Microbial response to warming and cellulose addition in a maritime Antarctic soil

Maritime Antarctic King George Island (South Shetland Islands) has experienced rapid warming in recent decades, but the impacts on soil organic matter (SOM) decomposition remain ambiguous. Most vegetation cover is dominated by bryophytes (mosses), whereas a few vascular plants, such as Deschampsia antarctica and Colobanthus quitensis grow interspersed. Therefore, SOM is mainly enriched with carbohydrates and C-alkyl, provided by mosses, which lack lignin as a precursor for aromatic compounds and humus formation. However, there is no clear answer to how substrate and temperature increase changes in Antarctic microbial respiration. We determined in what way SOM mineralization changes with temperature and substrate addition by characterizing the temperature sensitivity (Q(10)) of soil respiration in an open-top chamber warming experiment. We hypothesized that: (a) cold-tolerant microorganisms are well adapted to growing in maritime Antarctic soils (similar to 0 degrees C), so would not respond to low and moderate temperature increases because they undergo various metabolic mechanism adjustments until they experience increasing temperatures toward optimum growth (e.g., by enzyme production); and (b) cellulose, as a complex carbonaceous substrate of vegetated areas in Maritime Antarctic soils, activates microorganisms, increasing the Q(10) of soil organic carbon (SOC) mineralization. Soils (5-10 cm) were sampled after four consecutive years of experimental warming for SOC composition, microbial community structure, and C mineralization at 4, 12, and 20 degrees C with and without cellulose addition. Functional group chemoheterotrophs, represented mainly by Proteobacteria, decomposed more refractory SOC (aromatic compounds), as indicated by nuclear magnetic resonance (NMR) spectroscopy, in ambient plots than in warming plots where plants were growing. The C-CO2 efflux from the incubation experiment remained stable below 12 degrees C but sharply increased at 20 degrees C. Q(10) varied between 0.4 and 4 and was reduced at 20 degrees C, whereas cellulose addition increased Q(10). In conclusion, as confirmed during field studies in a climate scenario, cold-tolerant microorganisms in maritime Antarctic soils were slightly affected by increasing temperature (e.g., 4-12 degrees C), with reduced temperature sensitivity, as summarized in a conceptual model.