Isotope fractionation and 13C enrichment in soil profiles during the decomposition of soil organic matter

The mechanisms behind the C-13 enrichment of organic matter with increasing soil depth in forests are unclear. To determine if C-13 discrimination during respiration could contribute to this pattern, we compared delta C-13 signatures of respired CO2 from sieved mineral soil, litter layer and litterfall with measurements of delta C-13 and delta N-15 of mineral soil, litter layer, litterfall, roots and fungal mycelia sampled from a 68-year-old Norway spruce forest stand planted on previously cultivated land. Because the land was subjected to ploughing before establishment of the forest stand, shifts in delta C-13 in the top 20 cm reflect processes that have been active since the beginning of the reforestation process. As C-13-depleted organic matter accumulated in the upper soil, a 1.0 parts per thousand delta C-13 gradient from -28.5 parts per thousand in the litter layer to -27.6 parts per thousand at a depth of 2-6 cm was formed. This can be explained by the 1 parts per thousand drop in delta C-13 of atmospheric CO2 since the beginning of reforestation together with the mixing of new C (forest) and old C (farmland). However, the isotopic change of the atmospheric CO2 explains only a portion of the additional 1.0 parts per thousand increase in delta C-13 below a depth of 20 cm. The delta C-13 of the respired CO2 was similar to that of the organic matter in the upper soil layers but became increasingly C-13 enriched with depth, up to 2.5 parts per thousand relative to the organic matter. We hypothesise that this C-13 enrichment of the CO2 as well as the residual increase in delta C-13 of the organic matter below a soil depth of 20 cm results from the increased contribution of C-13-enriched microbially derived C with depth. Our results suggest that C-13 discrimination during microbial respiration does not contribute to the C-13 enrichment of organic matter in soils. We therefore recommend that these results should be taken into consideration when natural variations in delta C-13 of respired CO2 are used to separate different components of soil respiration or ecosystem respiration.