No thermal adaptation in soil extracellular enzymes across a temperate grassland region

Soil organic matter decomposition rates are thermally adapted, in relation to the ambient mean annual temperature of the soil environment. This could be because of temperature sensitivity of soil extracellular enzymes involved in decomposition. The objective of this study was to determine if soil extracellular enzyme activity and its temperature sensitivity (measured as Q10) were related to the mean annual temperature across a grassland region. We collected surface soil (0-10 cm) from 57 temperate grassland sites along a latitudinal gradient in northern China. The Q10 of soil extracellular enzymes (β-1,4-glucosidase, β-1,4-N acetylglucosaminidase, leucine aminopeptidase and acid phosphatase) was measured at four assay temperatures (5 °C, 15 °C, 25 °C, and 35 °C). There was no relationship between the mean annual temperature and standardized extracellular enzyme activity when other influential factors (soil pH, substrates, texture) were controlled, implying no thermal adaption of extracellular enzyme activity. Soil edaphic factors like soil pH, substrates and texture explained more of the spatial variation in the standardized extracellular enzyme activity than the mean annual temperature. Although the Q10 of all four enzymes increased significantly with mean annual temperature, the data converged at the maximum activity, and the average Q10 values were 1.17 for β-1,4-glucosidase, 1.21 for β-1,4-N acetylglucosaminidase, 1.11 for leucine aminopeptidase and 1.08 for acid phosphatase. The positive correlation between the Q10 of extracellular enzyme activity and mean annual temperature is not consistent with the thermodynamic theory, which predicts higher Q10 at sites with lower mean annual temperature. This may be due to the fact that extracellular enzymes operate outside of biological cells and thus are not directly related to physiological process rates of the microbial biomass. This study contributes to our understanding of the thermal adaptation of soil extracellular enzymes, and show that grassland sites with lower mean annual temperature do not necessarily induce higher temperature sensitivity of extracellular enzymes.