Soil Organic Carbon, Microbial Biomass, and Mineralizable Carbon and Nitrogen in Sorghum

Abstract

Abstract Quantifying changes in soil microbial biomass and mineralizable C and N is important in understanding the dynamics of the active soil C and N pools. Our objectives were to quantify long‐term and seasonal changes in soil organic C (SOC), soil microbial biomass C (SMBC) and N (SMBN), and mineralizable C and N in continuous sorghum [ Sorghum bicolor (L.) Moench] and sorghum‐wheat ( Triticum aestivum L.)/soybean [ Glycine max (L.) Merr.] sequences under conventional tillage (CT) and no tillage (NT) with and without N fertilization. A Weswood silty clay loam (fine, mixed, thermic Fluventic Ustochrept) in south‐central Texas was sampled after planting in April, during flowering in June, and following sorghum harvest in August. More crop residue C input was retained as SOC and SMBC under NT than under CT. Soil organic C, SMBC, SMBN, and mineralizable C and N were greatest at a depth of 0 to 50 mm under NT. Mineralizable C and SMBC averaged 18% greater in rotation than in monoculture, probably due to greater C input via crop roots and residues in rotation and a shorter fallow. Mineralizable N with N fertilization was 36% greater in continuous sorghum but not different in rotated sorghum. Mineralizable C and SMBC increased an average of 5%, but mineralizable N decreased 41% from planting to flowering, probably due to rhizodeposition. From planting to post‐harvest, mineralizable C and SMBC increased 9% but mineralizable N decreased 15% due to crop residue addition. Soil N availability was reduced by plant additions in the short term but enhanced in the long term.