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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]

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]

Weeds are a major threat to crop growth in sub-Saharan Africa (SSA), particularly in low-input cropping systems. We investigated the effect of different cropping systems and nitrogen fertilizer on weed infestation during the 2017/18 and 2018/19 cropping seasons in Goromonzi district, Zimbabwe. The objectives of this study were to (1) determine if and how weed composition and structure are affected by intercropping maize with either an improved or landrace cowpea variety compared to sole crops of maize and of improved or landrace cowpea; (2) if and how weed composition and structure are affected by nitrogen fertilization; and (3) how the subsequent changes in weed infestation affect grain yield. Weed parameters were measured at different dates during the cropping cycle, and grain yield was measured at harvest. Significant differences were observed for the density of the 21 weed species collected in two seasons; seven of these weed species had a relative density exceeding 5%. Significantly more weeds were recorded in sole maize compared to sole cowpea. Intercropping significantly reduced the density of C. benghalensis. In sole crops, application of mineral nitrogen fertilizer significantly affected weed density, diversity, richness, and groundcover. Maize grain yield was independent of weed biomass and density, in both sole crops and intercrops. However, increase in weed biomass significantly reduced cowpea yield. Our results suggest that growing sole cowpea reduces weed infestation, therefore, cowpea should be considered as one of the options for effective weed control in subsistence farming communities, where access to herbicides is lacking. • Sole cowpea effectively reduces weed growth in low-input systems. • Maize-cowpea intercropping significantly reduced the density of C. benghalensis. • Application of mineral nitrogen fertilizer in sole crops increased weeds. • Cowpea grain yield was correlated to weed biomass. • Maize grain yield was independent of weed biomass and density. [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]

Food insecurity in sub-Saharan Africa is partly due to low staple crop yields, resulting from poor soil fertility and low nutrient inputs. Integrated soil fertility management (ISFM), which 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 can help assess the performance and trade-offs of a range of crop management practices including ISFM, under current and future climate. Yet, uncertainty in model simulations can be high, resulting from poor model calibration and/or inadequate model structure. Multi-model simulations have been shown to be more robust than those with single models and help understand and reduce modelling uncertainty. In this study, we aim to perform the first multi-model comparison for long-term simulations of crop yield and SOC and their feedbacks in SSA. We evaluated the performance of 16 soil-crop models using data from four long-term maize experiments at sites in SSA with contrasting climates and soils. Each experiment had four treatments: i) no exogenous inputs, ii) addition of mineral nitrogen (N) fertilizer, iii) use of organic amendments, and iv) combined use of mineral and organic inputs. We assessed model performance in two steps: through blind calibration involving a minimum level of experimental data provided to the modeling teams, and subsequently through full calibration, which included a more extensive set of observational data. Model ensemble accuracy was greater with full calibration than blind calibration. Improvement in model accuracy was larger for maize yields (nRMSE 48 vs 18%) than for topsoil SOC (nRMSE 22 vs 14%). Model ensemble uncertainty (defined as the coefficient of variation across the 16 models) increased over the duration of the long-term experiments. Uncertainty of SOC simulations increased when organic amendments were used, whilst uncertainty of yield predictions was largest when no inputs were applied. Our study revealed large discrepancies among the models in simulating i) crop-to-soil feedbacks due to uncertainties in simulated carbon coming from roots, and ii) soil-to-crop feedbacks due to large uncertainties in simulated crop N supply from soil organic matter decomposition. These discrepancies were largest when organic amendments were applied. The results highlight the need for long-term experiments in which root and soil N dynamics are monitored. This will provide the corresponding data to improve and calibrate soil-crop models, which will lead to more robust and reliable simulations of SOC and crop productivity, and their interactions. • Multi-model assessment for long-term simulations of crop yield and soil carbon • Model ensemble uncertainty in simulating soil carbon increase with organic amendment • Model ensemble uncertainty in simulating yield increase in no-input treatments • Models largely differed in simulating root biomass and nitrogen mineralisation • More detailed experiments on soil crop feedback are needed to improve models [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]