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

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