Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 21, 2004
Publication Date: July 7, 2004
Citation: Nguyen, B., Olk, D.C., Cassman, K.G. 2004. Characterization of humic acid fractions improves estimates of nitrogen mineralization kinetics for lowland rice soils. Soil Science Society of America Journal. 68:1266-1277.
Interpretive Summary: Much of the plant-available nitrogen (N) in soil is derived not from applied fertilizers but from an immense pool of organic N that is bound in the soil organic matter and is released gradually through microbial activity into inorganic forms that can be taken up by the crop. However, the release patterns of organic N over time are not readily predictable. For tropical rice soils from Vietnam and the Philippines, the release patterns of organically bound N were determined during 84-day laboratory incubations. Parameters that mathematically described the release patterns were not predicted well by general properties of the rice soils, such as total N and carbon contents. To improve the predictive power, soil organic matter was chemically extracted from each soil as two fractions, and the quantities and chemical characteristics of these fractions were determined. The properties of the two fractions, in conjunction with the general soil properties, greatly improved the prediction of the parameters that described the release of organic N. The two humic fractions were chemically distinct in all soils, and their contributions to the release of soil organic N were related to their respective degrees of decomposition. Better understanding of the contribution of these organic matter fractions to short-term N cycling will improve the prediction of the soil N supply to rice crops, enabling more efficient application of N fertilizer.
Total soil carbon (C) and nitrogen (N) are poor predictors of N mineralization in tropical paddy rice soils. To better understand mineralization dynamics, two distinct humic acid (HA) fractions were chemically extracted from lowland rice soils of Vietnam and the Philippines. The influence of the properties of these HA fractions on N mineralization kinetics was evaluated in 84-day anaerobic incubations. Indicative of young organic matter, the mobile humic acid fraction had a smaller C:N ratio and optical density at 465 nm than did the calcium humate fraction, which was extracted after removal of polyvalent cations from the soil. Nitrogen content of each fraction was significantly correlated with total soil N and together represented 15-30% of total soil N. For 12 soils, 81% of the variation in the size of an initial, rapidly mineralizable N pool (N1) was explained through regression on soil organic C. In contrast, stepwise regressions using several whole soil variables explained only 27% of the variation in the rate constant for the rapidly mineralizable N pool and 36% of the variation in the rate constant of a slower phase. Prediction of the two rate constants was improved to 64% and 75%, respectively, and of N1 to 97% through their stepwise regressions against both soil and HA properties. All three mineralization parameters were negatively correlated with the degree of humification of the humic fractions. The degree of humification was positively associated with exchangeable polyvalent cations (calcium and magnesium) and negatively associated with exchangeable monovalent potassium, suggesting an influence of these soil cations on organic matter turnover. Better understanding of the contribution of these HA fractions to short-term N cycling will improve our ability to predict N supply in paddy rice soils.