Energy and matter dynamics in an estuarine soil are more sensitive to warming than salinization

Rising salinization of extended river-sides and estuary areas due to climate warming might alter microbial metabolic activity and cause unpredictable consequences for matter and energy turnover in soil. Therefore, we investigated the combined effects of salinization and warming on microbial activity and growth, examining CO₂ emissions (matter loss) and heat production (energy loss) during glucose metabolism. Soil from Elbe estuary was artificially salinized to medium (2.06 mS cm −1 ) and high (3.45 mS cm −1 ) levels, and ambient low salinity soil (0.93 mS cm −1 ) served as the control. We examined the influence of a comprehensive +2 °C climate warming (20 vs. 22 °C) on soil respiration (CO 2 emission), heat release, enzyme kinetics (cellobiohydrolase, β-glucosidase, acid phosphomonoesterase and leucine-aminopeptidase) and microbial carbon use efficiency (CUE) across the microbial growth. Increasing salinity did not impact respiration, heat release, microbial C and N content without glucose addition. However, activation of microorganisms with glucose brought force to the effect of salinity, and increasing salinity consistently retarded substrate uptake and growth. 2 °C warming affected substrate uptake and growth much more than increasing salinity. The calorespirometric ratio increased by 81–124% under high salinity compared to low salinity, with most of this increase occurring during the growth retardation stage. Enzyme activities increased by 68%–871% during the lag phase and remained relatively high throughout both the growth and retardation stages, regardless of salinity and temperature levels, suggesting the resistance of soil hydrolytic enzymes. The CUE gradually decreased and stabilized only at the very end of microbial growth, emphasizing the importance of considering the growth retardation for CUE estimation. Remarkably, disregarding the growth retardation stage resulted in a strong overestimation of the CUE accounting for 70%–98%. Our results highlight the importance of estimating the carbon budget of microbial growth by considering its dynamics when modeling carbon sequestration under global climate change. • 2 °C warming altered soil microbial activity stronger than 3.5-fold salinization. • Warming accelerated microbial growth, whereas elevated salinity delayed it. • Salinity had no significant effect on the activity of C-, N-, and P-cycling enzymes. • The carbon use efficiency is overestimated if growth retardation phase is ignored.