Nitrogen mineralization from humic acid fractions in rice soils depends on degree of humification

Although the chemical nature of soil organic matter (SOM) is thought to affect the mineralization rate of N bound in SOM, little direct evidence exists for such effects. To test the hypothesis that the N mineralization rate is affected by the degree of SOM humification, we added equivalent amounts of humic acid (HA) N as either the labile mobile humic acid (MHA) fraction or the more humified calcium humate (CaHA) fraction to two lowland rice (Oryza sativa L.) soils, which were subsequently incubated under anaerobic conditions for 6 wk. The HA fractions had been chemically extracted from seven irrigated lowland rice soils from Vietnam and the Philippines. In both incubation soils, the amount of N mineralized from the added HA fractions decreased exponentially as the optical density of the HA at 465 nm (E4, index of humification) increased (R2 = 0.94-0.98). Mineralization of humic N was also strongly negatively associated with the amount of N or C contained in each HA fraction per kilogram of soil extracted, suggesting a more recalcitrant nature of the fractions in those soils where conditions allowed them to accumulate. Nitrogen mineralization was less for both the MHA (by 26%) and CaHA (by 41%) when incubated in the International Rice Research Institute (IRRI) soil compared with the Tanhoi soil. The most plausible explanation for this decrease is increased stabilization of the added HA by the relatively abundant Ca(+2) in the IRRI soil. We conclude that the degree of humification of the MHA and CaHA fractions plays an important role in governing the rate of N mineralization in lowland rice soils because (i) N mineralization from these fractions was affected by their degree of humification, and (ii) the fractions are a small but important component of total soil N.