Biological N2-fixation and grain yield of pigeon pea: The role of biochar and conservation agriculture in low-input systems

Biological N 2 -fixation by legumes such as pigeon pea is a potentially important nitrogen input and a pathway for replenishing soil nitrogen (N) in maize dominant cropping systems. Biochar has been reported to affect N cycling, yet there are no studies on the synergism of conservation agriculture (CA) and biochar amendments on biological N 2 -fixation of pigeon pea in low input systems of sub-Saharan Africa. The objective of this study was to investigate the effect of biochar amendments under CA systems on biological N 2 -fixation, as these practices are promising sustainable intensification options in SSA. Specifically, we explored the effects of pigeon pea rotation and biochar amendments under CA on (1) biological N 2 -fixation, and (2) pigeon pea grain yield at three sites located in different agro-ecological regions of Uganda. We hypothesized that application of biochar under CA leads to greater biological N 2 -fixation and grain yield, than CA alone or conventional tillage. We set up three on-station experiments; one in Central Uganda (Mubende district), and two in Northern Uganda (Alebtong and Gulu districts) for two seasons. CA involved minimum tillage with planting basins established by hand hoeing. We investigated four treatments (1) conventional tillage with crop rotation (Conventional), (2) minimum tillage with crop rotation (CA), (3) CA and 2 Mg ha −1 of biochar, applied in both first and second year (CA+BC+BC), and (4) CA and 4 Mg ha −1 of biochar, applied once in the first year (CA+BC). Biological N 2 -fixation was high in Gulu, compared to Alebtong and Mubende; and this was mainly related to low soil N and greater percent of N derived from the atmosphere (%Ndfa), compared to other sites. Treatment effect on biological N 2 -fixation was variable across seasons, biochar application in CA systems did not affect biological N 2 -fixation in 2022 in Alebtong and Mubende. In 2023, biochar amendments in CA systems significantly increased biological N 2 -fixation in all sites. Grain yield was significantly increased by biochar application in CA systems compared to conventional practices, in all seasons and sites. A single dose application or split application of 4 Mg ha −1 biochar had similar effects on biological N 2 -fixation and grain yield. Biological N 2 -fixation was positively influenced by biochar application under CA systems. The additional N input associated with biological N 2 -fixation was up to 106 kg N ha −1 in Gulu and it significantly increased grain yield. There were no differences between once off application of 4 Mg ha −1 of biochar or applying 2 Mg ha −1 in the first year followed by 2 Mg ha −1 in the second year. • Combined effect of conservation agriculture (CA) and biochar on biological N 2 -fixation of pigeon pea in low input systems is not well understood. • Biological N 2 -fixation was variable across sites. • Biochar and CA enhanced biological N 2 -fixation and grain yield of pigeon pea. • Both single dose and split application of 4 Mg ha −1 biochar under CA had comparable results on biological N 2 -fixation and grain yield. • Biological N 2 -fixation was negatively correlated with soil N.