Dark microbial CO₂ fixation in temperate forest soils increases with CO₂ concentration

Abstract

Dark, that is, nonphototrophic, microbial CO2 fixation occurs in a large range of soils. However, it is still not known whether dark microbial CO2 fixation substantially contributes to the C balance of soils and what factors control this process. Therefore, the objective of this study was to quantitate dark microbial CO2 fixation in temperate forest soils, to determine the relationship between the soil CO2 concentration and dark microbial CO2 fixation, and to estimate the relative contribution of different microbial groups to dark CO2 fixation. For this purpose, we conducted a 13C‐CO2 labeling experiment. We found that the rates of dark microbial CO2 fixation were positively correlated with the CO2 concentration in all soils. Dark microbial CO2 fixation amounted to up to 320 µg C kg−1 soil day−1 in the Ah horizon. The fixation rates were 2.8–8.9 times higher in the Ah horizon than in the Bw1 horizon. Although the rates of dark microbial fixation were small compared to the respiration rate (1.2%–3.9% of the respiration rate), our findings suggest that organic matter formed by microorganisms from CO2 contributes to the soil organic matter pool, especially given that microbial detritus is more stable in soil than plant detritus. Phospholipid fatty acid analyses indicated that CO2 was mostly fixed by gram‐positive bacteria, and not by fungi. In conclusion, our study shows that the dark microbial CO2 fixation rate in temperate forest soils increases in periods of high CO2 concentrations, that dark microbial CO2 fixation is mostly accomplished by gram‐positive bacteria, and that dark microbial CO2 fixation contributes to the formation of soil organic matter.

Reviews and syntheses: Heterotrophic fixation of inorganic carbon – significant but invisible flux in environmental carbon cycling

“…Elevated partial pressure of CO 2 might stimulate dark DIC fixation. In temperate forest soils, rates of dark microbial CO 2 fixation were positive correlated with the CO 2 concentration (Spohn et al, 2019). Similarly, with increasing CO 2 concentrations, higher dark DIC fixation was observed in wetland soils affected by subcrustal CO 2 degassing (Beulig et al, 2015).…”

“…Miltner et al, 2005;Alonso-Saéz et al, 2010;Yakimov et al, 2014;Šantrůčková et al, 2018;Akinyede et al, 2020). Individual studies succeeded to follow the label of inorganic carbon in environmental samples into amino acids of microorganisms (Spohn et al, 2019;Spona-Friedl et al, 2020). In the future, the combined application of metabolomics and isotope tracing may help further develop this field of research.…”

Temperature sensitivity of dark CO2 fixation in temperate forest soils

“…Decomposition of soil organic carbon (SOC) provides one of the largest sources of carbon dioxide (CO 2 ) to the atmosphere (Rastogi et al, 2002;Lal, 2004). Microbes can refix 3 %-6 % of CO 2 in temperate forest mineral soils before its release to the atmosphere (Akinyede et al, 2020;Spohn et al, 2019), through so-called dark CO 2 fixation (Miltner et al, 2005;Šantrůčková et al, 2018). Dark CO 2 fixation in soils is mediated by chemolithoautotrophic bacteria, largely via the Calvin-Benson-Bassham (CBB) path-R. Akinyede et al: Temperature sensitivity of dark CO 2 fixation in temperate forest soils way (Niederberger et al, 2015;Wu et al, 2014), the Wood-Ljungdahl pathway (WLP), or the reverse tricarboxylic acid (rTCA) pathway (Beulig et al, 2016;Liu et al, 2018).…”

“…The biomass of microbial communities serves as the entry point of carbon fixed from CO 2 into SOC, which includes both the intact microbial biomass carbon (MBC) pool and released microbial residues (Miltner et al, 2004(Miltner et al, , 2005Spohn et al, 2019). Microbial residues constitute any non-living organic material of microbial origin including necromass and extracellular metabolites (Geyer et al, 2020).…”

Soil Microbes Transform Inorganic Carbon Into Organic Carbon by Dark Fixation Pathways in Desert Soil

“…These pathways were identified based on the occurrence of genes involved in autotrophic pathways. Moreover, heterotrophic microbial CO 2 fixation, for example, biosynthesis of fatty acids and anaplerotic reactions, for internal carbon cycling in the dark has been identified in arctic, cropland, temperate forest, and temperate soils (Miltner et al, 2004(Miltner et al, , 2005Nel & Cramer 2019;Spohn et al, 2020;Šantrůčková et al, 2018). This process may also contribute to the transformation of "SIC→SOC" in drylands; however, it remains largely unexplored in drylands.…”