FALLOW AND SESBANIA EFFECTS ON RESPONSE OF TRANSPLANTED LOWLAND RICE TO UREA

Although lowland soils in Asia are frequently fallowed between rice (O ryza sativa L.) crops, little is known about the effects Of fallow man agement 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 ros trata Bremek. & Oberm. grown in situ for either 45 or 60 d as green ma nure (SR). Weeds and S. rostrata were incorporated after soil flooding on 23 June of each year, and rice was transplanted on 30 June 1989 an d 4 July 1990. Four rates of urea-N (0, 40, 80, and 120 kg N ha-1) wer e applied to rice following each pre-rice treatment. The efficiency of added N with respect to dry matter accumulation (kg dry wt. kg-1 N ap plied) was reduced following WF and SR compared with that following WF F at 45 d after transplanting (DAT), about 10 to 15 d after panicle in itiation. Rice recovered from the initial adverse effect of incorporat ed plant residue, and growth rate after 45 DAT was greater following S R and WF than that following WFF. Grain yield without applied urea was 0.9 Mg ha-1 higher 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) r esulted in a lower rice N concentration at 45 DAT. Incorporated S. ros trata and weeds continued to provide N to rice after 45 DAT. When no p lant residues were incorporated (WFF), the application of only one-thi rd 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 fo llowing all pre-rice treatments. Results suggest that optimal distribu tion of urea among application timings might depend on whether plant r esidues are incorporated before transplanting.