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Soils are the largest terrestrial reservoir of organic carbon, yet they are easily degraded. Consistent and accurate monitoring of changes in soil organic carbon stocks and net greenhouse gas emissions, reporting, and their verification is key to facilitate investment in sustainable land use practices that maintain or increase soil organic carbon stocks, as well as to incorporate soil organic carbon sequestration in national greenhouse gas emission reduction targets. Building up on an initial review of monitoring, reporting and verification (MRV) schemes with a focus on croplands, grasslands, and forestlands we develop a framework for a modular, scalable MRV system. We then provide an inventory and classification of selected MRV methodologies and subsequently "score" them against a list of key characteristics. It appears that the main challenge in developing a unified MRV system concerns the monitoring component. Finally, we present a conceptual workflow that shows how a prototype for an operational, modular multi-ecosystem MRV tool could be systematically built. [ABSTRACT FROM AUTHOR]

No-till (NT) cropping systems have been proposed as a potential strategy to combat soil degradation and global warming by storing soil organic carbon (SOC) and nitrogen (N). Yet, there are ongoing debates about the real benefits of NT systems and factors influencing SOC and N accumulation. Assessing the dynamics of SOC and N on the long-term is needed to fill knowledge gaps and provide robust scientific evidence for potential additional SOC storage. We quantified the changes in SOC and N stocks and fractions down to 100 cm depth from three 13-year-old experiments in a tropical red Oxisol in Cambodia, comparing conventional tillage (CT) to NT monocropping and NT crop rotation systems using a diachronic and equivalent soil mass (ESM) approach. The three experiments comprised maize-, soybean-, and cassava-based cropping system trials, hereafter called MaiEx, SoyEx, and CasEx, respectively. Soil samples were collected in 2021, 10 years after the first sampling in 2011, at 7 depths: 0–5, 5–10, 10–20, 20–40, 40–60, 60–80, and 80–100 cm. Over the 10-year period (2011–2021), significant impacts on SOC stock and its vertical distribution differed among the NT systems and in the three experiments. In MaiEx and CasEx, the soils under all the NT systems significantly (P > 0.05) accumulated SOC stock across the soil depths, with the accumulation ranging from 6.97 to 14.71 Mg C ha-1 in the whole profile (0–100 cm). In SoyEx, significant increase in SOC stock was limited to the top 0–20 cm under NT monocropping, whereas NT crop rotation systems had significantly accumulating SOC stock from 0 to 80 cm depths. When considering 0–100 cm as a single stratum, the annual SOC cumulative rate in NT systems ranged from 0.86–1.47, 0.65–1.00, and 0.70–1.07 Mg C ha-1 yr-1 in MaiEx, SoyEx, and CasEx, respectively. In the top 0–10 cm, NT systems significantly increased C concentration in particulate organic matter (POM) by 115 %, 118 %, in MaiEx and SoyEx, respectively, and by 37 % in CasEx although not significantly. Similarly, at 0–10 cm depth, NT systems significantly enhanced C concentration in the mineral-associated organic matter (MAOM) by 33 %, 21 %, in MaiEx and SoyEx, respectively. Significant increase of C in MAOM was also observed from 0 to 40 cm in CasEx. In contrast, total N stock in NT systems increased in the surface 0–5 cm depth but decreased below 10 cm and in the whole profile (0–100 cm), particularly under NT monocropping with an annual loss rate of -0.10 and -0.17 Mg N ha-1 yr-1 in SoyEx and CasEx, respectively. Although NT systems increased N concentration in POM in the top 0–10 cm of MaiEx and SoyEx, a decreasing trend was observed below 10 cm depth. The N concentration in POM under NT systems in CasEx also decreased with soil depth. From 2011 to 2021, N concentration in MAOM under NT systems remained stable in MaiEx and SoyEx in the top 0–5 cm, but significant decreases in MaiEx and CasEx below 5 cm. Our findings suggest that adopting NT cropping systems with diverse crop and cover crop species and high biomass C inputs in the long-term leads to SOC accumulation not only in the surface but also in deeper layers, by increasing both the C pools in the POM and MAOM size fractions, even on the cassava-based system, which is believed to be an annual crop that could cause serious soil fertility depletion. This study highlights the potential of NT cropping systems to store SOC over time, but raises questions about soil N dynamics. [ABSTRACT FROM AUTHOR]

Shang et al. (2024) recently suggested to include nitrous oxide (N2O) emissions during the fallow period to better estimate N2O emission factors (EFs). We however highlighted several pitfalls of the proposed adjusted EFs for croplands in the specific case of dry subhumid, semiarid, and arid regions with dry fallow periods, these regions covering about 47% of the Earth's terrestrial area. [ABSTRACT FROM AUTHOR]

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 observed in NTM treatments at the two sites, and this could probably explain the positive effect on SOC stocks. Moreover, our results show that the combination of at least two CA principles including mulch is required to increase SOC stocks in these low-nitrogen-input cropping systems. [ABSTRACT FROM AUTHOR]

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 with the use of agroecological practices and adequate policy support. [ABSTRACT FROM AUTHOR]

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 about the impact of CA and its respective three principles on top and sub-soil organic carbon (SOC) stocks and on soil CO2 efflux in low input cropping systems of sub-Saharan Africa is rather limited. The study was conducted at two long-term experimental sites established in 2013 in Zimbabwe. The soil types were abruptic Lixisols at Domboshava Training Centre (DTC) and xanthic Ferralsol at the University of Zimbabwe farm (UZF). Six treatments, replicated four times were investigated: conventional tillage (CT), conventional tillage with rotation (CTR), NT, no-tillage with mulch (NTM), no-tillage with rotation (NTR), 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.). SOC concentration and bulk density were determined for samples taken from the 0–5, 5–10, 10–15, 15–20, 20–30, 30–40, 40–50, 50–75 and 75–100 cm depths. Gas samples were regularly collected using the static chamber method during the 2019/20 and 2020/21 cropping seasons and during the 2020/21 dry season. SOC stocks were significantly (p < 0.05) higher under NTM, NTR and NTMR compared to NT and CT in top 5 and 10 cm layers at UZF, while SOC stocks were only significantly higher under NTM and NTMR compared to NT and CT in top 5 cm at DTC. NT alone had a slightly negative impact on top SOC stock. Cumulative SOC stocks were not significantly different between treatments when considering the whole 100 cm soil profile. Regardless of larger organic carbon inputs in mulch treatments, there were no significant differences in CO2 efflux between treatments, but it was higher in maize rows than in inter-rows as a result of autotrophic respiration from maize roots. Our results show the overarching role of crop residue mulching in CA cropping systems in enhancing SOC storage but that this effect is limited to the topsoil. [ABSTRACT FROM AUTHOR]

The article challenges the notion that cover crops are effective in increasing soil organic carbon (SOC) stocks. The authors argue that the study's validity is questionable due to the exclusion of relevant published experimental data that show clear positive effects of cover crops on SOC stocks. They also criticize the authors for not following international standards for systematic reviews and meta-analyses. The article concludes that doubts about the positive effects of cover crops on SOC are unjustified and criticizes the authors' opinions on the relevance of policies and public subsidies. [Extracted from the article]

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. [ABSTRACT FROM AUTHOR]

Identifying the drivers of soil organic carbon (SOC) stock changes is of 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 with several semantic challenges. So far, a comprehensive thesaurus and classification of management practices in agriculture and forestry is lacking, especially while focussing 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 226 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) to enhance its findability, accessibility, interoperability and re-use 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 policy-makers to identify efficient agricultural and forest management practices to enhance SOC storage. [ABSTRACT FROM AUTHOR]

We have read with interest an opinion paper recently published in the European Journal of Soil Science (Berthelin et al., 2022). This paper presents some interesting considerations, at least one of which is already well known to soil scientists working on soil organic carbon (SOC), that is, a large portion (80%–90%) of fresh carbon inputs to soil is subject to rapid mineralization. The short‐term mineralization kinetics of organic inputs is well‐known and accounted for in soil organic matter models. Thus, clearly, the long‐term predictions based on these models do not overlook short‐term mineralization. We point out that many agronomic practices can significantly contribute to SOC sequestration. If conducted responsibly whilst fully recognising the caveats, SOC sequestration can lead to a win‐win situation where agriculture can both contribute to the mitigation of climate change and adapt to it, whilst at the same time delivering other co‐benefits such as reduced soil erosion and enhanced biodiversity. Highlights: Rapid mineralization of organic inputs is an important factor for soil carbon sequestration.Mineralization kinetics of organic inputs are well‐known and accounted for in soil organic matter models.Many agronomic practices can contribute significantly to SOC sequestration.SOC sequestration can lead to a win‐win situation where agriculture can both contribute to the mitigation of climate change and adapt to it. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

To respect the Paris agreement targeting a limitation of global warming below 2°C by 2100, and possibly below 1.5°C, drastic reductions of greenhouse gas emissions are mandatory but not sufficient. Large-scale deployment of other climate mitigation strategies is also necessary. Among these, increasing soil organic carbon (SOC) stocks is an important lever because carbon in soils can be stored for long periods and land management options to achieve this already exist and have been widely tested. However, agricultural soils are also an important source of nitrous oxide (N2O), a powerful greenhouse gas, and increasing SOC may influence N2O emissions, likely causing an increase in many cases, thus tending to offset the climate change benefit from increased SOC storage. Here we review the main agricultural management options for increasing SOC stocks. We evaluate the amount of SOC that can be stored as well as resulting changes in N2O emissions to better estimate the climate benefits of these management options. Based on quantitative data obtained from published meta-analyses and from our current level of understanding, we conclude that the climate mitigation induced by increased SOC storage is generally overestimated if associated N2O emissions are not considered but, with the exception of reduced tillage, is never fully offset. Some options (e.g. biochar or non-pyrogenic C amendment application) may even decrease N2O emissions. [ABSTRACT FROM AUTHOR]

Here, we present our collective musings on soil research challenges and opportunities and, in particular, the points raised by Philippe Baveye (Baveye, 2020a, 2020b) and Johan Bouma (Bouma, 2020) on I bypass i and I hyperbole i in soil science. Furthermore, developing a healthy and constructive post-publication peer-review system, where bypasses and hyperbolic approaches can be identified and discussed, would ultimately boost publication quality and contribute to a more open discourse in soil science. [Extracted from the article]

Agroforestry systems have been much studied for their potential to store soil organic carbon (SOC). However, few data are available on their specific impact on potential SOC mineralization, especially at depth in subsoils. Moreover, many soils of the world, especially in arid and semiarid environments, also contain large stocks of soil inorganic carbon (SIC) as carbonates. Consequently, the organic carbon dynamics have been poorly investigated in these soils due to the complexity of measurements and of the processes involved. To assess mineralization rates of SOC with depth, we incubated soil samples from an 18‐year‐old agroforestry system (both tree row and alley) and an adjacent agricultural plot established on a calcareous soil in France. Soil samples were taken at four different depths: 0–10, 10–30, 70–100 and 160–180 cm. Total CO2 emissions, the isotopic composition (δ13C, ‰) of the CO2 and microbial biomass were measured. The SIC concentrations ranged from 48 to 63 g C kg−1 soil and the SOC concentrations ranged from 4 to 17 g C kg−1 soil. The contribution of SIC‐derived CO2 represented about 20% in the topsoil and 60% in the subsoil of the total soil CO2 emissions. The microbial biomass and the SOC‐derived CO2 emissions were larger in the topsoil, but the decomposition rates (day−1) remained stable with depth, suggesting that only the size of the labile carbon pool was modified with depth. Subsoil organic carbon seems to be as prone to decomposition as surface organic carbon. No difference in CO2 emissions was found between the agroforestry and the control plot, except in the tree row at 0–10 cm. Our results suggest that the measurement of soil respiration in calcareous soils could be overestimated if the isotopic signature of the CO2 is not taken into account. It also advocates more in‐depth studies on carbonate dissolution–precipitation processes and their impact on CO2 emissions. Highlights: We measured SOC mineralization and inorganic carbon contribution to CO2 emissions in agroforestrySubsoil organic carbon was as prone to decomposition as surface organic carbonInorganic carbon contribution to CO2 emissions ranged from 20 to 60% depending on soil depthMeasurement of soil respiration in calcareous soils could be overestimated [ABSTRACT FROM AUTHOR]

Intercropping is a common practice among farmers in sub-Saharan Africa, regarded as a sustainable way of improving land productivity to meet food and nutritional requirements for a growing population, especially in marginal areas. Cowpea (Vigna unguiculata L. Walp) is often intercropped with major cereal crops, maize (Zea mays L.), sorghum (Sorghum bicolor L. Moench) and pearl millet (Pennisetum glaucumL.R.Br). Here we conducted a systematic literature review on cowpea intercropped with maize, sorghum or pearl millet reported in sub-Saharan Africa with the objectives (i) to determine yield and productivity of component crops and (ii) to quantify biological N2-fixation in sole or intercrops. We retrieved 60 unique publications combining 1196, 998 and 25 observations of yields, land productivity and N2-fixation, respectively, for crops grown as intercrops and monocrops. The major results are as follows: (1) land productivity of cowpea intercropped with maize, sorghum and pearl millet is favourable, with average land equivalent ratios of 1.42 ± 0.47, 1.26 ± 0.35 and 1.30 ± 0.32, respectively; (2) no significant differences between the proportion of nitrogen derived from the atmosphere (%Ndfa) for sole or intercropped cowpea were found, with average values of 56.00 ± 4.89 and 46.62 ± 7.05, respectively; (3) however, the total amount of fixed nitrogen was higher in cowpea monocropping systems due to higher biomass production; nitrogen fixation was 57 kg N ha−1 and 36 kg N ha−1 in monocrops and intercrops respectively. We conclude that cereal-cowpea intercropping is a pathway for intensification for the low nutrient input systems of smallholder farmers in sub-Saharan Africa. Our review also suggests potential for improvement of these systems, based on the choice of the associated varieties, planting patterns and sowing time, cowpea leaf harvesting as a vegetable, and fertilization. [ABSTRACT FROM AUTHOR]

Agroforestry is an increasingly popular farming system enabling agricultural diversification and providing several ecosystem services. In agroforestry systems, soil organic carbon (SOC) stocks are generally increased, but it is difficult to disentangle the different factors responsible for this storage. Organic carbon (OC) inputs to the soil may be larger, but SOC decomposition rates may be modified owing to microclimate, physical protection, or priming effect from roots, especially at depth. We used an 18-year-old silvoarable system associating hybrid walnut trees (Juglans regia ⨰ nigra) and durum wheat (Triticum turgidum L. subsp. durum) and an adjacent agricultural control plot to quantify all OC inputs to the soil - leaf litter, tree fine root senescence, crop residues, and tree row herbaceous vegetation - and measured SOC stocks down to 2m of depth at varying distances from the trees.We then proposed a model that simulates SOC dynamics in agroforestry accounting for both the whole soil profile and the lateral spatial heterogeneity. The model was calibrated to the control plot only. Measured OC inputs to soil were increased by about 40% (C 1.11 tCha-1 yr-1) down to 2m of depth in the agroforestry plot compared to the control, resulting in an additional SOC stock of 6.3 tCha-1 down to 1m of depth. However, most of the SOC storage occurred in the first 30 cm of soil and in the tree rows. The model was strongly validated, properly describing the measured SOC stocks and distribution with depth in agroforestry tree rows and alleys. It showed that the increased inputs of fresh biomass to soil explained the observed additional SOC storage in the agroforestry plot. Moreover, only a priming effect variant of the model was able to capture the depth distribution of SOC stocks, suggesting the priming effect as a possible mechanism driving deep SOC dynamics. This result questions the potential of soils to store large amounts of carbon, especially at depth. Deep-rooted trees modify OC inputs to soil, a process that deserves further study given its potential effects on SOC dynamics. [ABSTRACT FROM AUTHOR]

Agroforestry is an increasingly popular farming system enabling agricultural diversification and providing several ecosystem services. In agroforestry systems, soil organic carbon (SOC) stocks are generally increased, but it is difficult to disentangle the different factors responsible for this storage. Organic carbon (OC) inputs to the soil may be larger, but SOC decomposition rates may be modified owing to microclimate, physical protection, or priming effect from roots, especially at depth. We used an 18-year-old silvoarable system associating hybrid walnut trees (Juglans regia × nigra) and durum wheat (Triticum turgidum L. subsp. durum), and an adjacent agricultural control plot to quantify all OC inputs to the soil - leaf litter, tree fine root senescence, crop residues, and tree row herbaceous vegetation -, and measure SOC stocks down 2 m depth at varying distances from the trees. We then proposed a model that simulates SOC dynamics in agroforestry accounting for both the whole soil profile and the lateral spatial heterogeneity. OC inputs to soil were increased by about 40 % (+1.11 t C ha-1 yr-1) down to 2 m depth in the agroforestry plot compared to the control, resulting in an additional SOC stock of 6.3 t C ha-1 down to 1 m depth. The model described properly the measured SOC stocks and distribution with depth. It showed that the increased inputs of fresh biomass to soil explained the observed additional SOC storage in the agroforestry plot. Moreover, modeling revealed a strong priming effect that would reduce the potential SOC storage due to higher organic inputs in the agroforestry system by 75 to 90 %. This result questions the potential of soils to store large amounts of carbon, especially at depth. Deep-rooted trees modify OC inputs to soil, a process that deserves further studies given its potential effects on SOC dynamics. [ABSTRACT FROM AUTHOR]

Sustainable alternatives to slash-and-burn shifting cultivation in the (sub)humid tropics rely on the use of external nutrient inputs to address soil fertility decline. The use of organic inputs is widely accepted as a practice to improve soil fertility, in particular soil organic carbon (SOC). On the other hand, its combined use with mineral fertilizer has the potential to maintain or increase crop productivity through positive interactive effects between both resources. Few studies have investigated these effects in the long term. Therefore, the objective of this study was to investigate whether maize productivity and soil 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. Leguminous crops in the rotation were key for maize productivity, but probably due to effects non-related to supplementary N supply. • Maize yields are higher with than without compost addition and variability is reduced. • Without compost, 50% of initial SOC stock is lost within 20 years after land clearing. • Combined addition of compost and mineral N maintain productivity but not SOC stocks. • Without amendments, maize yield decreases with decreasing SOC concentration. • Leguminous crops in rotation are key for maize productivity but not due to N supply. [ABSTRACT FROM AUTHOR]

• Carbon sequestration potential through CA in Africa was overestimated with a factor of 10. • CA should be promoted for climate resilience benefits rather than for climate change mitigation. [ABSTRACT FROM AUTHOR]

• Two years on-farm trials on intercropping in poor soils in low inputs systems. • Land productivity increased by maize-cowpea intercropping. • Same proportion of N derived from the atmosphere for sole or intercropped cowpea. • Maize and cowpea grain mineral contents not impacted by treatment but by season. • Cowpea grain mineral contents (Fe, Zn, Mn, Cu, Ca, Mg, P, K) higher than for maize. Poor soil fertility and erratic rainfall constrain crop production in rain-fed smallholder farming systems in sub-Saharan Africa. Integration of drought tolerant and N 2 -fixing crops into maize-based cropping systems is a risk-averse strategy that also improves nitrogen cycling. A field experiment was carried out during the 2017/18 and 2018/19 cropping seasons in Goromonzi district in Zimbabwe. The trials were established on 14 farms, and on two field types (homefields and outfields), with eight treatments from a combination of cropping systems (maize and cowpea monocrops or maize/cowpea intercrops) and with or without nitrogen fertilizer (+N; -N). The trials were implemented on the same field plots for the two consecutive seasons. An improved cowpea variety and a landrace were used. The objectives were to determine 1) the productivity of the different cropping systems under variable soil fertility conditions, 2) N 2 -fixation of the two cowpea types when planted as monocrops or intercrops, and 3) mineral composition of maize and cowpea grains from intercrops and sole crops. Contrary to expected results, soil properties were not significantly different (P > 0.05) between field types. The land equivalent ratios (LER) were >1 for both seasons, implying improved land productivity under intercropping. Intercropping significantly reduced cowpea nodulation and active nodules, but not the total nodule weight, resulting in similar proportion of nitrogen derived from the atmosphere (%Ndfa) for cowpea grown as monocrop or in intercropping with maize. However, the total amount of fixed nitrogen was reduced in intercropping systems due to the smaller cowpea biomass compared to monocropping. Maize and cowpea grain mineral contents (Fe, Zn, Mn, Cu, Ca, Mg, P, K) were significantly affected by the cropping season only. We showed that intercropping maize with cowpea generally increases system productivity, in addition to substantial amounts of nitrogen being added to the system through N 2 -fixation. However, intercropping was not an agronomic biofortification option in these nutrient-depleted soils. Finally, annual variation in grain mineral quality can be larger than the annual variation in grain yield, potentially posing serious challenges to human nutrition. [ABSTRACT FROM AUTHOR]