Soil pH promoted respiration is stimulated by exoenzyme kinetic properties for a Pinus tabuliformis forest of northern China

Abstract

The trends in 21^st century climate change will be largely modulated by the amount of carbon respired via the enzymatic depolymerization of soil organic carbon (SOC). As soil pH serves as a key indicator of global change, understanding how soil respiration responds to pH induced changes in enzyme kinetic properties will provide valuable insights into the feedback of soil carbon to climate change. In a Pinus tabuliformis forest of northern China, a soil pH gradient ranging from 4.91 to 7.89 was constructed by applying ammonium nitrate at rates of 5, 10, 20, and 40 g N m^-2 yr^-1 (N5, N10, N20, and N40) and lime at rates of 50, 100, 200, and 400 g m^-2 yr^-1 (L50, L100, L200, and L400) since 2015. In August 2022, soil basal respiration, the β-glucosidase (BG) activity, and soil microbial properties were measured. Results revealed that soil basal respiration increased from 1.46 μmol CO₂ m^-2 s^-1 in N40 treatment to 2.36 CO₂ m^-2 s^-1 in L400 treatment, while the binding affinity of BG rose from 0.018 to 0.032 under the same treatments. The maximum activity of BG decreased from 119.82 nmol MUB·h^-1·g^-1 SOM in N40 treatment to 66.80 nmol MUB·h^-1·g^-1 SOM in L400 treatment. The temperature sensitivity of soil respiration showed a bell-shaped response to soil pH, with an optimal pH of about pH 6.7. Our findings demonstrated that it was the binding affinity instead of the activity of BG that positively promoted soil respiration across the established soil pH gradient. The underpinning mechanisms linking soil respiration with enzyme functions were ascribed to the soil acid-base microenvironment, which affected the bioavailability of key nutrient and the content of soil inorganic nitrogen. Additionally, these results will improve the understanding of enzymatic mechanisms in driving the biogeochemical cycle of SOC.