- مبلغ: ۸۶,۰۰۰ تومان
- مبلغ: ۹۱,۰۰۰ تومان
The occurrence of dark fixation of CO2 by heterotrophic microorganisms in soil is generally accepted, but its importance for microbial metabolism and soil organic carbon (C) sequestration is unknown, especially under Climiting conditions. To fill this knowledge gap, we measured dark 13CO2 incorporation into soil organic matter and conducted a 13C-labelling experiment to follow the 13C incorporation into phospholipid fatty acids as microbial biomass markers across soil profiles of four tundra ecosystems in the northern circumpolar region, where net primary productivity and thus soil C inputs are low. We further determined the abundance of various carboxylase genes and identified their microbial origin with metagenomics. The microbial capacity for heterotrophic CO2 fixation was determined by measuring the abundance of carboxylase genes and the incorporation of 13C into soil C following the augmentation of bioavailable C sources. We demonstrate that dark CO2 fixation occurred ubiquitously in arctic tundra soils, with increasing importance in deeper soil horizons, presumably due to increasing C limitation with soil depth. Dark CO2 fixation accounted on average for 0.4, 1.0, 1.1, and 16% of net respiration in the organic, cryoturbated organic, mineral and permafrost horizons, respectively. Genes encoding anaplerotic enzymes of heterotrophic microorganisms comprised the majority of identified carboxylase genes. The genetic potential for dark CO2 fixation was spread over a broad taxonomic range. The results suggest important regulatory function of CO2 fixation in C limited conditions. The measurements were corroborated by modeling the long-term impact of dark CO2 fixation on soil organic matter. Our results suggest that increasing relative CO2 fixation rates in deeper soil horizons play an important role for soil internal C cycling and can, at least in part, explain the isotopic enrichment with soil depth.
Our results demonstrate that dark CO2 fixation is common in all arctic soils investigated and anaplerotic reactions are mainly responsible for this. Many anaplerotic pathways in heterotrophic CO2 incorporation do not lead to any net C assimilation and biomass production (Alonso-Saez et al., 2010). Microbial biomass did not increase in soils without addition of bioavailable substrate either, which further suggests that dark IC fixation may only enable microorganisms to maintain metabolic activity even in C poor conditions. Inorganic C incorporation into OC only corresponds to a few percent of net soil respiration, but still it can play an important role in supporting microbial metabolism and organic matter transformation. We further demonstrate a positive impact of bioavailable soil organic compounds on inorganic C incorporation, implying that increases in plant litter decomposition induced by projected warming and input of root-derived compounds may also enhance C incorporation via dark C fixation.