TILLAGE INTENSITY AND CROP RESIDUE EFFECTS ON NITROGEN AND CARBON CYCLING IN A VERTISOL

The objective of this study was to examine the impact of tillage inten sity and crop residue on carbon (C) and nitrogen (N) cycling in a Vert isol. Soil samples (0-10- and 10-20-cm depth) were collected from a Ho uston Black soil (fine, montmorillonitic, thermic Udic Pellusterts) wi th three different levels of tillage intensity, varying from no tillag e to complete residue burial. The experiment was a split plot design w ith five replications. The main plots were three crop species [corn (Z ea mays L.), grain sorghum (Sorghum bicolor [L.] Moench), and soybean (Glycine max [L.] Merr.)] and the subplots were three levels of tillag e intensity (chisel tillage, reduced tillage, and no tillage). Total N , organic C, inorganic N, and C:N ratio were measured on soil samples as well as the potential C mineralization, N mineralization, C turnove r, and C:N mineralization ratio during a 7- and 30-d incubation. Chise l tillage reduced total N, C mineralization, and C turnover at the 0-1 0-cm depth compared to the other conservation tillage systems. Followi ng corn production, soil organic C increased and C mineralization and C turnover were decreased at the 10-20-cm depth compared to the other crop species. This data indicates that, in the short term, tillage sys tems may control soil organic C at the soil surface, while changes in plant rooting may control soil organic C storage at deeper soil depths in Texas Vertisols.