Predicting Nitrogen Availability to Rice: I. Comparison of Methods for Determining Available Nitrogen to Rice from Field and Reservoir Soils

Abstract

Abstract Organic matter or available N in soil before or after incubation under waterlogged conditions each was used to predict soil N availability to ‘Nato’ rice ( Oriza sativa L.) plants in a greenhouse experiment. The 42 silt loams had much lower organic matter and available N contents than the 19 clays (or reservoir soils) used in the study. Soluble + extractable NH 4 + ‐N after 6 days incubation accounted for 91% of the variation in yield on the 19 clay (or reservoir) soils, but accounted for only 18% on the 42 silt loams. Initial NH 4 + ‐N, initial NO 3 − ‐N, NH 4 + ‐N production during 6 days incubation, and NH 4 + ‐N production during 6 to 12 days incubation, as independent variables in a multiple linear regression, were used to predict grain yield. Little was gained by including NO 3 − ‐N or 6 to 12 day NH 4 + ‐N production. Of the independent variables included, NH 4 + ‐N production during 0 to 6 days incubation alone accounted for the greatest percentage of yield variation but including initial NH 4 + ‐N together with 6 day NH 4 + ‐N production (also initial NH 4 + ‐N + that produced during 6 days) improved the prediction. Organic matter or methods which measure total N in soil alone were poor predictors of N availability to rice. Organic matter accounted for 41 % of the variation in yield on clay soils and < 1% on silt loams, whereas total N accounted for 56 and < 1% of the yield variation on the clays and silt loams, respectively.