Characterization of Humic Acid Fractions Improves Estimates of Nitrogen Mineralization Kinetics for Lowland Rice Soils

Boone, 1994). Despite these efforts, no single fractionation method has been shown to cleanly distinguish Nitrogen mineralization in irrigated lowland rice (Oryza sativa L.) SOM fractions based on their contributions to soil N soils is poorly predicted by total soil C and N. To improve its prediction, two humic acid fractions were extracted with NaOH from lowland mineralization across a wide range of soil types and rice soils of Vietnam and the Philippines to evaluate their influence environments. Organic matter has a rich diversity of on N mineralization kinetics during 84-d anaerobic incubations. Indicorganic subunits and bond types, which are apparently ative of young organic matter, the mobile humic acid (MHA) fraction arranged in random patterns (MacCarthy, 2001), so it had a smaller C/N ratio and optical density at 465 nm (E4) than is possible that no single fractionation method can adedid the calcium humate (CaHA) fraction, which was extracted after quately distinguish labile SOM from recalcitrant SOM removal of polyvalent cations from the soil. Nitrogen contents of both in all soils. fractions combined represented 15 to 30% of total soil N. Nitrogen Whether chemical extractions of SOM can provide mineralization followed a two-pool firstand zero-order kinetic model fractions that differ clearly in their turnover rates and during the incubations. More than 80% of the variation in the size contributions to nutrient cycling has been questioned of a rapidly mineralizable N pool (N1) was explained by soil organic C. However, stepwise regressions using whole soil variables explained by several scientists (Ladd et al., 1977; Cadisch et al., only 27% of the variation in the rate constant for the rapidly mineraliz1996). Yet stabilization of SOM by chemical agents is able N pool and 36% of the variation in the rate constant for a slower well known and seems likely to influence the cycling steady-state phase. Prediction of the two rate constants was improved of SOM-bound nutrients. The longevity of chemically to 64 and 75%, respectively, and of N1 to 97% by including MHA and stabilized SOM (Jenkinson and Rayner, 1977; Campbell CaHA properties in the stepwise regressions. All three mineralization et al., 1967) has been attributed to binding with exparameters were negatively correlated with the degree of humification changeable or structural cations (Martin and Haider, in these humic fractions. The degree of humification was positively 1986; Anderson, 1979), most commonly Ca 2, Fe 3, and associated with exchangeable polyvalent cations (Ca 2 and Mg 2) and Al 3. Calcium in particular has been found to stabilize negatively associated with exchangeable K , suggesting an influence organic materials against microbial degradation under of exchangeable cations on the turnover of young organic matter. We conclude that better understanding the contribution of these humic laboratory, greenhouse, and field conditions (Baldock fractions to short-term N cycling will improve our ability to predict and Skjemstad, 2000; Bruckert et al., 1994; Kononova, N supply in lowland rice soils. 1961). Accordingly, in some SOM studies soil was extracted first with NaOH to recover the more loosely bound A proportion of N taken up by crops is MHA fraction, and then acid-washed to displace polyvaderived from mineralization of soil organic matter lent cations before a second NaOH extraction to recover (SOM). Although reliable prediction of N mineralizathe CaHA fraction. Carbon-14 dating found the CaHA tion rates would enable more efficient use of N fertilizer fraction to be older than the MHA and in cases considerwhile ensuring an adequate crop N supply, development ably older (Campbell et al., 1967; Olk et al., 1995, 1996). of accurate prediction methods has proven to be an In studies of tropical lowland rice soils, the MHA was elusive goal. For example, total soil N has been shown less humified than the CaHA in its chemical forms of to be a poor predictor of actual soil N availability to C (Olk et al., 1995, 1996, 1998), N (Mahieu et al., 2000a), the rice crop in irrigated lowland rice systems of the and P (Mahieu et al., 2000b), and in the concentrations tropics (Cassman et al., 1996a). of phenolic moieties, functional groups, and free radicals One approach to investigating soil N mineralization associated with humification (Olk et al., 1999, 2000). has been to extract labile SOM fractions, quantify their Changes in recent crop management regime that rerelative contributions to N mineralization, and validate sulted in increased SOM content caused a greater prothese contributions under field conditions. A number portional increase in the content of MHA than CaHA, of soil fractionations have been evaluated, including and both fractions increased more than did total soil C those based on particle size (Christensen, 1992), aggreor N (Olk et al., 1996, 1998). During one season in a gation (Feller and Beare, 1997), and densitometry lowland rice field, 15N-labeled urea N cycled into and out of the MHA and CaHA faster than for bulk SOM B.V. Nguyen, Dep. of Crop Sciences, Can Tho Univ., Can Tho, Vietnam; (Olk and Cassman, 2002). Similar trends in N cycling D.C. Olk, USDA-ARS, National Soil Tilth Laboratory, 2150 Pammel were found in California rice soils by Devêvre and HorDr., Ames, IA 50011; K.G. Cassman, Dep. of Agronomy and Horticulwath (2001) and Bird et al. (2003), who also fractionated ture, Univ. of Nebraska, Lincoln, NE 68583-0915. Journal Series No. 14347 from the Agricultural Research Division, Univ. of Nebraska. Abbreviations: BIARC, Bicol Integrated Agricultural Research CenAReceived 22 Nov. 2003. *Corresponding author ([email protected]). ter; CaHA, calcium humates; E4, optical density at 465 nm; HA, humic acid; IRRI, International Rice Research Institute; LTFE, Long-Term Published in Soil Sci. Soc. Am. J. 68:1266–1277 (2004).  Soil Science Society of America Fertility Experiment; MHA, mobile humic acids; PhilRice, Philippine Rice Research Institute Research; SOM, soil organic matter. 677 S. Segoe Rd., Madison, WI 53711 USA