Impact of Soil Texture on the Distribution of Soil Organic Matter in Physical and Chemical Fractions Alain F. Plante,* Richard T. Conant, Catherine E. Stewart, Keith Paustian, and Johan Six ABSTRACT Previous research on the protection of soil organic C from decom- position suggests that soil texture affects soil C stocks. However, dif- ferent pools of soil organic matter (SOM) might be differently related to soil texture. Our objective was to examine how soil texture differ- entially alters the distribution of organic C within physically and chemi- cally defined pools of unprotected and protected SOM. We collected samples from two soil texture gradients where other variables influ- encing 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 (nonhydrolyz- able 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 uni- versal predictor of whole-soil C content. S OIL ORGANIC MATTER can be protected from decom- position and stabilized in soils by different mechan- isms, including chemical protection by association with mineral surfaces, physical protection by occlusion within aggregates, and biochemical protection by recalcitrance (Jastrow and Miller, 1997; Six et al., 2002; Krull et al., 2003). Chemical stabilization of organic molecules through mineral-organic matter binding is well established (Ladd et al., 1985; Gonzalez and Laird, 2003). Even labile organic material that would otherwise decompose quickly can be protected from decomposition by close association with silt and clay particles (Sørensen, 1972). Analyses synthe- sizing multiple studies suggest that stabilization capacity is dictated by soil silt and clay content and the surface area and reactivity of mineral soil particles (Hassink, 1997; Kiem et al., 2002; Kiem and Ko ¨ gel-Knabner, 2002; Six et al., 2002). Several studies have shown that soil tex- ture influences aggregation (Kemper and Koch, 1966; Chaney and Swift, 1984; Schlecht-Pietsch et al., 1994) such that increased clay contents were associated with in- creased aggregation or aggregate stability. In increasing soil aggregation, soil clay content indirectly affects soil C storage by occluding organic materials, making them inaccessible to degrading organisms and their enzymes. Therefore, soil texture (particularly soil clay content) plays direct and indirect roles in chemical and physical protec- tion mechanisms. Unprotected soil C is not intimately associated with soil mineral particles and is not occluded within aggre- gates. Unprotected soil C can be defined operationally as free particulate organic matter (POM), which includes rapidly metabolized plant and associated microbial car- bohydrates and more recalcitrant molecules derived from resistant plant materials and microbial decompo- sition products (Golchin et al., 1994; Six et al., 2001). Biochemically resistant C, defined operationally as or- ganic C resistant to acid hydrolysis (Leavitt et al., 1996), is an average of 1300 to 1500 yr older than whole-soil C (Paul et al., 1997; Paul et al., 2001). Even in the presence of cometabolites, specialized enzymes, and optimum en- vironmental conditions, decomposition of this material is slow, resulting in turnover times on the order of cen- turies to millennia. This pool of organic C is often as- sociated with silt and clay minerals (Paul and Clark, 1989) but is protected from decomposition primarily due to its complex chemical structure rather than by the mineral association. These observations suggest soil texture affects chemi- cal and physical protection of soil C stocks, whereas unprotected C and biochemically protected C should vary largely independent of soil texture. The principle of soil texture altering soil C levels and decomposition kinetics has been integrated into several biogeochemi- cal models (e.g., van Veen and Paul, 1981; Parton et al., 1987) but has not been fully evaluated across a con- trolled soil textural sequence. In addition, the means by which texture alters C dynamics in these models reflect only the conceptual chemical protection and do not en- compass physical protection mechanisms. There is an increasing demand for new models that incorporate measurable fractions rather than conceptual pools (e.g., Christensen, 1996; Arah and Gaunt, 2001), which has been met with varying degrees of success (e.g., Sohi et al., 2001; Skjemstad et al., 2004). The goal of this work A.F. Plante, R.T. Conant, C.E. Stewart, K. Paustian, and J. Six, Natu- ral Resource Ecology Lab., Colorado State Univ., Fort Collins, CO 80523; K. Paustin, Dep. of Soil and Crop Sciences, Colorado State Univ., Fort Collins, CO 80523; J. Six, Dep. of Plant Sciences, Univ. of California, Davis, CA 95616. Received 23 Nov. 2004. *Corresponding author (alainfplante@hotmail.com). Published in Soil Sci. Soc. Am. J. 70:287–296 (2006). Nutrient Management & Soil & Plant Analysis doi:10.2136/sssaj2004.0363 ª Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: CPOM, coarse particulate organic matter . 250 mm in size; d-clay, easily dispersed clay-sized fraction; d-silt, easily dispersed silt-sized fraction; fPOM, fine particulate organic matter 53–250 mm in size; POM, particulate organic matter; magg-clay, microaggregate- derived clay-sized fraction; magg-silt, microaggregate-derived silt-sized fraction. Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved. 287 Published online January 6, 2006