LONG-TERM EFFECTS OF TILLAGE AND FERTILIZATION ON SOIL ORGANIC-MATTERDYNAMICS

Understanding the effects of long-term management practices on soil C and N pools and activities is essential for sustaining soil productivi ty. The objectives of this study were to evaluate long-term and season al changes in soil organic C (SOC), soil microbial biomass C (SMBC) an d N (SMBN), and mineralizable C and N in continuous corn (Zea mays L.) under conventional tillage (CT), moldboard (MB), chisel (CH), minimum tillage (MT), and no-tillage (NT) with low (45 kg N, 10 P kg ha(-1)) and high (90 N kg, 10 P kg ha(-1)) N fertilization. An Orelia sandy cl ay loam (fine-loamy, mixed, hyperthermic Typic Ochraqualf) in south Te xas was sampled before corn planting in February, during pollination i n May, and following corn harvest in July. No-tillage and MT retained more corn residue C input as SOC and SMBC than the more intensive till age systems. Soil organic C, SMBC, SMBN, and mineralizable C and N wer e greatest in the surface 0 to 50 mm with NT and MT. Seasonal distribu tions of SMBC and mineralizable C were consistently greater in reduced -tillage systems (NT and MT), averaging 22 and 34% greater than plowed treatments at planting, 45 and 53% larger at pollination, and 36 and 34% higher at harvest, respectively, at a depth of 200 mm. The greater amount of crop residues remaining with MT and NT may have provided av ailable substrate for maintenance of the larger SMB pool and the highe r C and N mineralization in the 0- to 200-mm depth during the growing season. Higher N fertilization increased seasonal mineralizable C and N, but did not consistently affect SOC and SMB. Reduced tillage system s that promote surface residue accumulation provide an opportunity for increasing sequestration of C and mineralizable nutrients within SMB.