Tillage impacts on microbial biomass and soil carbon and nitrogen dynamics of corn and cotton rotations

Long-term no tillage (NT) may enhance soil C sequestration and alter soil C and N dynamics. The objectives of this study were to investigate the impacts of tillage on soil C and N sequestration and microbial C and N dynamics of corn (Zea mays L.) and cotton (Gossypium hirsutum L.) cropping sequences after 20 years of management. Tillage regimes included conventional tillage (CT), moldboard plow (MP), minimum tillage (MT), and NT. No tillage increased soil organic carbon (SOC) and nitrogen (SON) concentrations in surface soil (0-2.5 cm) for cotton but not for corn. Few tillage effects on SOC and SON were observed in subsurface soils. For corn, SOC and SON were 11 and 21% higher under NT than other tillage regimes at 0-2.5 cm, but were 22 and 12% lower under NT from 2.5 to 20 cm. Averaged between depths, SOC and SON for cotton were 8 and 7% greater under NT than CT, while NT and MT had 24 and 43% greater SOC and SON than MP. Soil organic C and SON were significantly greater for com than cotton, but this did not result in greater microbial biomass and mineralizable C and N than for cotton. Microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN) were often highest under NT and MT in surface soils, but few tillage impacts were observed at 2.5-20 cm. Mineralizable C and N were highest under NT and MT in surface soils for corn and cotton, and in subsurface soils for cotton. Even though SOC and SON were greater for com than cotton, cotton exhibited greater soil mineralizable C and N under NT and MT than com, especially in subsurface soils. These results indicate a greater potential supply of N for the cotton than com crop during the growing season. Increased SOM content in surface soils under reduced tillage may increase N mineralization and the nutrient supply to crops, but the potential of these soils for C and N sequestration appeared limited. (c) 2004 Elsevier B.V. All rights reserved.