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.V., Olk, D.C., Cassman, K.G. 2004. Nitrogen mineralization from humic acid fractions in rice soils depends on degree of humification. Soil Science Society of America Journal. 68:1278-1284.
Interpretive Summary: Much of the plant-available nitrogen (N) in soil is derived 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. The factors that control the release rates, though, are poorly understood. To determine whether the degree of aging, or decomposition, of the organic matter is one such factor, two organic matter fractions were extracted from seven irrigated lowland rice soils from Vietnam and the Philippines. Their chemical characteristics were correlated with the amount of inorganic N they released during 6-week incubations, and considerable variation existed among the seven extracted soils and between the two organic matter fractions. In general, more N was released from younger, less decomposed organic matter. Results indicate that (i) the chemical nature of organic matter can vary predictably among soils and cropping systems, and (ii) their chemical nature affects the release rate of organic N into inorganic forms. Acquired knowledge allows identification of soils where older organic matter is accumulating and hence where problems may occur with the release of organic N. It also allows scientists to better link basic chemical and biological processes in the soil to N availability. Better understanding of the controls of organic N release will improve the prediction of the soil N supply to rice crops, enabling rice farmers to more efficiently apply N fertilizer.
Although the chemical nature of soil organic matter (SOM) is thought to affect the mineralization rate of nitrogen (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 weeks. The HA fractions had been chemically extracted from seven irrigated lowland rice soils from Vietnam and the Philippines. In both incubation soils, the amounts of N mineralized from the added HA decreased as the optical density of the HA at 465 nm (E4) increased (R2=0.94 to 0.98), and E4 is a positive index of humification. Mineralization of humic N was also strongly negatively correlated with the amount of N or C contained in each HA fraction on a whole soil basis, suggesting that accumulation of these fractions in lowland rice soils is associated with more recalcitrant HA forms. Nitrogen mineralization was less for both the MHA (26%) and CaHA (41%) when incubated in the Philippine soil compared to the Vietnam 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 together these fractions represent 15-30% of total soil N.