Smallholder farms have and can store more carbon than previously estimated.

Increasing soil organic carbon (SOC) stocks is increasingly targeted as a key strategy in climate change mitigation and improved ecosystem resiliency. Agricultural land, a dominant global land use, provides substantial challenges and opportunities for global carbon sequestration. Despite this, global estimates of soil carbon sequestration potential often exclude agricultural land and estimates are coarse for regions in the Global South. To address these discrepancies and improve estimates, we develop a hybrid, data-augmented database approach to better estimate the magnitude of SOC sequestration potential of agricultural soils. With high-resolution (30 m) soil maps of Africa developed by the International Soils Database (iSDA) and Malawi as a case study, we create a national adjustment using site-specific soil data retrieved from 1160 agricultural fields. We use a benchmark approach to estimate the amount of SOC Malawian agricultural soils can sequester, accounting for edaphic and climatic conditions, and calculate the resulting carbon gap. Field measurements of SOC stocks and sequestration potentials were consistently larger than iSDA predictions, with an average carbon gap of 4.42 ± 0.23 Mg C ha-1 to a depth of 20 cm, with some areas exceeding 10 Mg C ha-1 . Augmenting iSDA predictions with field data also improved sensitivity to identify areas with high SOC sequestration potential by 6%-areas that may benefit from improved management practices. Overall, we estimate that 6.8 million ha of surface soil suitable for agriculture in Malawi has the potential to store 274 ± 14 Tg SOC. Our approach illustrates how ground truthing efforts remain essential to reduce errors in continent-wide soil carbon predictions for local and regional use. This work begins efforts needed across regions to develop soil carbon benchmarks that inform policies and identify high-impact areas in the effort to increase SOC globally.

Can sub-Saharan Africa feed itself?

Although global food demand is expected to increase 60% by 2050 compared with 2005/2007, the rise will be much greater in sub-Saharan Africa (SSA). Indeed, SSA is the region at greatest food security risk because by 2050 its population will increase 2.5-fold and demand for cereals approximately triple, whereas current levels of cereal consumption already depend on substantial imports. At issue is whether SSA can meet this vast increase in cereal demand without greater reliance on cereal imports or major expansion of agricultural area and associated biodiversity loss and greenhouse gas emissions. Recent studies indicate that the global increase in food demand by 2050 can be met through closing the gap between current farm yield and yield potential on existing cropland. Here, however, we estimate it will not be feasible to meet future SSA cereal demand on existing production area by yield gap closure alone. Our agronomically robust yield gap analysis for 10 countries in SSA using location-specific data and a spatial upscaling approach reveals that, in addition to yield gap closure, other more complex and uncertain components of intensification are also needed, i.e., increasing cropping intensity (the number of crops grown per 12 mo on the same field) and sustainable expansion of irrigated production area. If intensification is not successful and massive cropland land expansion is to be avoided, SSA will depend much more on imports of cereals than it does today.

Response of common bean (Phaseolus vulgaris L.) to nitrogen, phosphorus and rhizobia inoculation across variable soils in Zimbabwe.

Common bean is an important crop with potential to curb malnutrition in poor Sub-Saharan African populations. Yields of common bean (Phaseolus vulgaris L.) are, however poor, limited by low soil phosphorus (P), nitrogen (N) and poor biological N2-fixation. On-farm experiments were carried out to study the effect of N, P and rhizobia inoculation on common bean yield and yield components during the 2014/2015 and 2015/2016 cropping seasons in Eastern Zimbabwe. Experiments were conducted on five farmers' fields located in two agroecologies; three fields were considered to be degraded with soil organic carbon (SOC) < 4 g kg-1 and available P < 6 mg kg-1, while the two non-degraded sites had SOC > 7 g kg-1 and available P > 15 mg kg-1. Two common bean varieties (Gloria and NUA45) were tested in a split-plot arranged in randomized complete block design. The main plot factor was the combination of N (0 and 40 kg ha-1) and P (0 and 20 kg ha-1), and the sub-plot factors were variety (Gloria and NUA 45) and inoculation with Rhizobium tropici strain CIAT899 (+/- inoculum). At planting, both N and P were applied at 20 kg ha-1, with an additional 20 kg ha-1 N top dressing applied at flowering. Analysis of variance indicated common bean did not respond to rhizobia inoculation (P > 0.05) whilst P significantly increased the number of nodules and active nodules per plant (P < 0.001), and grain yield. Application of 40 kg ha-1 N significantly increased the number of pods per plant, number of seeds per pod, and grain yields. A significant NP interaction was only observed on grain yield for non-degraded soils. Co-application of N and P in non-degraded sites increased grain yields from 0.27 to 1.48 Mg ha-1during the first season and from 0.37 to 2.09 Mg ha-1during the second season. On degraded sites, NP application resulted in uninspiring grain yield gains of 0.09 to 0.19 Mg ha-1 during the first season, and from 0.16 to 0.28 Mg ha-1 in the second season. In general, effects of N or P were not significantly different, suggesting that farmers could invest in either of these nutrients for increased common bean grain yields. Strategically, P investments would be more logical as residual P effects to rotational cereals improve overall cropping system performance. The response of common bean to inoculation in Zimbabwe still needs to be widely investigated for these and other varieties.

Short-term impacts of soil nutrient management on maize (Zea mays L.) productivity and weed dynamics along a toposequence in Eastern Zimbabwe.

Poor soil fertility and weed infestation are among major constraints to maize production in southern Africa. Nutrient and weed management strategies that are products of empirical research, are needed to improve efficiencies on farms. A field experiment was carried out in Eastern Zimbabwe on three smallholder farms positioned on upper, middle and lower catena. The farms differed in soil organic carbon (SOC) content, 3.9, 6.4 and 8.9 g kg-1 (hereafter referred to as low, medium and high), respectively, and are located within one km distance. The objective of the study was to investigate short-term (6 years) repeated application of soil nutrient amendments on maize productivity and weed dynamics across a soil fertility gradient. Treatments included strategic combinations of NPK fertiliser, cattle manure, and lime. On each farm, a randomised complete block design with three replicates was used. Multivariate, Principal Component Analysis, was used to establish the relationship between season, SOC content, nutrient management, and weed density. Maize yield was strongly linked to SOC content, with six-year mean maize grain yields of 1.31, 2.47, and 2.75 Mg ha-1 for low, medium, and high SOC content, respectively. Maize grain yields with cattle manure (CM) or NPK application were only 0.25 and 0.60 Mg ha-1, respectively for the poorest SOC content field. However, when manure was combined with NPK fertiliser, yields at the site substantially increased to 1.5 Mg ha-1 while in medium and high SOC recorded 2.47 and 2.75 Mg ha-1 respectively. Weed density, and biomass were larger in the medium, and high SOC content. Richardia scabra, Melinis repens, and Cyperus sp. were associated with low SOC. Luecus martinicensis, Bidens pilosa, and Galinsoga parviflora were linked to medium and high SOC content. Results from this study suggest site-specific weed management approach. On soils critically poor in SOC content, maize yield gains are only achieved when organic and mineral fertilisers are combined. Our results also suggest that farmers must increase vigilance and intensity of weed management in soils with medium and high SOC content, particularly after application of CM.

Smallholder Farms and the Potential for Sustainable Intensification.

The sustainable intensification of African agriculture is gaining momentum with the compelling need to increase food and agricultural production. In Southern Africa, smallholder farming systems are predominately maize-based and subject to erratic climatic conditions. Farmer crop and soil management decisions are influenced by a plethora of complex factors such as market access resource availability, social relations, environment, and various messages on sustainable farming practices. Such factors pose barriers to increasing sustainable intensification in Africa. This paper characterizes smallholder farming practices in Central Malawi, at Africa Research in Sustainable Intensification for the Next Generation (Africa RISING) project sites. We present findings from a survey of 324 farmers, located within four Africa RISING sites selected in a stratified random manner to represent (1) low agricultural potential (high evapotranspiration, variable rainfall), (2) medium agricultural potential (two sites), and (3) high agricultural potential (well-distributed rainfall). Soil fertility was low overall, and certain farming practices appeared to limit the sustainability of agricultural production. Nearly half of farmers did not value legume residues as a high nutrient value resource for soil amelioration, as legume residues were removed (17.9%) or burned (21.4%). Conversely, maize residues were rarely removed (4.5%) or burned (10.4%). We found that farmers do not allocate soil amendment resources to legume fields (zero instances of mineral fertilizer or manure application to legumes compared to 88 and 22% of maize systems, respectively). Policy makers in Malawi have led initiatives to intensify agricultural systems through subsidizing farmer access to mineral fertilizer as well as maize hybrid seed, and only rarely to improved legume seed. In this survey, farmers allocate mineral fertilizer to maize systems and not legume systems. There is urgent need to invest in education on sustainable reinvestment in natural resources through complementary practices, such as maximization of biological nitrogen fixation through improved legume agronomy and better organic resource and crop residue management. Recent efforts by Malawi agricultural services to promote doubled-up legumes as a sustainable intensification technology are encouraging, but benefits will not accrue unless equal attention is given to an extension campaign on management of organic resources such as crop residues.