SOIL CARBON AND NITROGEN DYNAMICS AS AFFECTED BY LONG-TERM TILLAGE AND NITROGEN-FERTILIZATION

Quantifying seasonal dynamics of active soil C and N pools is importan t for understanding how production systems can be better managed to su stain long-term soil productivity especially in warm subhumid climates . Our objectives were to determine seasonal dynamics of inorganic soil N, potential C and N mineralization, soil microbial biomass C (SMBC), and the metabolic quotient of microbial biomass in continuous corn (Z ea mays L.) under conventional (CT), moldboard (MB), chisel (CH), mini mum tillage (MT), and no-tillage (NT) with low (45 kg N ha(-1)) and hi gh (90 kg N ha(-1)) N fertilization. An Orelia sandy clay loam (fine-l oamy, mixed, hyperthermic Typic Ochraqualf) in south Texas, United Sta tes, was sampled before corn planting in February, during pollination in May, and following harvest in July. Soil inorganic N, SMBC: and pot ential C and N mineralization were usually highest in soils under NT, whereas these characteristics were consistently lower throughout the g rowing season in soils receiving MB tillage. Nitrogen fertilization ha d little effect on soil inorganic N, SMBC, and potential C and N miner alization. The metabolic quotient of microbial biomass exhibited seaso nal patterns inverse to that of SMBC. Seasonal changes in SMBC, inorga nic N, and mineralizable C and N indicated the dependence of seasonal C and N dynamics on long-term substrate availability from crop residue s. Long-term reduced tillage increased soil organic matter (SOM), SMBC , inorganic N, and labile C and N pools as compared with plowed system s and may be more sustainable over the long term. Seasonal changes in active soil C and N pools were affected more by tillage than by N fert ilization in this subhumid climate.