Rice‐growing ( Oryza sativa L.) soils in tropical Asia are frequently aerobic during the dry season but become waterlogged through irrigation or rainfall in the wet season. Field studies were conducted at three locations in the Philippines to measure the accumulation of soil NO 3 during a dry season mungbean [ Vigna radiata (L.) Wilczek] crop and subsequent weedy fallow prior to flooding the soil for wet season rice. On a silty clay (Vertic Tropaquept), no soil NO 3 was detectable following a wet season rice crop. During the subsequent dry season mungbean crop, 25 and 18 kg NO 3 ‐N ha −1 accumulated in 1986 and 1987, respectively, in the top 0.6‐m soil layer. After a subsequent 47‐d (1986) or 42‐d (1987) fallow period with no weed control, 52 and 77 kg soil NO 3 ‐N ha −1 were present in 1986 and 1987, respectively. Soil NO 3 in the top 0.6 m correlated inversely with aboveground weed dry matter ( r = −0.80**, significant at the 0.01 probability level) and aboveground weed N ( r = −0.84**). Soil NO 3 disappeared after flooding for the succeeding wet season rice. An incubation study with 15 N‐labeled NO 3 revealed 99 to 100% disappearance of 15 µg NO 3 ‐N cm −3 in 9 d and 5% or less reduction of NO 3 ‐N to NH 4 ‐N and organic N. On a clay (Vertic Tropaquept) and silty clay (Lithic Mollic Tropaquept) in 1987, the accumulated soil NO 3 in the top 0.6 m after dry season mungbean and weedy fallow were 91 and 39 kg N ha −1 , respectively. Nitrate accumulation during the dry season and subsequent loss by denitrification or leaching following flooding for wet season rice might represent a major mechanism of N loss in tropical rainfed and partially irrigated lowland rice environments.
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Buresh et al. (1989) studied this question.