ABSTRACT

The objective of  this study was to investigate to what extent the potential C dynamics of soil organic matter (SOM) are related to the degree of ¹³C enrichment with increasing depth in soil profiles under permanent grassland. The evolution of the C content and the ¹³C natural abudance  (δ¹³C value) of SOM  has been 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 δ¹³C 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 δ¹³C value of SOM at the surface. The relation between the ¹³C enrichment and total organic C content at different depths in the profiles (down to 40cm depth in the loam and  a clay loam soil, down to 25 cm depth in the loamy sand soil) could be well described by the Rayleigh equation. The enrichment factors e associated with the Rayleigh approximation of the data, ranged  from –1.57‰ (clay loam soil),-1.64‰ (loamy sand soil) and –1.91‰ (loam soil) The potential C dynamics in four depth intervals from the profile (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  (drc_{c =} 0.22Δδ ¹³ C + 0.019, R² = 0.75, P<0.001, n=12) with the corresponding Δδ¹³C values (change of the δ¹³C value per depth increment). These results suggest that the Δδ¹³C 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)