Short-term fates of carbon-13-depleted cowpea shoots in no-till and standard tillage soils

The fate of C from C-13-depleted cowpea [Vigna unguiculata (L.) Walp. ssp. unguiculata] shoots was examined to assess factors affecting residue C dynamics in soil cores taken from plots under no-till (NT) vs. standard tillage (ST). Intact soil cores were incubated in the greenhouse after removing them from field plots, and then C-13-depleted residue was incorporated at 0- to 15-cm depth in ST cores, or laid on the surface of NT cores. Cores were taken from both the planted and unplanted zones in NT, and from an intermediate position in the ST system, where residues had always been evenly incorporated. Cores without soil were used as a control. After 31 d, higher C from the residue was observed as microbial biomass C (MBC) and more nematodes were found in the 0- to 15-cm layer in ST than in NT cores. For NT treatments, after 31 d, higher C from residue was observed in microbial biomass in the 0- to 15-cm layer in cores from planted than in those from the unplanted zones. Despite the low assimilation of newly added residue C in NT soil, similar amounts of CO2 were emitted from ST and NT, probably due to high activity of microbes in the phyllosphere of the residue in NT. In the short term, ST more effectively assimilated shoot residue into soil microbial and nematode C than NT. These results were unexpected given the typical increase in soil C that is observed in the surface layer of NT systems, and suggest that lack of disturbance, changes in microenvironment, or the composition of soil biota in NT must enhance C storage from shoot residue during longer time periods than measured here.