Physical, Chemical and Molecule Fractions of Soil Organic Carbon in a Subtropical Long-Term Cotton Field As Influenced By Different Fertility Management.

Agricultural management such as tillage, crop rotation, cover crop and fertilization influence carbon (C) sequestration in soils. In this study, effects of different field practices on physical, chemical and molecular fractions of SOC were evaluated. Soil samples were collected from a long-term cotton rotation experiment established in a Pacolet fine sandy loam soil (fine, kaolinitic, thermic Typic Kanhapludults) located at the boundary between Coastal Plain and Southern Piedmont Plateau Physiographic regions of the eastern part of Alabama (32°N, 85°W), Auburn University. Field treatments include 1) Continuous cotton with no winter legume/no N (C); 2) Continuous cotton with winter legume (C+WL); 3) Cotton-corn rotation with winter legume (C-C+WL); 4) Cotton-corn rotation with winter legume and fertilizer N as ammonium nitrate at a rate of 134 kg N ha^-1yr^-1 (C-C+WL+N); and 5) Continuous cotton only with 134 kg N ha^-1yr^-1 ammonium nitrate (C+N). Total carbon (TC) and total nitrogen (TN) were analyzed based on combustion; Acid-hydrolysis Carbon (HC) was determined using a C analyzer following 6 N HCl extraction. Results showed that the crop rotation with cover crop (C-C+WL) and that with N fertilizer (C-C+WL+N) increased TC, dissolved organic C (DOC), HC, and TN in bulk soil samples as well as in different aggregate fractions. The treatment with winter cover crop and with N fertilizer also increased TC, TN and HC but at a less degree. It was found that that the proportion of soil macroaggregates (>250µm) were higher than microaggregates (52~250µm) and silt and clay (<53µm) fractions in all treatments. Overall, the results indicated that the cotton-corn rotation, legume cover crop and N fertilization have significant influence on different SOC pools.