Achieving carbon isotope mass balance in northern forest soils, soil respiration and fungi
Sammanfattning: Northern forests contain a large part of the global terrestrial carbon pool and it is unclear whether they will be sinks or sources for atmospheric carbon if the climate warms as predicted. Stable isotope techniques provide unique tools to study the carbon cycle at different scales but the interpretation of the isotope data is impaired by our inability to close the carbon isotope mass balance of ecosystems. This involves the paradox that the soil organic matter becomes increasingly 13C-enriched with increasing soil depth relative to the carbon input, plant litter, at the same time as soil respiration, the major carbon outflow from the soil, and fungi, organisms dependent on plant derived carbon, both are relatively 13C-enriched. I have determined the δ13C of the respired CO2 and the organic matter from different ecosystem components in a Norway spruce forest aiming at finding an explanation to the observed carbon isotope pattern.In the first study the soil surface respiration rate and isotopic composition was found to be governed by aboveground weather conditions the preceding 1-6 days. This suggests there is a fast flux of recent photosynthates to root respiration. In the second study I compared the respired CO2 from decomposition with the δ13C of the root free soil organic matter sampled from the litter layer down to 50 cm depth. Discrimination against 13C during respiration could not explain the 13C enrichment of soil organic matter with depth because the δ13C of the respired CO2 became increasingly 13C-enriched relative to the organic matter with soil depth. However, ~1.5‰ of the 2‰ 13C-gradient could be explained by the 13C depletion of atmospheric CO2 that has proceeded since the beginning of the 18th century due to the burning of fossil fuels and deforestation. The remaining shift was hypothesized to be due to a belowground contribution of 13C-enriched ectomycorrhizal derived carbon. In the third study I compared the δ13C of respired CO2 and sporocarps of ectomycorrhizal and saprotrophic fungi sampled in the spruce forest. The δ13C of respired CO2 and sporocarps were positively correlated and the differences in δ13C between CO2 and sporocarps were small, <±1‰ in nine out of 16 species, although three out of six species of ectomycorrhizal basidiomycetes respired 13C-enriched CO2 (up to 1.6‰), whereas three out of five species of polypores respired 13C-depleted CO2 (up to 1.7‰; P<0.05). Loss of 13C-depleted CO2 may have enriched the biomass of some fungal species in 13C. However, the consistent 13C enrichment of fungal sporocarps and respired CO2 relative to the plant materials implies that other processes must be found to explain the consistent 13C-enrichment of fungal biomass compared to plant materials. In the final study, compound specific stable isotope analyses provided further evidence for the hypothesis that the biomass of ectomycorrhizal fungi are 13C-enriched relative to host biomass because the carbon provided by the host is 13C-enriched Furthermore, ectomycorrhizal fungi showed lower average δ13C values of metabolites than saprotrophs which gives further support for the so-called saprotrophic-mycorrhizal divide. I conclude that a belowground allocation of 13C-enriched carbon to ectomycorrhizal fungi closes the carbon isotope mass balance in boreal and temperate forest soils and explains the 13C-enriched soil respiration.
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