The role of nitrogen fixation and crop N dynamics on performance and legacy effects of maize-grain legumes intercrops on smallholder farms in Tanzania

Maize-grain legume intercrops form an important component of the cropping systems of smallholder farmers in many parts of sub-Saharan Africa. However, the effects of cropping system and fertilizer use on nitrogen fixation and nitrogen uptake of component crops in maize-legume intercrops are not well understood. Our study addressed the questions: (i) What is the capacity of pigeonpea and lablab to fix atmospheric nitrogen (N₂) in sole crop and when intercropped with maize on smallholder farms across different agro-ecological conditions?; (ii) How does productivity and N-uptake by sole and intercropped maize and legumes respond to N and P fertilizer?; and (iii) What are the residual effects of the sole crops, intercrops and fertilizer treatments on the productivity of a succeeding maize crop? We studied additive intercropping systems on eight farms in Babati, Tanzania: maize-long duration pigeonpea, maize-medium duration pigeonpea and maize-lablab, with separate sole crops, at three fertilizer rates: no fertilizer; 40 kg P ha⁻¹; and 90 kg N ha⁻¹ + 40 kg P ha⁻¹. Whereas P fertilizer was applied on maize and the legumes, the N fertilizer was only applied on maize. Maize and pigeonpea were sown simultaneously, while lablab was relay-planted one month later. N₂-fixation was quantified using the ¹⁵N natural abundance method. N₂-fixation differed among the legume species. Sole long-duration pigeonpea fixed significantly more N₂ (20–63 kg ha⁻¹ more) than all other cropping systems, corresponding to the greater shoot dry matter and N yield of this system. The combined N uptake of maize and legume in intercrops was consistently larger than that of sole maize or the legume. Application of fertilizer resulted in enhanced N uptake both in the current season (up to 40 kg N ha⁻¹ more) and in a succeeding maize crop (up to 71 kg N ha⁻¹ more). We observed positive associations between grain yield, dry matter production and total N uptake of a succeeding maize crop, and the N-fixed by legume species in the preceding season. Each kg of legume shoot N yield was associated with up to 14 kg ha⁻¹ extra grain yield, 29 kg ha⁻¹ extra dry matter and 0.5 kg ha⁻¹ extra total N uptake of the succeeding maize crop. Our results show that maize-legume intercrops enhance N-uptake. Furthermore, N₂-fixation by sole or intercropped legumes confers strong residual benefits on productivity of a succeeding maize crop.