Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 7, 2006
Publication Date: May 1, 2006
Citation: Olk, D.C. 2006. A chemical fractionation for structure-function relations of soil organic matter in nutrient cycling. Soil Science Society of America Journal. 70(3) 1013-1022. Interpretive Summary: Large amounts of plant-essential nutrients in soil are chemically bound into soil organic matter as part of molecules that are not readily available for uptake by crops. To better understand the processes of the chemical binding and hence the later release of bound nutrients into forms that crops access, soil organic matter is often extracted for chemical analyses. Many extraction methods are used yet none is ideal. In this review I summarized the new knowledge on soil nutrient cycling that was gained through one extraction procedure that distinguishes two fractions of soil organic matter based on whether they are bound to soil calcium. I demonstrated the potentially broad relevance of this method for explaining soil nutrient cycling: the effects of farming strategies on the quantities and chemical natures of the two fractions contributed in separate studies to an understanding of nutrient cycling and associated yield problems in production of tropical rice and California cotton and rice. This report explained the strengths and weaknesses of the extraction method, which will help scientists identify the situations where it will be a useful technique. When used properly it will help scientists elucidate the interactions of nutrient cycling with soil organic matter, thereby leading to the identification of farming strategies that maximize the benefits of soil nutrients to crop production while minimizing environmental degradation and fertilizer waste.
Technical Abstract: Chemical processes contribute to the stabilization of soil organic matter (SOM). Chemical extractions of SOM are therefore capable of distinguishing labile humic material from recalcitrant material, an essential step for elucidating SOM dynamics. Yet their use in studying SOM dynamics has been sparse. To illustrate potential applications of chemical SOM extractions to nutrient cycling issues, this report reviews studies in which the extraction of SOM fractions was based on their binding to polyvalent soil cations. For several soil types and land uses, carbon (C) and nitrogen (N) isotopic data indicated that the unbound mobile humic acid (MHA) fraction cycled faster than did the cationic-bound calcium humate (CaHA) fraction. Quantities of both fractions were generally appreciable. Analyses for elemental concentrations and biochemical compounds demonstrated that the MHA consisted of younger, less humified materials than did the CaHA. Quantities and chemical natures of both fractions responded to recent crop management, especially those of the MHA. Characterization of the MHA and CaHA contributed toward a process-level understanding of nutrient cycling in (i) tropical lowland soils under continuous rice cropping, (ii) California cotton soils amended with animal manure, and (iii) California rice soils with different modes of straw management. This fractionation is well suited for studying N dynamics, especially in soils enriched in phenolic lignin residues. It worked well in C-rich submerged soils but needs further evaluation in soils with regular aeration and modest C content. Distinction of the MHA and CaHA fractions enabled the linkage of SOM function with its chemical nature. Further contributions may be achieved by its integration with biological and physical extractions.