Season-dependence of soil extracellular enzyme activities in a Pinus koraiensis forest on Changbai Mountain

Changbai Mountain, central in the distribution of Pinus koraiensis, supports a virgin Korean pine forest with vertical gradient distribution. Soil extracellular enzyme activity (EEA) and enzyme stoichiometry (ES) are reliable indicators of the energy and nutrients utilized by microbial communities and of soil nutrient changes. We measured four representative soil EEAs (sucrase, cellulase, urease, acid phosphatase) at two soil layers (A: 0–5 cm and B: 5–10 cm) beneath Korean pine forest at five elevations on Changbai Mountain during growing season. The vertical and seasonal variations of EEAs were analyzed by soil enzyme stoichiometry to quantify the role of soil microorganism in the nutrient cycling process. The activities of four soil extracellular enzymes and the ratios of enzyme activity to soil microbial biomass carbon (EA/ SMBC) did not vary with elevation. The first partition point of multiple regression trees was in September, and the second branch was split by elevation. Seasonal change had more influence on soil enzyme activity (A layer: 75.6%; B layer: 71.3%) than did change in elevation (A layer: 7.8%; B layer: 7.5%). Over one entire growing season, both vector length and vector angle were unchanged by elevation, but varied significantly by month. Among the soil physicochemical factors, available phosphorus and pH were the main factors affecting the four soil EEAs. The ratio of basal area of the coniferous tree to broad-leaved tree species (Scon/Sbr), soil microbial biomass carbon (MBC) and nitrogen (MBN) influenced the four soil EEAs. The results of vector analysis revealed that C and N sources were generally sufficient, but P was limiting (vector angle > 45°). The vector angle for September was significantly higher than for other months. This result verified that phosphorus was the limiting factor affecting soil microorganism function in nutrient metabolism and cycling. Soil enzyme stoichiometry proved to be an efficient index for quantifying soil microorganism-mediated nutrient cycling in the Korean pine ecosystem.