TILLAGE INTENSITY AND FERTILITY LEVEL EFFECTS ON NITROGEN AND CARBON CYCLING IN A VERTISOL

Because of erosion problems, an effort has been undertaken to evaluate the effect of tillage intensity on carbon (C) and nitrogen (N) cyclin g on a vertisol. Soil samples at 0-10, 10-20, and 20-30 cm depth were collected from a split plot experiment with five different levels of t illage intensity on Houston Black soil (fine, montmorillonitic, thermi c Udic Pellusterts). The experiment was a split plot design with 5 rep lications. The main plots were chisel tillage, reduced tillage, row ti llage, strip tillage, and no tillage. The subplots were soil fertility levels with either high or low fertilizer application rate. Total N, total phosphorus (P), organic C, inorganic N, and C:N ratio were measu red on soil samples as well as the potential C mineralization, N miner alization, C turnover, and C:N mineralization ratio during a 30 d incu bation. Total P and organic C in soil were increased, with 0.9 and 0.8 kg P ha(-1) and 20.6 and 20.0 kg C ha(-1), for high and low soil fert ility, respectively. Fertilizer application had no effect on either to tal N at the 0-10 cm depth, or on soil nutrient status below 10 cm. Po tential soil N mineralization was decreased at the 0-10 cm depth and i ncreased at the 20-30 cm depth by the high fertilizer treatment. Chise l tillage decreased total N and P in the 0-10 cm depth, with 1.4 and 1 .6 kg N ha(-1) and 0.8 and 0.9 kg P ha(-1). However, chisel tillage in creased total N and p at the 10-20 cm depth, with 1.3 and 1.2 kg N ha( -1), and 0.72 and 0.66 kg P ha(-1) for chisel tillage and no tillage, respectively. Tillage intensity increased C mineralization and C turno ver, but reduced N mineralization at the 0-10 cm depth. The results in dicate that intensively tilled soil had a greater capacity for C miner alization and for reductions in soil organic C levels compared to less intensively tilled systems.