Whole soil warming decreases abundance and modifies community structure of subsoil microorganisms

Abstract:

The microbial community composition in subsoils remains understudied and it is largely unknown whether subsoil microorganisms show a similar response to global warming as microorganisms at the soil surface. Since microorganisms are key drivers of soil carbon fluxes, this knowledge gap causes uncertainty in predictions of future carbon fluxes from the enormous subsoil carbon pool (>50 % of the soil organic carbon stocks are below 20 cm soil depth) to the atmosphere.

In the Blodgett forest field warming experiment (California, USA) we investigated how +4°C warming the whole soil profile to 100 cm soil depth for 4.5 years has affected the abundance and community structure of microorganisms. We used proxies for bulk microbial biomass carbon (MBC) and functional microbial groups based on lipid biomarkers, such as phospholipid fatty acids (PLFAs) and branched glycerol dialkyl glycerol tetraethers (brGDGTs).

We found that microbial biomass concentration decreased with warming in the subsoil by 27%, but increased or was not affected in the topsoil and organic horizons. The microbial community response to warming was depth dependent: for example, the relative abundance of Actinobacteria increased in subsoil, and Gram+ bacteria in subsoils adapted their cell-membrane structure to warming induced stress as indicated by a ratio of specific PLFAs. Our results show for the first time that subsoil microorganisms are likely more affected by warming compared to topsoil microorganisms. We hypothesize that these microbial responses in the subsoil could be related to changes in carbon availability. This is because carbon concentrations were significantly lower in the warmed subsoils. If subsoil microorganisms start feeding on other carbon sources as a consequence of carbon limitation, previously stable subsoil carbon pools might become more vulnerable to microbial decomposition under global warming.