The integrated use of legume green manure or legume residue with inorganic N fertilizer could offer rice (Oryza sativa L.) farmers an opportunity to reduce their expensive inorganic fertilizer inputs. A 2‐yr field experiment was conducted on an Andaqueptic Haplaquoll in the Philippines to determine the effect of cowpea [Vigna unguiculata (L.) Walp.] cropping systems on response of a subsequent rice crop to applied urea and effectiveness of several urea management practices for rice. The pre‐rice cropping systems including fallow, cowpea incorporated at the flowering stage as a green manure, and cowpea grown to maturity with either grain and pods removed or all aboveground vegetation removed prior to soil puddling for rice. The mean aboveground N content was 66 kg N ha−1 for green manure and 54 kg N ha−1 for mature cowpea without grain and pods. Rice yield responses following fallow and cowpea cropping with removal of all aboveground cowpea vegetation were similar, but mean rice grain yields were 0.9 and 0.7 Mg ha−1 higher following incorporation of green manure and residue, respectively. The green manure substituted for 34 and 54 kg urea‐N ha−1 in 1986 and 1987, respectively. Residue substituted for 44 and 50 kg urea‐N ha−1 in 1986 and 1987, respectively. Early application of urea (two‐thirds basal and one‐third at 5 d before panicle initiation) was superior to delayed application (one‐half at 14 d after transplanting and one‐half at 10 d after panicle initiation), even when supplemental N as green manure or residue was incorporated 15 d before transplanting. Cowpea grown for green manure or for grain, with incorporation of residues remaining after harvest, made similar positive contributions to rice yield, but growing cowpea to maturity offered the advantage of attaining approximately 1.0 Mg ha−1 cowpea grain production.

A field study was conducted on a clay soil (Andaqueptic Haplaquoll) in the Philippines to directly measure the evolution of (N2 + N20)-Z~N from 98 atom % ~SN-labeled urea broadcast at 29kg N ha -~ into 0.05-m-deep floodwater at 15 days after transplanting (DT) rice. The flux of (N2 + N20)-~SN during the 19 days following urea application never exceeded 28g N ha ~ day -~. The total recovery of (N 2 + N20)-~SN evolved from the field was only 0.51% of the applied N, whereas total gaseous tSN loss estimated from unrecovered ~SN in the ~SN balance was 41% of the applied N. Floodwater (nitrate + nitrite)-N in the 5 days following urea application never exceeded 0.14 g N m 3 or 0.3% of the applied N. Prior cropping ofcowpea [Vigna unguiculata (L.) Walp.] to flowering with subsequent incorporation of the green manure (dry matter = 2.5 Mg ha-~, C/N = 15) at 15 days before rice transplanting had no effect on fate of urea applied to rice at 15DT. The recovery of (N 2 + N20)-~SN and total ~SN loss during the 19 days following urea application were 0.46 and 40%, respectively. Direct recovery of evolved (N2 + N20)-~SN and total tSN loss from 27kg applied nitrate-N ha ~ were 20% and 53% during the same 19-day period. The failure of directly-recovered (N2 + N20)-~SN to match total ~SN loss from added nitrate-JSN might be due to entrapment of denitrification end products in soil or transport of gaseous end products to the atmosphere through rice plants. The rapid conversion of added nitrate-N to (N2 + N20)-N, the apparently sufficient water soluble soil organic C for denitrification (101 #g C g-~ in the top 0.15-m soil layer), and the low floodwater nitrate following urea application suggested that denitrification loss from urea was controlled by supply of nitrate rather than by availability of organic C.

Cowpea, Vigna unguiculata (L.) Walp., is well adapted to acid upland soil and can be grown for seed, green manure, and fodder production. A 2-yr field experiment was conducted on an Aeric Tropaqualf in the Philippines to determine the effect of cowpea management practice on the response of a subsequent upland rice crop to applied urea. Cowpea was grown to flowering and incorporated as a green manure or grown to maturity with either grain and pods removed or all aboveground vegetation removed before sowing rice. Cowpea green manure accumulated on average 68kgN ha -t, and aboveground residue after harvest of dry pods contained on average 46 kg N ha -1. Compared with a pre-rice fallow, cowpea green manure and residue increased grain yield of upland rice by 0.7 Mg ha-I when no urea was applied to rice. Green manure and residue substituted for 66 and 70 kg urea-N haon upland rice, respectively. In the absence of urea, green manure and residue increased total aboveground N in mature rice by 12 and 14 kg N ha ~, respectively. These increases corresponded to plant recoveries of 13% for applied green manure N and 24% for applied residue N. At 15 d after sowing rice (DAS), 33% of the added green manure N and 16% of the added residue N was recovered as soil (nitrate + ammonium)-N. At 30 DAS, the corresponding recoveries were 20 and 37% for green manure N and residue N, respectively. Cowpea cropping with removal of all aboveground cowpea vegetation slightly increased (p < 0.05) soil (nitrate + ammonium)-N at 15 DAS as compared with the pre-rice fallow, but it did not increase rice yield. Cowpea residue remaining after harvest of dry pods can be an effective N source for a subsequent upland rice crop.

Little is known about whether the high N losses from inorganic N fertilizers applied to lowland rice (Oryza sativa L.) are affected by the combined use of either legume green manure or residue with N fertilizers. Field experiments were conducted in 1986 and 1987 on an Andaqueptic Haplaquoll in the Philippines to determine the effect of cowpea [Vigna unguiculata (L.) Walp.] cropping systems before rice on the fate and use efficiency of ~SN-labeled urea and neem cake (Azadirachta indica Juss.) coated urea (NCU) applied to the subsequent transplanted lowland rice crop. The pre-rice cropping systems were fallow, cowpea incorporated at the flowering stage as a green manure, and cowpea grown to maturity with subsequent incorporation of residue remaining after grain and pod removal. The incorporated green manure contained 70 and 67 kg N ha-J in 1986 and 1987, respectively. The incorporated residue contained 54 and 49kgNha -l in 1986 and 1987, respectively. The unrecovered ~SN in the ~SN balances for 58 kg N ha-~ applied as urea or NCU ranged from 23 to 34% but was not affected by pre-rice cropping system. The partial pressure of ammonia, pNH3, and floodwater (nitrate + nitrite)-N following application of 29kgNha l as urea or NCU to 0.05-m-deep floodwater at 14 days after transplanting was not affected by pre-rice cropping system. In plots not fertilized with urea or NCU, green manure contributed an extra 12 and 26 kg N ha-~ to mature rice plants in 1986 and 1987, respectively. The corresponding contributions from residue were 19 and 23 kg N ha -1, respectively. Coating urea with 0.2 g neem cake per g urea had no effect on loss of urea-N in either year; however, it significantly increased grain yield (0.4Mgha -l) and total plant N (11 kgha -~) in 1987 but not in 1986.

A field experiment was conducted at the Division of Agronomy, Indian Agricultural Research Institute, New Delhi during 1983‐84 and 1984‐85 to study the effect of lentil residues on the yield and response of succeeding rice to nitrogen. Lentil residues benefitted the succeeding rice and gave longer and heavier panicles, more grains per panicle, higher 1000‐grain weight and higher grain and straw yield of rice as compared to fallow. Rice responded well to applied nitrogen both in the absence and presence of lentil residues. The effect of lentil root residues and root residues plus incorporation of lentil straw was equivalent to 11 and 33 kg N ha−1 when 60 kg N ha−1 was applied to rice; the corresponding values being 54 and 60 kg N ha−1 when 120 kg N ha−1 was applied to rice.