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Chemical extractions of soil organic matter (SOM) have not been widely used to elucidate the dynamics of SOM in field settings, especially to address issues of nutrient cycling. To illustrate potential applications of chemical extractions to nutrient issues, this report reviews studies in which the extraction of SOM fractions was based on their binding to polyvalent soil cations. Radiocarbon ages and cycling rates of C and N indicated that the unbound mobile humic acid (MHA) fraction cycled faster than did the cationic-bound calcium humate (CaHA) fraction. Analyses for C, N, H, and O concentrations and for biochemical groups including carboxyl, phenol, amino, diester P, and free radicals demonstrated that the MHA consisted of more labile and less humified materials than did the CaHA. Quantities and chemical natures of both fractions responded to recent crop management, especially those of the MHA. Three case studies are described in which characterization of the MHA and CaHA contributed toward a process-level understanding of nutrient cycling: (i) a phenol accumulation in the MHA fraction was linked to an inhibition of N mineralization in tropical lowland soils under continuous rice (Oryza sativa L.) cropping, (ii) addition of the MHA to California cotton (Gossypium hirstum L.) soils in laboratory studies resulted in increased K availability and plant K uptake, reproducing the benefit of animal manure application in field conditions, and (iii) effects of straw management and winter flooding on N cycling in California rice soils were elucidated by studying a fraction comparable to the MHA fraction. This fractionation is well suited for studying N dynamics, especially in soils enriched in phenolic compounds, and it enabled the linkage of SOM function with chemical nature. It worked well in C-rich flooded soils but needs further evaluation in upland aerobic soils. Further insight into chemical structure and function relations might be achieved by its integration with physical and biological extractions. IN THE SURFACE LAYER of most soils, SOM contains .90% of the total N and S and up to 75% of the total P (Stevenson, 1994). The chemical nature of SOM is thought to influence the mineralization of these organic nutrients into plant-available forms because some chemical compounds are considered more resistant to microbial degradation than others (Stevenson and Cole, 1999). Few SOM studies, though, have demonstrated such a chemical structure–function relation for a range of soil types. Progress toward this goal has been slowed by the fact that the most advanced spectroscopic analyses for the chemical structure of SOM, especially nuclear magnetic resonance (NMR) spectroscopy, are best suited to purely organic samples. Their precision is diminished by the presence of paramagnetic elements and other mineral components. This liability impairs chemical characterization of whole soils and many of the SOM fractions that are recovered through physical extractions of soil. Chemical extractions for SOM fractions are advantageous in this respect by providing purely organic fractions that are free of mineral components. Several different chemical extractants have been used to obtain distinct SOM fractions, which can contain substantial proportions of total SOM (Stevenson, 1994). However, chemical fractionations also have disadvantages. One concern is that chemical fractionations are often designed to maximize the quantity of extracted material to facilitate subsequent analyses for its chemical composition. The intent to extract as much material as possible could lead to coextraction of material frommultiple SOM pools that differ in their chemical nature or function. For example, NaOH is a popular chemical extractant because it generally extracts large quantities of humic material (Hayes et al., 1975; Stevenson, 1994; Swift, 1996). Extraction of humic substances by NaOH is yet more efficient when the soil is initially washed in an acid solution, normally HCl, before the NaOH extraction. The excess protons replace soil cations that bridge SOM to mineral surfaces and that provide internal binding within the organic macromolecule, stabilizing SOM against microbial degradation. The acid wash is most important for removal of polyvalent cations, especially Ca (Mortensen, 1965; Stevenson, 1994; Swift, 1996), which are not completely removed from the soil by NaOH and are often the most abundant cations on soil mineral surfaces. Consequently, the acid wash generally results in much more efficient extraction of SOM, and it has become a common first step in extraction procedures involving NaOH. Because the material extracted by NaOH after the acid wash was by definition chemically stabilized, it might conceivably differ from humic material that was not stabilized by polyvalent cations. The bound material may include SOM that cycles only slowly: recalcitrant SOM pools cycle over decades to centuries (Jenkinson, 1981), while N and other SOM-bound nutrients cycle on a daily basis. Consequently, the properties of the polyvalent cation-bound material may not fully capture the effects of recent crop management on SOM, and its analysis might not fully elucidate the causes of perturbed nutrient cycling. Yet, the initial step of acid wash results in coextraction of this material with SOM that is not stabilized by polyvalent cations and is conceivably more USDA-ARS, National Soil Tilth Lab., 2150 Pammel Dr., Ames, IA 50011. Received 5 Apr. 2005. *Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 70:1013–1022 (2006). Symposium: Meaningful Pools in Determining Soil C and N Dynamics doi:10.2136/sssaj2005.0108 a Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: CaHA, calcium humate; DAEHA, directly alkaliextractable humic acid; MHA, mobile humic acid; NMR, nuclear magnetic resonance; RMBHA, reducible metal-bound humic acid; SOM, soil organic matter. R e p ro d u c e d fr o m S o il S c ie n c e S o c ie ty o f A m e ri c a J o u rn a l. P u b lis h e d b y S o il S c ie n c e S o c ie ty o f A m e ri c a . A ll c o p y ri g h ts re s e rv e d . 1013 Published online April 19, 2006