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Nitrogen dynamics of grain legume-weedy fallow-flooded rice sequences in the tropics

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Dry-season (DS) grain legume-weedy fallow-wet-season (WS) flooded rice is a common cropping sequence in the rainfed lowlands of tropical Asia. To better manage N in this cropping system we need to understand N dynamics and balances as influenced by the aerobic-anaerobic soil aeration sequence legume cropping biological N2 fixation (BNF) and recycling of legume residues. To understand N dynamics under a range of N derived from BNF (15N-estimated) harvested in pods and left in residues we conducted a 2-yr experiment on a Philippine Alfisol using cowpea [Vigna unguiculata (L.) Walp.] mungbean [V. radiata (L.) Wilcz.] nodulating and nonnodulating soybean [Glycine max (L.) Merr.] and weeds. The main portion of soil mineral N (0 to 60 cm) was NO3 in the dry season and NH4 in the wet season. The sum of soil NO3 and soil N uptake at legume harvest exceeded the decrease in soil NO3 from legume seeding to harvest by 81 kg ha -1 indicating the continued production and legume uptake of soil NO3. The large differences in total N of legumes (46 to 238 kg N ha-1) however were associated with differences in N derived from BNF (0 to 176 kg N ha-1). When pod N was excluded legume N balance was in most cases negative. The average soil N depletion was 40 kg ha-1 from nonnodulating soybean compared with 8 kg ha-1 from N2-fixing legumes. In terms of WS rice grain and N yields legume cropping did not differ from weedy fallowing despite greater (by up to 46 kg N

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