Whole soil warming decreases abundance and modifies community\nstructure of microorganisms in subsoil but not in surface soil

Abstract

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 do microorganisms at the soil surface. Since microorganisms are key drivers of soil organic carbon decomposition, this knowledge gap causes uncertainty in predictions of future carbon cycling in the subsoil carbon pool (>50\u2009% of the soil organic carbon stocks are below 30\u2009cm soil depth). In the Blodgett forest field warming experiment (California, USA) we investigated how +4\u2009°C warming the whole soil profile to 100\u2009cm 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). Microbial biomass decreased and community composition changed with depth. Our results show that the concentration of PLFAs decreased with warming in the subsoil (below 30\u2009cm) by 28\u2009% but was not affected in the topsoil. Phospholipid fatty acid concentrations changed in concert with soil organic carbon. The microbial community response to warming was depth dependent. 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 the ratio of anteiso to iso PLFAs. Our results show for the first time that subsoil microorganisms can be more affected by warming as compared to topsoil microorganisms. These microbial responses could be explained by the observed decrease in subsoil organic carbon concentration in the warmed plots. A decrease in microbial abundance in warmed subsoils might reduce the magnitude of the respiration response over time. The shift in the subsoil microbial community towards more actinobacteria might disproportionately enhance degradation of previously stable subsoil carbon, as this group is able to metabolize complex carbon sources.\n