Sustaining and enhancing soil organic carbon (SOC) and nitrogen (N) is critical to crop health and productivity, particularly in the low-input, resource-limited smallholder agriculture widely found across sub-Saharan Africa (SSA). In the managed environment of the agroecosystem, farmer practices directly shape soil processes. Farmer surveys and soil and agronomic analyses conducted in Uganda and Malawi detail and elucidate farmer practices and their impacts on SOC and N cycling and overall soil health. In western Uganda, use of fertilizer and external inputs is extremely low, and legume crops like groundnut provide the main source of N input into soil, making management of legume residues key to soil N and C gains. A partial N-balance of groundnut fields constructed from collected field and soils data, farmer responses, and values from the literature, found that groundnut residues retained on fields could potentially contribute substantial N inputs, but that burning and removal of groundnut residues in approximately half of the surveyed fields conferred minimal N contributions. Chi-square analysis revealed a relationship between farmer perception and valuation of groundnut residues and residue management practices. A comparison to the uncultivated soils of adjacent Kibale National Park showed that SOC and total N were lower in groundnut field soils and that groundnut residue management practices did not have an observed effect. Within the same region in western Uganda, detailed agronomic surveys and soil sampling were conducted to quantify and map the flow of organic resources and measure SOC and N within 19 case study farms. Home banana plantations located directly next to homesteads received the majority of organic inputs and had positive N balances, while fields further away received few inputs and had negative N balances, even when cropped to legumes. Despite receiving more inputs, home banana plantations did not have higher SOC and N, and there was no evidence of a management gradient related to field distance from the homestead. Farms with greater resource endowments had more organic resources, but did not fully utilize available resources or have soils with higher SOC or N compared to farms with less resources, highlighting the heterogeneity of soils across the landscape, as well as the importance of other factors, such as timing of planting and harvesting, labor availability, and seed quality. As in Uganda, smallholder farmers in Malawi intercrop and rotate with legumes. On-farm, participatory trials established in three agroecological zones in Central Malawi in 2012 provided the opportunity to evaluate impacts of crop diversification on SOC pools within a “doubled-up” legume rotation system compared to simplified systems. After six years of trial establishment, SOC was measured in bulk soils, aggregate fractions and in faster cycling C pools that respond more rapidly to management practices. The groundnut and pigeonpea DLR system accumulated more SOC than sole pigeonpea or sole groundnut in rotation with maize, and all legume systems acquired more SOC than continuous maize. Cropping treatment differences were not seen in bulk SOC or total N, but differences were apparent in SOC pools characterized by a shorter turnover time. Readily decomposable and biologically active SOC pools are shown to be early indicators of SOC dynamics and the effects of crop rotation and diversification.
Read
