Relationships among forest soil C isotopic composition, partitioning, and turnover times

The purpose of this research was to test the hypothesis that vertical enrichment of soil δ13C values is related to rates of soil C turnover in undisturbed, mature forest ecosystems. Soil C and N were measured at nine sites along an altitudinal gradient in the southern Appalachian Mountains (Tennessee and North Carolina, USA). Measurements indicated greater labile and total soil C stocks with increasing altitude. Laboratory incubations (3 days) of rewetted, air-dry soils indicated potential soil C mineralization (micrograms CO2 produced·g-1 soil C) declined with elevation. A principal component analysis indicated N availability increased with altitude. At each site, there was a significant relationship between δ13C and log-transformed C concentrations in the soil profile (30 cm deep). Enrichment factors (εe) from the Rayleigh equation were also equally useful for describing soil δ13C profiles at each site. Soil C partitioning and turnover times along the gradient were correlated with 13C-enrichment factors. Greater rates of change in δ13C through the soil profile were correlated with faster soil C turnover. Environmental factors, soil C partitioning, and the rate of vertical change in soil 13C abundance are interrelated such that δ13C measurements are a potential indicator of C dynamics in undisturbed forest soils.