Long-term effects of tillage and fertilization on soil organic matter dynamics

Understanding the effects of long-term management practices on soil C and N pools and activities is essential for sustaining soil productivity. The objectives of this study were to evaluate long-term and seasonal changes in soil organic C (SOC), soil microbial biomass C (SMBC) and 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 clay loam (fine-loamy, mixed, hyperthermic Typic Ochraqualf) in south Texas was sampled before corn planting in February, during pollination in 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 tillage systems. Soil organic C, SMBC, SMBN, and mineralizable C and N were greatest in the surface 0 to 50 mm with NT and MT. Seasonal distributions 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 available substrate for maintenance of the larger SMB pool and the higher 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 systems that promote surface residue accumulation provide an opportunity for increasing sequestration of C and mineralizable nutrients within SMB.