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Symposium: Meaningful Pools in Determining Soil C and N Dynamics

A Chemical Fractionation for Structure–Function Relations of Soil Organic Matter in Nutrient Cycling

USDA-ARS, National Soil Tilth Lab., 2150 Pammel Dr., Ames, IA 50011

^* Corresponding author (olk{at}nstl.gov)

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 ¹³C and ¹⁵N 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.

Abbreviations: CaHA, calcium humate • DAEHA, directly alkali-extractable humic acid • MHA, mobile humic acid • NMR, nuclear magnetic resonance • RMBHA, reducible metal-bound humic acid • SOM, soil organic matter

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				D. C. Olk and E. G. Gregorich Overview of the Symposium Proceedings, "Meaningful Pools in Determining Soil Carbon and Nitrogen Dynamics" Soil Sci. Soc. Am. J., April 19, 2006; 70(3): 967 - 974. [Abstract] [Full Text] [PDF]