Abstract

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.

Key words: Intercropping, Groundnut, Pigeonpea, Nitrogen fixation, ¹⁵N natural abundance.