Soil carbon dynamics in a nitrogen-enriched grassland

Soil organic carbon (SOC) dynamics were investigated in a grassland where 13 years of N fertilization shifted vegetation from C₄ to C ₃ species. The fraction of organic matter (OM) input retained in the soil ranged from 17 to 28%, and was unaffected by litter quality (C:N ratio). Isotopic composition (δ13C) of soil ranged from −19.7 to −24.9‰; there was no isotopic fractionation during formation of SOC from plant inputs. δ13C of active and slow SOC pools described by the CENTURY model (δ13C _active and δ13C_slow, respectively) were estimated from soil incubation δ13C_slow was strongly correlated to δ13C of OM inputs during 13 years. The size of the active pool ranged from 1.9 to 13.6% of total SOC and was related to plant growth and species diversity. δ13C _active, did not reflect δ13C of recent OM inputs. Any interpretation of δ13C_active, must consider the plant and physical conditions when sampling occurred.