Nitrogen Mineralization from Humic Acid Fractions in Rice Soils Depends on Degree of Humification

such effects, especially for chemically extracted SOM fractions. In this study, we explore the relationship beAlthough the chemical nature of soil organic matter (SOM) is tween the chemical nature of extracted HA fractions thought to affect the mineralization rate of N bound in SOM, little direct evidence exists for such effects. To test the hypothesis that the and their pattern of N mineralization when the fractions N mineralization rate is affected by the degree of SOM humification, were added to two test soils and incubated. The same we added equivalent amounts of humic acid (HA) N as either the two fractions were examined as were used by Nguyen labile mobile humic acid (MHA) fraction or the more humified calet al. (2004) to study soil N mineralization in lowland cium humate (CaHA) fraction to two lowland rice (Oryza sativa L.) rice: (i) the MHA, which is a young HA fraction that is soils, which were subsequently incubated under anaerobic conditions relatively N-rich and can be extracted from soil without for 6 wk. The HA fractions had been chemically extracted from seven prior removal of polyvalent cations, and the CaHA fracirrigated lowland rice soils from Vietnam and the Philippines. In both tion, which is somewhat older than the MHA and stabiincubation soils, the amount of N mineralized from the added HA lized by polyvalent cations (Olk et al., 1995, 1996). fractions decreased exponentially as the optical density of the HA The primary objective of these experiments was to at 465 nm (E4, index of humification) increased (R2 0.94–0.98). Mineralization of humic N was also strongly negatively associated test the hypothesis that the rate of N mineralization with the amount of N or C contained in each HA fraction per kilogram from these HA fractions was influenced by their degree of soil extracted, suggesting a more recalcitrant nature of the fractions of humification, which varies due to soil management in those soils where conditions allowed them to accumulate. Nitrogen practices and the environmental conditions in these lowmineralization was less for both the MHA (by 26%) and CaHA (by land rice fields (Olk et al., 1996, 1998; Mahieu et al., 41%) when incubated in the International Rice Research Institute 2002; Nguyen et al., 2004). A secondary objective was (IRRI) soil compared with the Tanhoi soil. The most plausible explato determine whether N mineralization from these fracnation for this decrease is increased stabilization of the added HA tions was related to other chemical properties of these by the relatively abundant Ca 2 in the IRRI soil. We conclude that fractions or properties of the soils from which the fracthe degree of humification of the MHA and CaHA fractions plays tions were extracted. 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 MATERIALS AND METHODS important component of total soil N. Selection of Soils The MHA and CaHA were extracted from seven of the 14 M of indigenous soil N is an important lowland rice soils used by Nguyen et al. (2004). To ensure a representative subset of the 14 soils, we hypothesized that issue in irrigated lowland rice systems because N potential N mineralization of the humic fractions was related mineralization from SOM provides a substantial portion to the MHA-N fraction, which is the ratio of the amount of of the total N taken up by a rice crop. In a companion N contained in the MHA per kilogram of extracted soil paper (Nguyen et al., 2004), we used stepwise regression (MHA-N) to the sum of MHA-N and CaHA-N. This hypotheto predict parameters of net N mineralization from ansis was based on the interpretation by Nguyen et al. (2004) aerobic incubation of 12 lowland rice soils. The predicand Olk et al. (1998) that in the better-aerated, low-N soils tion was improved when the independent variables of (such as the Guimba and Philippine Rice Institute [PhilRice] the regression included not only whole soil properties, soils), young humic materials such as the MHA are more such as total soil N, but also the properties and amounts readily decomposed than in soils that remain under anoxic of specific HA fractions in each soil. These results point conditions for most of the year, resulting in accumulation of a more recalcitrant form of young humic materials in the to the relevance of chemically extracted HA fractions to anoxic soils. Thus, the 14 soils were grouped by their MHA-N achieving better understanding of process-level controls fraction and two to three soils were selected from each of on soil N mineralization in lowland rice. three groups: (i) MHA-N fraction 0.50, (ii) MHA-N fraction Although the chemical nature of SOM is thought to ≈0.50, and (iii) MHA-N fraction 0.50. Of the seven selected affect the mineralization rate of SOM-bound N (Stevensoils, Guimba is from a farmer’s field in Central Luzon, Philipson and Cole, 1999), little direct evidence exists for pines, PhilRice is from the Long-Term Fertility Experiment (LTFE) at the Philippine Rice Research Institute in Central B.V. Nguyen, Dep. of Crop Sciences, Can Tho Univ., Can Tho, Vietnam; Luzon, IRRI is from the LTFE at the International Rice ReD.C. Olk, USDA-ARS, National Soil Tilth Laboratory, 2150 Pammel search Institute in Luzon, Cuulong is from a long-term experiDr., Ames, IA 50011; and K.G. Cassman, Dep. of Agronomy and ment in the Mekong Delta of Vietnam, and Cantho, Huongso, Horticulture, Univ. of Nebraska, Lincoln, NE 68583-0915. Journal Seand Tanhoi are from farmers’ fields in the Mekong Delta and ries No. 14403 from the Agricultural Research Division, Univ. of Nebraska. Received 22 Nov. 2003. *Corresponding author ([email protected]). Abbreviations: CaHA, calcium humates; E4, optical density at 465 nm; HA, humic acid; IRRI, International Rice Research Institute; Published in Soil Sci. Soc. Am. J. 68:1278–1284 (2004).  Soil Science Society of America LTFE, Long-term Fertility Experiment; MHA, mobile humic acids; PhilRice, Philippine Rice Research Institute; SOM, soil organic matter. 677 S. Segoe Rd., Madison, WI 53711 USA