Hydrolase kinetics to detect temperature-related changes in the rates of soil organic matter decomposition

To evaluate the importance of the role of temperature on the decomposition of soil organic matter (SOM), we investigated the SOM decomposition rates and the kinetics of two hydrolases involved in carbon (C) and nitrogen (N) cycling, namely β-1,4-glucosidase (βG) and β-1,4-N-acetylglucosaminidase (NAG), and their sensitivity to temperature in representative temperate forests. Soils were collected from three spatially-separate replicate plots at study sites distributed at heights of 1233, 1060, and 825 m a.s.l. along an elevation gradient on the southern slopes of Laotuding Mountain, Northeast China. Soils were incubated at temperatures between 4 and 40 °C at 6 °C intervals for 7 days in our laboratory. Decomposition rates of SOM responded positively to temperature, and, at the same temperature, were highest in the soil from 1233 m a.s.l. than in the soils from the other elevations. SOM decomposition rates were positively correlated with the maximum activity (Vmax) of βG and the contents of total and particulate organic C, but were negatively correlated with the soil silt and clay contents. The Vmax and the Michaelis constant (Km) of the two hydrolases were positively correlated and were also correlated with increases in temperature, suggesting that the Km values could offset increases in Vmax with increases in temperature. These correlations also highlight the enzymatic tradeoff between the maximum catalytic rate and the substrate binding affinity for the two hydrolases. The catalytic efficiencies of the two hydrolases were highest at 1060 m a.s.l., followed by 1233 m a.s.l., and were lowest at 825 m a.s.l. The catalytic efficiencies were positively correlated with the soil water contents and macroaggregate contents (>250 μm), but negatively with the soil C/N ratios. The temperature sensitivities (Q10) of the SOM decomposition rates were similar at the different elevations (P > 0.05), but generally increased as the temperature increased (P < 0.05). The Q10(Vmax)/Q10(Km) values of βG and NAG increased significantly as the temperature increased from 22 to 40 °C (P < 0.001), and were generally similar between elevations (P > 0.05). Our results suggest that, in spite of the negative effects of increased temperatures on enzyme substrate affinity, increases in hydrolytic activity will lead to accelerated SOM decomposition in temperate forests.