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Legumes form a very important component in Malawi’s cropping systems because of their roles in food security, income generation and soil fertility improvement through biological nitrogen fixation (BNF). They are commonly grown in various cropping systems including sole cropping, cereal-legume intercrops and legume-legume intercrops (also commonly referred to as “doubled-up”). However, information on BNF by the pigeon pea and cowpea under doubled-up system is scanty. Therefore, a study was conducted at two sites of Lilongwe and Dowa in the 2013/14 growing season, to quantify and compare the amounts of biologically fixed nitrogen in the three legume cropping systems. The experiments were laid out in a randomized complete block design and BNF was estimated using the modified nitrogen difference method. Results showed that there were significant differences (P < 0.05) in nodule numbers, nodule dry weights, and quantities of N2 fixed per unit area due to cropping systems’ effects at both sites. Sole cropped pigeon pea produced the highest N2 fixed (92.9 kg N ha-1) which was significantly higher by 86, 30 and 36% than the amounts fixed in the cowpea-maize intercrop (13.1 kg N ha-1), sole cowpea (62.5 kg N ha-1) and pigeon pea-maize intercrop (59.9 kg N ha-1), respectively, at the Dowa site. On the other hand, the total sum of the amounts of N2 fixed (82.9 kg N ha-1) by the component crops in the pigeon pea-cowpea “doubled-up” was comparable to that by sole cropped pigeon pea at the Dowa site. However, for Lilongwe site the doubled-up cropping system total amount of biologically fixed nitrogen (57.4 kg N ha-1) was significantly lower than that by the sole cropped pigeon pea (85.7 kg N ha-1) by 33%. From this study it can be noted that all three legume cropping systems can lead to substantial amounts of biologically fixed nitrogen, but their implementation should consider both combinations and environmental factors for specific sites.       Key words: Biological nitrogen fixation, cowpea; cropping systems, cereal-legume intercrop; legume-legume intercrop, pigeon pea.

Biological nitrogen fixation by pigeon pea and cowpea in the doubled-up and other cropping systems on the Luvisols of Central Malawi