Tillage and Crop Effects on Seasonal Soil Carbon and Nitrogen Dynamics

Abstract

Abstract Seasonal changes in the active fractions of soil C and N are important in understanding mineralization‐immobilization processes that influence soil N availability. Our objective was to determine the effect of tillage (conventional tillage [CT] and no tillage [NT]) in monoculture sorghum [ Sorghum bicolor (L.) Moench], wheat ( Triticum aestivum L.), and soybean [ Glycine max (L.) Merr.] on the seasonal dynamics of in situ inorganic soil N, potential C and N mineralization, and soil microbial biomass C (SMBC). A Weswood silty clay loam (fine‐silty, mixed, thermic Fluventic Ustochrept) located in south‐central Texas was sampled to a depth of 200 mm 57 times during a 2‐yr period. In situ soil NO 3 − accumulated at a rate of ≈0.03 g N m −2 d −1 following harvest when temperature and moisture were adequate. Potential soil C mineralization was 35, 39, and 53% greater under NT than under CT when averaged across months in wheat, sorghum, and soybean, respectively. However, following harvest of each crop, potential soil C mineralization increased at a faster rate under CT than under NT and differences between tillage regimes were reduced for several months, which caused temporary N immobilization in CT. Periods of net N immobilization at the end of the growing season and beginning of the fallow period were often associated with an increasing SMBC pool. Large seasonal changes in active soil C and N pools could be related to rhizodeposition, crop residue addition, and the long fallow period. The type, placement, and timing of residue input significantly altered the seasonal dynamics of the active soil C and N pools and their effect on soil N availability.