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Nutrient Management & Soil & Plant Analysis

Impact of Soil Texture on the Distribution of Soil Organic Matter in Physical and Chemical Fractions

^a Natural Resource Ecology Lab., Colorado State Univ., Fort Collins, CO 80523
^b Dep. of Soil and Crop Sciences, Colorado State Univ., Fort Collins, CO 80523
^c Dep. of Plant Sciences, Univ. of California, Davis, CA 95616

^* Corresponding author (alainfplante{at}hotmail.com)

Previous research on the protection of soil organic C from decomposition suggests that soil texture affects soil C stocks. However, different pools of soil organic matter (SOM) might be differently related to soil texture. Our objective was to examine how soil texture differentially alters the distribution of organic C within physically and chemically defined pools of unprotected and protected SOM. We collected samples from two soil texture gradients where other variables influencing soil organic C content were held constant. One texture gradient (16–60% clay) was located near Stewart Valley, Saskatchewan, Canada and the other (25–50% clay) near Cygnet, OH. Soils were physically fractionated into coarse- and fine-particulate organic matter (POM), silt- and clay-sized particles within microaggregates, and easily dispersed silt- and clay-sized particles outside of microaggregates. Whole-soil organic C concentration was positively related to silt plus clay content at both sites. We found no relationship between soil texture and unprotected C (coarse- and fine-POM C). Biochemically protected C (nonhydrolyzable C) increased with increasing clay content in whole-soil samples, but the proportion of nonhydrolyzable C within silt- and clay-sized fractions was unchanged. As the amount of silt or clay increased, the amount of C stabilized within easily dispersed and microaggregate-associated silt or clay fractions decreased. Our results suggest that for a given level of C inputs, the relationship between mineral surface area and soil organic matter varies with soil texture for physically and biochemically protected C fractions. Because soil texture acts directly and indirectly on various protection mechanisms, it may not be a universal predictor of whole-soil C content.

Abbreviations: CPOM, coarse particulate organic matter > 250 µm in size • d-clay, easily dispersed clay-sized fraction • d-silt, easily dispersed silt-sized fraction • fPOM, fine particulate organic matter 53–250 µm in size • POM, particulate organic matter • µagg-clay, microaggregate-derived clay-sized fraction • µagg-silt, microaggregate-derived silt-sized fraction

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