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This paper describes the application of mid-infrared (MIR) spectroscopy and partial least-squares (PLS) analysis to predict the concentration of organic carbon fractions present in soil. The PLS calibrations were derived from a standard set of soils that had been analysed for total organic carbon (TOC), particulate organic carbon (POC), and charcoal carbon (char-C) using physical and chemical means. PLS calibration models from this standard set of soils allowed the prediction of TOC, POC, and char-C fractions with a coefficient of determination (R2) of measured v. predicted data ranging between 0.97 and 0.73. For the POC fraction, the coefficient of determination could be improved (R2=0.94) through the use of local calibration sets. The capacity to estimate soil fractions such as char-C rapidly and inexpensively makes this approach highly attractive for studies where large numbers of analyses are required. Inclusion of a set of soils from Kenya demonstrated the robustness of the method for total organic carbon and charcoal carbon prediction.

Rice‐growing (Oryza sativaL.) 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 NO3during 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 NO3was detectable following a wet season rice crop. During the subsequent dry season mungbean crop, 25 and 18 kg NO3‐N ha−1accumulated 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 NO3‐N ha−1were present in 1986 and 1987, respectively. Soil NO3in 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 NO3disappeared after flooding for the succeeding wet season rice. An incubation study with 15N‐labeled NO3revealed 99 to 100% disappearance of 15 µg NO3‐N cm−3in 9 d and 5% or less reduction of NO3‐N to NH4‐N and organic N. On a clay (Vertic Tropaquept) and silty clay (Lithic Mollic Tropaquept) in 1987, the accumulated soil NO3in 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.