One goal of soil C sequestration is to increase the mass of C stored in agricultural soils. Reducing soil disturbance, e.g., no-till management, facilitates soil fungal growth and results in higher C sequestration rates; however, the specific mechanisms associated with short-term plant residue C and N retention are less clear. We applied 13C- and 15N-enriched grain sorghum ( Sorghum bicolor) residue to no-till (NT) and conventional tillage (CT) soils, and measured the 13C and 15N retention in the soil and in aggregate fractions, along with soil microbial dynamics, during a growing season. The field site was located at Ashland Bottoms near Manhattan, Kansas. The added plant residue mineralized rapidly in both tillage systems, with similar decomposition kinetics, as indicated by 13C data. Mass balance calculations indicated that approximately 70% of the added 13C was mineralized to CO 2 by 40 days. The total Gram positive and Gram negative bacteria and fungal phospholipid fatty acids were higher under NT 0-5 cm during the most active period of residue mineralization compared with the CT 0-5 or 5-15 cm depths. No changes were observed in the NT 5-15 cm depth. The >1000-m aggregate size class retained the most 13C, regardless of tillage. The NT >1000-m aggregates retained more 15N at the end of the experiment than other NT and CT aggregates size classes. Data obtained indicate higher biological activity associated with NT soils than under CT, and increased retention of plant residue C and N in macroaggregates.
