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

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 (GHG) 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 dynamics 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, crop rotation and residue retention, whereas conventional tillage involved overall ploughing. 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. Across two growing seasons, area-weighted cumulative N2O emissions for 279 d ranged from 0.46 kg kg N ha−1 in CA + BC treatment to 0.88 kg N ha−1 in the Conventional treatment, respectively. CA + BC reduced area-weighted N2O emissions by 33 % and 66 % compared to Conventional treatment in the first and second season, respectively. In addition, biochar amendments in CA systems also reduced yield-scaled N2O emissions by 48 % across two seasons. 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 reducing pigeon pea yield, in the first year after field clearing.