SEASONAL-CHANGES IN SOIL MICROBIAL BIOMASS AND MINERALIZABLE C AND N IN WHEAT MANAGEMENT-SYSTEMS

Crop management strategies can affect the short-term dynamics of the a ctive C and N pools of soil organic matter (SOM) by altering the timin g, placement, quantity, and quality of crop root and residue input, as well as nutrient status and environmental conditions (i.e. soil tempe rature and water content). Our objectives were to quantify seasonal ch anges in soil microbial biomass (SMB) and mineralizable C and N in con tinuous wheat (Triticum aestivum L.), continuous wheat-soybean [Glycin e max (L.) Merr.], and wheat-soybean-sorghum [Sorghum bicolor (L.) Moe nch.] sequences under conventional tillage (CT) and no tillage (NT) wi th or without N fertilization. Soil classified as a Weswood silty clay loam (fine, mixed, thermic Fluventic Ustochrept) located in southcent ral Texas was sampled shortly after planting, during flowering, and fo llowing harvest of wheat. Soil microbial biomass C (SMBC) increased by 18% and mineralizable C increased by 37% from planting to flowering w hen averaged across crop sequence, tillage, and N fertilization regime s. At harvest, SMBC and mineralizable C had returned to the amount at planting in all crop sequences, except in continuous wheat, in which d ecomposition proceeded without C input during the long fallow. Mineral izable C was 64, 28, and 15% greater at flowering compared to planting under NT and 45, 38, and 29% greater under CT at depths of 0 to 50, 5 0 to 125, and 125 to 200 mm, respectively. The greater increase in min eralizable C near the surface may be related to the abundance of crop roots, rhizosphere products, and more optimal air-filled porosity. Wit h N fertilization, mineralizable N followed the same seasonal pattern as SMBC and mineralizable C. Without N fertilization, mineralizable N did not change during the growing season, despite increased SMBC and m ineralizable C at flowering, indicating greater immobilization of N at flowering. Seasonal inputs of crop roots, rhizosphere products, and c rop residues significantly altered SMB and mineralizable C and N of th is soil, illustrating the dependence of N dynamics on short-term C inp uts. Seasonal changes in the active C and N pools of SOM depended upon (i) crop sequence for the quantity, quality, and frequency of C input , (ii) tillage for the depth distribution of added substrates, and (ii i) N fertilization for the quantity and quality of substrates. These s easonal changes can alter N availability and conservation.