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The global nitrogen (N) cycle is markedly, and increasingly, influenced by anthropogenic inputs. A large unknown remains the quantity of biological N fixation (BNF) inputs derived from agriculture. This leads to major uncertainties in modeling reactive N interactions with climate change, and understanding N biogeochemical processes. Understanding N dynamics is central to enhancing productivity in cropping systems. To fill this gap, we used 15N natural abundance to quantify BNF and yield of groundnut and pigeonpea – on 18 on-farm sites in Ekwendeni, Northern Malawi. The study was conducted over the 2007/08 (2008) and 2008/09 (2009) cropping seasons under farmer management, for a range of edaphic environments. Overall, the soils are largely sandy with low to moderate organic carbon (0.12-1.56%), pH (5.5-6.5), and very low to moderately high inorganic P (3 to 85 mg kg-1). Intercropping was efficient at utilization of growth resources than sole cropping as evidences by land equivalent ration (LER) >1. The main drivers of BNF were plant density, inorganic P and interspecific competition. The proportion of N derived from the atmosphere (22-99%) was influenced by soil P status across seasons and crop species, but not by cropping system. The mean proportion of BNF was high in both groundnut (75%) and pigeonpea (76%). Total N fixed, on the other hand, differed with cropping system in the dry year, where intercropping was associated with low levels of N fixed by pigeonpea (15 kg N ha-1) compared to sole pigeonpea (32 kg N ha-1). A short rainfall season could not support biomass production of pigeon pea, and this has negative implications for relying on BNF to drive productivity on smallholder farms.

Biological nitrogen fixation and yield of pigeonpea and groundnut: Quantifying response on smallholder farms in northern Malawi