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.