Impact of slope exposure on chemical and microbiological properties of Norway spruce deadwood and underlying soil during early stages of decomposition in the Italian Alps

Mountain forest ecosystems are particularly sensitive to changing environmental conditions that affect the rate of deadwood decay and, thus, also soil carbon turnover and forest productivity. Little is known about how slope exposure and climate influence microbial abundance and activity in general, and wood-inhabiting bacteria during deadwood decomposition in particular. Therefore, a field experiment using open mesocosms was carried out along an altitudinal gradient (from 1200 to 2000 m above sea level) in the Italian Alps to evaluate the impact of exposure (north (N)- vs. south (S)-facing sites) on microbial biomass (double stranded DNA, dsDNA); microbial abundance (real-time PCR-based: fungi; dinitrogen reductase, nifH; ammonia-monooxygenase, amoA); and several hydrolytic enzyme activities involved in the main nutrient cycles during decomposition of Picea abies wood blocks (2 ×5 ×5 cm) over a 2-year period. In addition, soil physico-chemical and (micro)biological properties were determined at each site. The cooler, moister and more acidic conditions at north-facing slopes led to an increase in the wood and soil fungal abundance. Furthermore, soil nitrogen-related bacterial functional genes (nifH and amoA) gave a contrasting response to exposure in terms of abundance: nifH (N > S, altitude and decay time-dependent); ammonia-oxidising bacteria (AOB; S > N, altitude and decay time-dependent); ammonia-oxidising archaea (AOA; N > S, only at 2000 ma.s.l.). The AOB and AOA abundance, however, was below the detection limit in the wood blocks. Soil microbial biomass was in general higher at the north-facing slopes, whereas in the wood exposure affected to a lesser extent the microbial biomass. Overall, the exposure effect on the microbial biomass and abundance as well as for most of the enzymatic activities was altitude- and decay time-dependent.