Differential response of soil characteristics and extracellular enzyme activities along an altitude gradient in a subtropical forest ecosystem, eastern China

Variations in altitude profoundly affect soil characteristics and extracellular enzyme activities (EEAs) due to the distinct vertical zonation of mountains. However, understanding the nature and strength of the interactive linkages among soil characteristics, C:N:P stoichiometry, and EEAs along altitudinal gradients in subtropical forest ecosystems remains elusive. Here, we designed an altitude gradient that covered five altitude levels on Tianmu Mountain, located in the transition zone between the central and northern subtropics, eastern China. This study explores how the soil characteristics and EEAs vary with altitude and aims to clarify the nature and strength of the interactive linkages among soil characteristics, C:N:P stoichiometry, and EEAs along an altitude gradient. Soil sand ratio, soil water content (SWC), electrical conductivity (EC), soil organic carbon (SOC), total nitrogen (TN), protease (PRO), urease (URE), and acid phosphatase (ACP) activities exhibited fluctuating upward trends with increasing altitude, peaking at the 1500 m asl. Conversely, silt ratio and pH levels demonstrated a downward trajectory, reaching their nadir at the same altitude. Meanwhile, the clay ratio, ammonium, and nutrient stoichiometric ratio initially increase but then decline with their highest value at 800 m or 1000 m. On the contrary, soil nitrate and total phosphorus (TP), first decrease and then increase again with increasing altitude, reaching their lowest value at 1000 m and 800 m, respectively. Soil catalase (CAT) and invertase (INV) activity exhibited a pronounced upward surge followed by a stabilization as altitude increased. Moreover, the altitude-dependent differences in the soil EEAs were primarily affected by SOC, SWC, TN, sand, silt, and pH, with explained variation higher than 30 % (P < 0.01). Overall, this study demonstrates a clear altitude effect on soil characteristics and EEAs, and the alterations in soil characteristics across altitudes are essential in shaping the soil EEAs in subtropical forest ecosystems.