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Nitrogen mineralization in irrigated lowland rice (Oryza sativa L.) soils is poorly predicted by total soil C and N. To improve its prediction, two humic acid fractions were extracted with NaOH from lowland rice soils of Vietnam and the Philippines to evaluate their influence on N mineralization kinetics during 84-d anaerobic incubations. Indicative of young organic matter, the mobile humic acid (MHA) fraction had a smaller C/N ratio and optical density at 465 nm (E4) than did the calcium humate (CaHA) fraction, which was extracted after removal of polyvalent cations from the soil. Nitrogen contents of both fractions combined represented 15 to 30% of total soil N. Nitrogen mineralization followed a two-pool first- and zero-order kinetic model during the incubations. More than 80% of the variation in the size of a rapidly mineralizable N pool (N1) was explained by soil organic C. However, stepwise regressions using 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 for a slower steady-state phase. Prediction of the two rate constants was improved to 64 and 75%, respectively, and of N1 to 97% by including MHA and CaHA properties in the stepwise regressions. All three mineralization parameters were negatively correlated with the degree of humification in these humic fractions. The degree of humification was positively associated with exchangeable polyvalent cations (Ca+2 and Mg+2) and negatively associated with exchangeable K+, suggesting an influence of exchangeable cations on the turnover of young organic matter. We conclude that better understanding the contribution of these humic fractions to short-term N cycling will improve our ability to predict N supply in lowland rice soils.