| Abstrakt: |
Although lowland soils in Asia are frequently fallowed between rice (Oryza sativaL.) crops, little is known about the effects of fallow management on N cycling and growth of a subsequent lowland rice crop. A 2‐yr field study was conducted in the Philippines on a Typic Tropaquept to determine the interactive effects of pre‐rice land management and urea fertilization on rice growth, N uptake, and response to applied N. Management treatments before transplanting wet‐season rice were (i) weedy fallow (WF), (ii) weed‐free fallow (WFF), and (iii) Sesbania rostrataBremek. & Oberm. grown in situ for either 45 or 60 d as green manure (SR). Weeds and S. rostratawere incorporated after soil flooding on 23 June of each year, and rice was transplanted on 30 June 1989 and 4 July 1990. Four rates of urea‐N (0, 40, 80, and 120 kg N ha‐1) were applied to rice following each pre‐rice treatment. The efficiency of added N with respect to dry matter accumulation (kg dry wt. kg−1N applied) was reduced following WF and SR compared with that following WFF at 45 d after transplanting (DAT), about 10 to 15 d after panicle initiation. Rice recovered from the initial adverse effect of incorporated plant residue, and growth rate after 45 DAT was greater following SR and WF than that following WFF. Grain yield without applied urea was 0.9 Mg ha‐1higher following WF than that following WFF. Incorporated weed‐N (35 kg N ha‐1) substituted for 40 kg urea‐N ha‐1. High initial rice growth rate in the absence of incorporated plant residue (WFF) resulted in a lower rice N concentration at 45 DAT. Incorporated S. rostrataand weeds continued to provide N to rice after 45 DAT. When no plant residues were incorporated (WFF), the application of only one‐third of the total urea‐N at panicle initiation was insufficient to meet the N requirements of rice. Basal urea‐N (two‐thirds of the total urea‐N) failed to increase rice dry weight or N concentration at 21 DAT following all pre‐rice treatments. Results suggest that optimal distribution of urea among application timings might depend on whether plant residues are incorporated before transplanting. |
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