Relationship between soil organic C degradability and the evolution of the δ13C signature in profiles under permanent grassland

Abstract The main objective of this research was to investigate to what extent the potential C dynamics of soil organic matter (SOM) are related to the degree of 13 C enrichment with increasing depth in soil profiles under permanent grassland. The evolution of the C content and the 13 C natural abundance ( δ 13 C value) of SOM were investigated in three soil profiles (0–40 cm depth) under permanent grassland of varying texture (a loamy sand, a loam and a clay loam soil). The δ 13 C value of the SOM showed a gradual increase with increasing depth and decreasing C content in the profiles, ranging from 1.9‰ (loamy sand soil), 2.9‰ (clay loam soil) and 4‰ (loam soil) in relation to the δ 13 C value of SOM at the surface. The relationship between the 13 C enrichment and total organic C content at different depths in the profiles (down to 40 cm depth in the loam and clay loam soil, down to 25 cm depth in the loamy sand soil) could be well described by the Rayleigh equation. The enrichment factors ε , associated with the Rayleigh approximation of the data, ranged from −1.57‰ (clay loam soil) to −1.64‰ (loamy sand soil) and −1.91‰ (loam soil). The potential C dynamics in four depth intervals from the profiles (0–10, 10–20, 20–30 and 30–40 cm depth) were determined by means of an incubation experiment. The C decomposition rate constants from the four sampling depths in the profiles showed a significant, positive correlation (y = 0.21x + 0.018, R 2 = 0.66, p < 0.005) with the corresponding Δ δ 13 C values (change of the δ 13 C value per depth increment). A better correlation was obtained when only the data from the upper 20 cm in the profiles (y = 0.21x + 0.019, R 2 = 0.78, p < 0.05) were considered. These results suggest that the Δ δ 13 C values in the surface layers of profiles under permanent grassland may serve as an indicator of the potential degradability or the stability of the SOM (in terms of C decomposition rate constants). Copyright © 2003 John Wiley & Sons, Ltd.