Crop Residue Management and Its Cascading Effects on Soil Carbon Sequestration, Greenhouse Gas Emissions, and Horticultural Productivity in the West African Sahel: A Multi-Site Study

Crop residue management decisions in horticultural systems of the West African Sahel have cascading consequences for soil carbon sequestration, greenhouse gas emissions, and crop productivity, yet these interconnected processes have rarely been studied simultaneously in North West Nigerian horticultural contexts. This dissertation investigates the effects of four crop residue management strategies, complete removal, surface mulching, incorporation, and biochar conversion, on soil carbon dynamics, net greenhouse gas balance, and horticultural productivity across multi-site trials in Katsina, Sokoto, and Zamfara states over three growing seasons. Soil carbon stocks were monitored using standardized sampling protocols at 0-10 cm, 10-30 cm, and 30-60 cm depths. Greenhouse gas emissions of carbon dioxide, methane, and nitrous oxide were measured using static chamber methods at weekly intervals. Horticultural crop yield data for onion, tomato, and melon were collected in each season at all sites. Structural equation modeling linked residue management to carbon, emissions, and yield outcomes simultaneously. Results indicate that biochar conversion produced the highest soil carbon sequestration (mean +0.84 Mg C/ha/year) and the most favorable greenhouse gas balance, while surface mulching delivered the best trade-off between yield maintenance and soil carbon accumulation under resource-limited conditions. Complete residue removal consistently degraded soil carbon stocks across all sites. The study provides multi-site evidence supporting biochar-based residue management as a climate-smart horticultural practice for the Nigerian Sahel. Keywords: crop residue, soil carbon sequestration, greenhouse gas, horticultural productivity, West African Sahel