Mineralization of 14 C-Labelled Plant Residues and Microbial Assimilation in Conventional Tillage and No-Tillage Systems.

Tillage practices cause changes in soil C dynamics through their influence on soil biological properties and quality and quantity of soil organic matter. In this study, soils from two contrasting tillage systems were compared with respect to C mineralization of a common ¹⁴C-labelled substrate, and the efficiency of soil microbial community, within a long-term winter wheat-corn-soybean rotation. Intact core samples were collected from 0-5 cm for conventional tillage (CT) and from 0-5 and 10-15 cm for no-tillage (NT) in May 2007 before fertilizer application and corn planting. For each tillage and soil depth, 330.473 mg of ground ¹⁴C-labeled wheat residue with a specific activity of 0.084 kBq ¹⁴C mg^-1C was applied on the surface of cores, or was uniformly incorporated with the soil and repacked into the core. Cores were incubated at 25^oC for 86 days and cumulative carbon mineralization was then determined by trapping CO₂ in alkali. Microbial biomass C (MBC) was measured on four occasions. After 86 days from addition of ¹⁴C-labeled wheat residues, 7 to 11% of added residues were respired as CO₂-¹⁴C by soil microbial biomass. Greater cumulative respired CO₂-¹⁴C (10%) and potentially mineralizable ¹⁴C (¹⁴C₀) (20%) at 0-5 cm was measured in CT compared with NT. There was no significant residue placement or sampling depth effect or their interaction effects on cumulative respired CO₂-¹⁴C and ¹⁴C₀. Microbial biomass ¹⁴C accounted for 0.4 to 5% of the ¹⁴C derived from wheat residues. Incorporation of residues with soil resulted in 39% greater MB¹⁴C and 61% greater qCO₂-¹⁴C compared with surface application of residues averaged across sampling dates, tillage systems and sampling depths. The lack of a significant effect of tillage system on the metabolism of ¹⁴C-labelled residues or the size MB¹⁴C suggests that the metabolic capacity of the microbial community was not influenced by tillage system.