Isotope fractionation and 13C enrichment in soil profiles during the decomposition of soil organic matter |
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Authors: | Björn Boström Daniel Comstedt Alf Ekblad |
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Affiliation: | Department of Natural Sciences, Orebro University, 701 82, Orebro, Sweden. Bjorn.Bostrom@nat.oru.se |
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Abstract: | The mechanisms behind the 13C enrichment of organic matter with increasing soil depth in forests are unclear. To determine if 13C discrimination during respiration could contribute to this pattern, we compared δ13C signatures of respired CO2 from sieved mineral soil, litter layer and litterfall with measurements of δ13C and δ15N 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 δ13C in the top 20 cm reflect processes that have been active since the beginning of the reforestation process. As 13C-depleted organic matter accumulated in the upper soil, a 1.0‰ δ13C gradient from −28.5‰ in the litter layer to −27.6‰ at a depth of 2–6 cm was formed. This can be explained by the 1‰ drop in δ13C 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‰ increase in δ13C below a depth of 20 cm. The δ13C of the respired CO2 was similar to that of the organic matter in the upper soil layers but became increasingly 13C enriched with depth, up to 2.5‰ relative to the organic matter. We hypothesise that this 13C enrichment of the CO2 as well as the residual increase in δ13C of the organic matter below a soil depth of 20 cm results from the increased contribution of 13C-enriched microbially derived C with depth. Our results suggest that 13C discrimination during microbial respiration does not contribute to the 13C enrichment of organic matter in soils. We therefore recommend that these results should be taken into consideration when natural variations in δ13C of respired CO2 are used to separate different components of soil respiration or ecosystem respiration. |
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Keywords: | C:N ratio δ 13C Forest soil organic matter Isotopic discrimination Microbial respiration |
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