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Published online 19 April 2006
Published in Soil Sci Soc Am J 70:1013-1022 (2006)
DOI: 10.2136/sssaj2005.0108
© 2006 Soil Science Society of America
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A Chemical Fractionation for Structure–Function Relations of Soil Organic Matter in Nutrient Cycling

Daniel C. Olk*

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


Figure 1
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  		Fig. 1. Changes in the {delta} 13C value of the mobile humic acid fraction for the surface layer of a Philippine soil under two crop rotations. DS is dry season, WS is wet season. Soil samples were collected at the midtillering crop growth stage and at shortly before harvest in each season from 1994 to 1996. Vertical bars at each data point represent standard errors.

 

Figure 2
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  		Fig. 2. Solid-state 13C nuclear magnetic resonance spectra for the mobile humic acid (MHA) fraction and calcium humate (Ca-HA) fraction, extracted from the surface layer of a calcareous California soil planted to a cotton-based rotation. Reprinted from Olk et al. (1995) with permission from Elsevier Publishers.

 

Figure 3
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  		Fig. 3. Solid-state nuclear magnetic resonance spectra of 13C bonded to 14N in the (a) mobile humic acid fraction and (b) whole soil organic matter for the surface layer of a triple-cropped rice soil in the Philippines. The N–C=O signal is indicative of amides, the N–Carom signal represents the aromatic N form anilide, and the N–CH signal is an aliphatic resonance of peptides. Figure 3a is reprinted from Schmidt-Rohr et al. (2004) with permission from the National Academy of Sciences of the USA. Figure 3b is from K. Schmidt-Rohr (2005, unpublished data).

 

Figure 4
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  		Fig. 4. Levels of 1 M NH4–extractable K in the surface layer of a California cotton soil incubated for 7 d following addition of four K rates and the (a) mobile humic acid (MHA) fraction or (b) calcium humate (CaHA) fraction. The humic acid subscript is the amount of humic acid added (g kg–1 soil). Standard error bars are smaller than the data points if not visible. Redrawn from Olk and Cassman (1995).

 




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