understanding of the climate mitigation potential of agroforestry and facilitate the incorporation of agroforestry into climate goals. Although agroforestry offers high potential as a climate solution, delivering on that promise requires a more precise understanding of how much carbon can actually be captured, based on the best available data.

Following the Paris agreement (COP21), France set a carbon neutrality objective by 2050. Given its contribution to national GHG emissions (20%), the French agricultural sector is expected to play its part in achieving this ambitious target. It can act on three levers: N2O and CH4 emissions reduction, renewable energy production, and carbon storage in biomass and soil, the latter being less studied. The recent controversy on the 4 per 1000 initiative has also emphasized the need for a quantitative assessment of the potential for and cost of additional C storage in agricultural soils at the national level. Eight carbon storing practices were identified based on a literature review: expansion of cover crops; mobilization of new C inputs (not already spread on agricultural soils under current management practices); expansion of temporary grasslands (instead of silage maize); agroforestry; hedges; moderate intensification of extensive grasslands (+50kgN/ha); animal grazing instead of mowing and grass cover of vineyard. We assessed and mapped their potential for additional carbon storage in soils at a very fine spatial scale (1 km2), using a crop model (STICS) and a grassland model (PASIM). The additional C storage was calculated as the difference between the simulated soil C stock under i) the C storing practices and ii) the current management practices (i.e. the baseline), after a 30 years period. Using public statistics, we then assessed for each C storing practice: its potential applicability, its implementation cost, and its efficiency at the regional level. Using an economic model (BANCO), we finally compute the cost-effective allocation of the additional carbon storage effort, i.e. the uptake level of each storing practice in each region that minimizes the total cost of achieving a given total additional C storage target in mainland France. By varying this national storage target, we are able to depict a marginal carbon storage cost curve. Our results show a potential for additional C storage of about 8.43 Mt C/year for a total cost of 2.3 bn€/yr, which would compensate 6.8% of national GHG emissions and 41% of the agricultural sector emissions. This potential is mainly found in arable soils, where initial soil C stocks are low. Moreover, 96% of this potential can be achieved at a lower cost than 250€/tCO2e (the 2030 target carbon price in France) and for a total cost of 1.6 bn€/yr, which corresponds to only 18% of the current French budget for the Common Agricultural Policy. Reaching high soil C storage targets implies the full implementation of cover crops and agroforestry, together with the expansion of temporary grasslands in crop rotations. The costs and potential for additional C storage vary both between practices and across regions. This is why there is no “one-size-fits- all” solution, rather a combination of good practices at the right place. Our results thus provide useful information for the design of efficient climate change mitigation policies. Besides highlighting the potential for additional C storage in arable soils, where initial stocks are low, our study also underlines the need to maintain soil C stocks where they are high, i.e. in grasslands and forest soils.

L'initiative "4 pour mille : les sols pour la sécurité alimentaire et le climat" propose d'augmenter chaque année d'un quatre millième le stock de carbone présent dans tous les sols du monde. À la demande de l'Ademe et du ministère de l'Agriculture et de l'Alimentation, l'Inra (devenu aujourd'hui INRAE) a conduit une étude, centrée sur la France métropolitaine, visant à estimer le potentiel de stockage de carbone des sols agricoles et forestiers et, in fine, à mesurer la contribution potentielle de ce levier à l'objectif de réduction des émissions nettes de gaz à effet de serre. Diverses pratiques candidates (cultures intermédiaires, apport de nouvelles ressources organiques, gestion des prairies, agroforesterie…) ont été évaluées. Les résultats obtenus ont montré une forte variabilité du stockage additionnel de carbone. L'étude a également permis d'estimer le coût supplémentaire, pour les agriculteurs, de mise en oeuvre de ces pratiques de stockage, puis une allocation de l'effort de stockage entre les pratiques et les régions a été effectuée. Ces données permettront aux différents acteurs concernés de faire les meilleurs choix pour stocker davantage de carbone dans les sols. Cet ouvrage s'adresse aux décideurs chargés de l'élaboration des politiques publiques climatiques dans le domaine agricole, aux responsables territoriaux, aux aménageurs, aux ingénieurs et techniciens, aux agriculteurs et à l'ensemble des citoyens intéressés par la problématique de l'agriculture et du changement climatique.

International audience; L’initiative « 4 pour 1 000 : les sols pour la sécurité alimentaire et le climat » propose d’augmenter chaque année d’un quatre millième le stock de carbone présent dans tous les sols du monde. À la demande de l’Ademe et du ministère de l’Agriculture et de l’Alimentation, l’Inra (devenu aujourd’hui INRAE) a conduit une étude, centrée sur la France métropolitaine, visant à estimer le potentiel de stockage de carbone des sols agricoles et forestiers et, in fine, à mesurer la contribution potentielle de ce levier à l’objectif de réduction des émissions nettes de gaz à effet de serre. Diverses pratiques candidates (cultures intermédiaires, apport de nouvelles ressources organiques, gestion des prairies, agroforesterie…) ont été évaluées. Les résultats obtenus ont montré une forte variabilité du stockage additionnel de carbone. L’étude a également permis d’estimer le coût supplémentaire, pour les agriculteurs, de mise en œuvre de ces pratiques de stockage, puis une allocation de l’effort de stockage entre les pratiques et les régions a été effectuée. Ces données permettront aux différents acteurs concernés de faire les meilleurs choix pour stocker davantage de carbone dans les sols. Cet ouvrage s’adresse aux décideurs chargés de l’élaboration des politiques publiques climatiques dans le domaine agricole, aux responsables territoriaux, aux aménageurs, aux ingénieurs et techniciens, aux agriculteurs et à l’ensemble des citoyens intéressés par la problématique de l’agriculture et du changement climatique.

L’initiative « 4 ‰ sur les sols pour la sécurité alimentaire et le climat », lancée par la France à l’occasion de la Conférence de Paris sur le climat (COP-21), propose d’augmenter chaque année d’un quatre millième le stock de carbone présent dans tous les sols du monde afin de compenser les émissions anthropiques de CO2. Cet objectif, très ambitieux, nécessite des évolutions profondes des pratiques agricoles et des modes de gestion sylvicoles, certaines pouvant s’accompagner de modifications de systèmes de production et, éventuellement, des modes d’usage des sols. C'est dans ce contexte que le Ministère de l’Agriculture et de l’Alimentation (MAA) et l’Agence De l'Environnement et de la Maîtrise de l'Energie (ADEME) ont demandé à l'INRA de conduire la présente étude "4 pour mille France". Les objectifs étaient : i) d’identifier des pratiques agricoles et sylvicoles plus "stockantes" que les pratiques actuellement mises en œuvre ; ii) de chiffrer le potentiel de stockage additionnel associé, de le cartographier, de quantifier les autres effets induits liés à l’adoption de ces pratiques stockantes (pertes ou gains de rendement, émissions de N2O, lixiviation de nitrate, utilisation de produits phytosanitaires...) ; iii) de chiffrer leur coût de mise en œuvre et de proposer une stratégie coût-efficace de stockage. Le présent document constitue le rapport scientifique de cette étude.Ce travail a fait l'objet de documents de synthèse et d'un colloque public de restitution, qui s'est tenu à Paris le 13 juin 2019.