| Abstract: | Active fractions of soil carbon (C) and nitrogen (N) can undergo seasonal changes due to environmental and cultural factors, thereby influencing plant N availability and soil organic matter (SOM) conservation. Our objective was to determine the effect of tillage (conventional and none) on the seasonal dynamics of potential C and N mineralization, soil microbial biomass C (SMBC), specific respiratory activity of SMBC(SRAC), and inorganic soil N in a sorghum Sorghum bicolor (L.) Moench]-wheat (Triticum aestivum L.)/soybean Glycine max (L.) Merr.] rotation and in a wheat/soybean double crop. A Weswood silty clay loam (fine, mixed, thermic Fluventic Ustochrept) in southcentral Texas was sampled to 200 mm depth 57 times during a 2-yr period. Potential C mineralization was lowest (≈?2 to 3 g · m?2 · d?1) midway during the sorghum and soybean growing seasons and highest (≈?3 to 4 g · m?2 · d?1) at the end of the wheat growing season and following harvest of all crops. Addition of crop residues increased SMBC for one to three months. Potential N mineralization was coupled with potential C mineralization, SRAC, and changes in SMBC at most times, except during the wheat growing season and shortly after sorghum and soybean residue addition when increased N immobilization was probably caused by rhizodeposition and residues with low N concentration. Seasonal variation of inorganic soil N was 19 to 27%, of potential C and N mineralization and SRAC was 8 to 23%, and of SMBC was 7 to 10%. Soil under conventional tillage experienced greater seasonal variation in potential C and N mineralization, SRAC, bulk density, and water-filled pore space than under no tillage. High residue input with intensive cropping and surface placement of residues were necessary to increase the long-term level of active C and N properties of this thermic-region soil due to rapid turnover of C input. |
