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 active C and N pools of soil organic matter (SOM) by altering the timing, placement, quantity, and quality of crop root and residue input, as well as nutrient status and environmental conditions (i.e. soil temperature and water content). Our objectives were to quantify seasonal changes in soil microbial biomass (SMB) and mineralizable C and N in continuous wheat (Triticum aestivum L.), continuous wheat-soybean [Glycine max (L.) Merr.], and wheat-soybean-sorghum [Sorghum bicolor (L.) Moench.] sequences under conventional tillage (CT) and no tillage (NT) with or without N fertilization. Soil classified as a Weswood silty clay loam (fine, mixed, thermic Fluventic Ustochrept) located in southcentral Texas was sampled shortly after planting, during flowering, and following harvest of wheat. Soil microbial biomass C (SMBC) increased by 18% and mineralizable C increased by 37% from planting to flowering when averaged across crop sequence, tillage, and N fertilization regimes. At harvest, SMBC and mineralizable C had returned to the amount at planting in all crop sequences, except in continuous wheat, in which decomposition proceeded without C input during the long fallow. Mineralizable 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, 50 to 125, and 125 to 200 mm, respectively. The greater increase in mineralizable C near the surface may be related to the abundance of crop roots, rhizosphere products, and more optimal air-filled porosity. With 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 mineralizable C at flowering, indicating greater immobilization of N at flowering. Seasonal inputs of crop roots, rhizosphere products, and crop residues significantly altered SMB and mineralizable C and N of this soil, illustrating the dependence of N dynamics on short-term C inputs. 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 (iii) N fertilization for the quantity and quality of substrates. These seasonal changes can alter N availability and conservation.