Tillage Effects On the Distribution of Soil Organic Carbon and Nitrogen in Particle-Size Fractions.

The non-labile soil organic matter pool associated with clay particles is a vital reservoir for long-term C sequestration. Particle-size fraction concentrations of C and N were determined from the 0 to 10 cm soil depth of long-term experiments under continuous NT and conventional tillage in a Wickham sandy loam (fine-loamy, mixed, semi active, thermic Typic Hapludult), Delanco fine-sandy loam (fine-loamy, mixed, mesic, Aquic Hapludult), and Wedowee sandy clay loam (fine, kaolinitic, thermic Typic Kanhapludult) in North Carolina Coastal Plain (9 yr), Mountain (14 yr), and Piedmont (24 yr) locations, respectively. Significant treatment effects were most evident in the Piedmont location; where the C concentrations under NT and moldboard plow (MBP) were, respectively, 29.4 and 17.4 g/kg in fine clay; and 18.1 and 10.3 g/kg in silt+coarse clay fractions. The corresponding N concentrations were, respectively, 2.9 and 2.2 g/kg in fine clay under NT and 1.7 and 1.4 g/kg in silt+coarse clay under MBP. Erosive forces likely caused significant SOC losses in particle-size fractions under MBP relative to NT over 24 years in the Piedmont; whereas 9- and 14-yr tillage in the Coastal plain and Mountain, respectively, resulted in non-significant differences between CT and NT. The results demonstrate that a given tillage system’s duration can be a factor determining particle-size fraction C sequestration in the soil depth studied; and that NT can potentially increase C sequestration by increasing the C content associated with fine clay. The fractionation procedure used in this study can differentiate C and N contents in fine clay and silt+coarse clay fractions in diverse agroecosystems.