Altitudinal Gradients Modify the Ecological Stoichiometry and Enzyme Activities of Soil in a Natural Secondary Quercus spp. Forest of the Dabie Mountains

Understanding the vertical distribution and driving mechanisms behind soil carbon (C), nitrogen (N), and phosphorus (P) contents and enzyme activities along elevation gradients is of great significance for the healthy and sustainable management of forest ecosystems. For this study, the 0–20 cm soil-layer samples of different natural Quercus spp. secondary forests from eight altitude gradients (ranging from 250 to 950 m) were investigated to quantify their physicochemical properties, ecological stoichiometry characteristics, and enzyme activities. The results indicated that the soil nutrient content of natural secondary Quercus spp. forests in the Dabie Mountains was low, with average soil organic carbon (SOC) and total phosphorus (TP) contents of 19.86 ± 3.56 g·kg−1 and 0.68 ± 0.10 g·kg−1, respectively, which were 19.14% and 12.82% lower, respectively, than the Chinese average. In terms of vertical spatial distribution, the SOC, total nitrogen (TN), and TP contents of the soil at high altitudes (≥750 m) were greater than those at middle- and low-altitude areas and reached the maximum value at or near the top of the mountain (850–950 m). The stoichiometric attributes of the soil ecosystem fluctuated with the higher altitudes in vertical space; however, the fluctuation range was not significant. The C:N, N:P, and C:P ratios reached their maximum values at altitudes of 250, 750, and 850 m, respectively. However, the overall average value remained generally lower than the national average; thus, in forest management, attention should be paid to the supplementation of the soil with C and P. The activities of soil sucrase, urease, acid phosphatase, and catalase were interconnected across the overall space, and increased with altitude. The SOC, TP, and pH were the main factors that influenced the changes in soil enzyme activities.