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

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.