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Nitrogen Dynamics of Grain Legume-Weedy Fallow-Flooded Rice Sequences in the Tropics

(IRRI-NifTAL Collaborative Program)
IRRI, P.O. Box 933, Manila, Phillippines
(IRRI), ICRAF Southeast Asia Regional Programme, Jalan Gunung Batu 5, P.O. Box 161 Bogor 16001, Indonesia
IRRI, P.O. Box 933, Manila, Philippines

^* Corresponding author (Email: in % t.george{at}cgnet.com).

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^–l 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 ha^–1) quantities of legume residue N in some instances. Large amounts of legume residues, however, were associated with reduced legume grain yields, thus decreasing the harvestable grain N output. Fertilizer N, compared with residue N, had a greater effect on WS rice grain and N yields. The use of legumes in lowland rice-based cropping systems must maximize harvestable N while effectively using soil, BNF, and applied N sources.

Received for publication March 31, 1994.