Effects of soil warming on soil microbial extracellular enzyme activities with different depths in a young Chinese fir (Cunninghamia lanceolata)plantation of subtropics.

Extracellular enzyme activitie (EEAs) are a sensitive indicator of microbial function and soil organic matter decomposition in response to climate warming. Up to now, most studies of climate warming and their effects on EEAs have been restricted on the relatively carbon rich topsoil (the upper 20 cm of the soil), whereas little is known about EEAs in subsoil (below 30 cm depth). This study focused on the responses of EEAs to soil warming in a subtropical forest at depths of 0-10 cm, 10-20 cm, 20-40 cm and 40-60 cm. The examined extracellular enzymes included ß-glucosidase (BG), cellobiohydrolase (CBH), phenoloxidase (PHO) and peroxidase (PEO), all being involved in the C-cycle. The results showed that, 1) warming significantly increased all EEAs (18%-69%) at the depth of 0-10 cm and 10-20 cm. Below the depth of 20 cm, warming did no affect or suppressed EEAs (13%-31%), except increasing PHO (10%) at 20-40 cm. 2) Results from the redundancy analysis showed that the EEAs were mainly driven by ammonium nitrogen (NH4+-N) and soil moisture (M) in organic carbon rich topsoil. Warming enhanced nutrient competition between soil microorganisms and plants. Thus, it increased EEAs to meet NH4+-N demands of microorganisms. In subsoil with relatively low substrate availability, the EEAs were dominated by dissolved organic matter and microbial biomass (MBC). Warming increased dissolved organic matter and thus provided more substrates for microorganisms, which relieved the dependence of microbes on EEAs. Consequently, warming diminished EEAs in subsoils. Our results suggested that EEAs at the four depths showed different responses to warming. In addition, environmental factors accounting for the variances in EEAs under soil warming condition were different at topsoil and subsoil. Paying more attention to microbes at different soil depths has important implications to precisely predict ecosystem C cycling in response to global warming.