Search

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]

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]

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]

The respective contribution of conservation agriculture (CA) principles (no-tillage, permanent soil cover/mulch and crop rotations) on greenhouse gas (GHG) emissions is still unclear. This study was conducted at two long-term experimental sites established in 2013 in Zimbabwe, on an abruptic Lixisol at Domboshava Training Center (DTC) and on a xanthic Ferralsol at the University of Zimbabwe Farm (UZF). The purpose of the study was to unravel the individual and combined effects of tillage, mulching and rotation on N 2 O and CH 4 emissions in low nitrogen (N) input maize-based cropping systems (< 60 kg N ha−1) and to compare emissions within maize rows and between maize rows. We hypothesised that integrating no tillage, mulch and cereal-legume rotation would enhance N 2 O emissions. Six treatments, replicated four times were investigated: conventional tillage, conventional tillage with rotation, no-tillage, no-tillage with mulch, no-tillage with rotation, no-tillage with mulch and rotation. The main crop was maize (Zea mays L.) and treatments with rotation included cowpea (Vigna unguiculate L. Walp.). Gas samples were regularly collected using the static chamber method in the maize row and inter-row spaces during the 2019/20 and 2020/21 cropping seasons and during the 2020/21 dry season. Soil moisture and mineral N were measured in the 0–20 cm soil depth. In 2019/20, cumulative total N 2 O emissions were significantly higher in mulch treatments at DTC, while at UZF N 2 O emissions were higher with cowpea rotation. Cumulative total N 2 O emissions ranged from 215 to 496 g N 2 O-N ha−1 yr−1 and from 226 to 395 g N 2 O-N ha−1 yr−1, at DTC and UZF, respectively. In 2020/21, N 2 O emissions were much lower and no differences were found between treatments on both sites (145 to 179 g N 2 O-N ha−1 yr−1 and 83 to 136 g N 2 O-N ha−1 yr−1 at DTC and UZF, respectively). A significant relationship was found between soil nitrate and daily N 2 O emissions. At UZF, highest N 2 O emissions were observed at a water-filled pore space of 60–70%. There were no significant differences in yield-scaled N 2 O emissions between treatments at both sites for the two seasons. DTC was a net source of CH 4 (694 g CH 4 -C ha−1 yr−1 on average), while UZF was a net sink of CH 4 (−494 g CH 4 -C ha−1 yr−1 on average). No evidence was found for in situ CH 4 production at DTC, and an external source is most likely. Our study indicates that for low N input cropping systems in the sub-humid tropics, N loss through N 2 O is low. • Cowpea rotation increases maize cumulative area-scaled N 2 O emissions in a heavy soil. • Mulch increase cumulative area-scaled N 2 O emissions in a dry year in a sandy soil. • Yield-scaled N 2 O emissions are similar among the treatments. • Nitrogen loss through N 2 O emissions is very low and lower than IPCC default values. • CH 4 emissions or consumption linked to soil type and not to treatment. [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]