Altitudinal, seasonal and interannual shifts in microbial communities and chemical composition of soil organic matter in Alpine forest soils

The study of soil microbial communities and chemical properties of soil organic matter (SOM) over altitudinal mountain gradients have been regarded as especially useful to better predict and help mitigate the effects of climate change. However, there is a lack of surveys considering altitudinal, seasonal and interannual variations at the same time. Here, we investigated four forest sites, along an altitude gradient (545–2000 m a.s.l.), in spring and autumn during two consecutive years (2014 and 2015) regarding i) soil temperature and physicochemical properties; ii) SOM chemical composition using pyrolysis; iii) soil microbial activity (basal respiration, potential enzyme activities, community level physiological profiles (CLPP)); iv) archaeal, bacterial and fungal abundance (qPCR, phospholipid fatty acid (PLFA) analysis, numbers of culturable heterotrophic bacteria); and v) microbial community structure (PLFA analysis). Sites at high altitudes (alpine, subalpine) showed lower mean, maximum and minimum soil temperatures respect to those at lower altitudes (montane, submontane) and were characterized by increased levels of SOM and nutrients, higher archaeal, bacterial and fungal abundance as well as higher microbial activities. Soils at the sites at higher altitudes presented a higher humification degree of SOM, as demonstrated by an enhanced content of aromatic compounds and fatty acids. The seasonal effect determined a rise in the content of SOM, some soil nutrients, basal respiration and microbial abundance in autumn respect to spring over the altitudinal gradient as well as a change in the structure of microbial community and SOM molecular composition. Interannuality had a significant effect on the relative abundance of various chemical groups of SOM compounds and on the ratios of microbial groups, and produced an increase in microbial abundance in the second study year, as a consequence of the increased soil maximum temperatures recorded in this year. CLPP showed to be site-specific with limited seasonal and interannual variations. Altitudinal, seasonal and interannual changes in chemical composition of SOM were explained by variations in PLFA-based microbial community structure as well as in the abundance of bacteria and fungi, which, in turn, were affected by climatic conditions.