Plant-Soil Interactions And Acclimation To Temperature Of Microbial-Mediated Soil Respiration May Affect Predictions Of Soil CO2 Efflux

It is well known that microbial-mediated soil respiration, the major source of CO₂ from terrestrial ecosystems, is sensitive to temperature. Here, we hypothesize that some mechanisms, such as acclimation of microbial respiration to temperature and/or regulation by plant fresh C inputs of the temperature sensitivity of decomposition of soil organic matter (SOM), should be taken into account to predict soil respiration correctly. Specifically, two hypotheses were tested: (1) under warm conditions, temperature sensitivity (Q₁₀) and basal rates of microbial-mediated soil respiration (Bs_20, respiration at a given temperature) would be primarily subjected to presence/absence of plant fresh C inputs; and (2) under cold conditions, where labile C depletion occurred more slowly, microbial-mediated soil respiration could adjust its optimal temperatures to colder temperatures (acclimation), resulting in a net increase of respiration rates for a given temperature (Bs₂₀). For this purpose, intact soil cores from an oak savanna ecosystem were incubated with sufficient water supply at two contrasting temperatures (10 and 30°C) during 140 days. To study temperature sensitivity of soil respiration, short-term temperature cycles (from 5 to 40°C at 8 h steps) were applied periodically to the soils. Our results confirmed both hypotheses. Under warm conditions ANCOVA and likelihood ratio tests confirmed that both Q₁₀ and Bs₂₀ decreased significantly during the incubation. Further addition of glucose at the end of the incubation period increased Bs₂₀ and Q₁₀ to initial values. The observed decrease in temperature sensitivity (Q₁₀) in absence of labile C disagrees with the broadly accepted fact that temperature sensitivity of the process increases as quality of the substrate decreases. Our experiment also shows that after 2 months of incubation cold-incubated soils doubled the rates of respiration at cold temperatures causing a strong increase in basal respiration rates (Bs₂₀). This suggest that microbial community may have up-regulated their metabolism at cold conditions (cold-acclimation), which also disagrees with most observations to date. The manuscript discusses those two apparent contradictions: the decrease in temperature sensitivity in absence of labile C and the increase in microbial-mediated soil respiration rates at cold temperatures. While this is only a case study, the trends observed could open the controversy over the validity of current soil respiration models.