Nitrous oxide emission from pigeon pea – maize rotation in response to conservation agriculture and biochar amendments in a Ferralsol, northern Uganda

Résumé

Abstract. Smallholder agriculture in sub-Saharan Africa (SSA) commonly involves limited use of mineral or organic fertilizer, often resulting in severe nutrient limitation. Conservation Agriculture (CA), including crop rotation with legumes and biochar amendments, has been advocated to enhance soil fertility and plant available nitrogen (N). However, CA may affect nitrous oxide (N2O) emissions even in unfertilized agroecosystems. N2O is an important greenhouse gas, and understanding the trade-offs between N2O emissions and crop yields in N-poor agroecosystems in SSA is essential. Here we studied crop yield, soil N and N2O emissions in a double cropping system (pigeon pea – maize rotation) throughout two consecutive cropping seasons (April–October 2023 and October 2023–January 2024) in a Ferralsol in Northern Uganda. The study, conducted at a site which had been left fallow for 3 years, involved pairwise comparison of conventionally tilled systems under crop rotation (Conventional) and continuous maize monocropping (ConventMM). In addition, the effect of tillage systems (Conventional, CA and CA+biochar) under pigeon pea – maize rotation was investigated. We defined CA as reduced tillage with planting basins and crop residue retention, whereas conventional tillage involved overall ploughing. Grain yield was not significantly affected by rotation or tillage system. N2O fluxes were small, ranging from 1.02–51.19 µg N m2 h-1 over the entire period. Short-lived emission peaks were observed following pigeon pea harvest in the crop rotation, which were absent in maize monocropping. Overall, across all seasons, cumulative growing-season (279 days) N2O emissions ranged from 0.44–1.11 kg N ha-1. Biochar amendments in CA systems did not affect daily N2O emissions in planting basins. In the first season, yield-scaled N2O emissions and N yield scaled N2O emissions were significantly smaller in CA systems with biochar compared to conventional tillage, suggesting that CA and biochar was effective in minimising emissions without penalising pigeon pea productivity.

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