Microbial hotspot areas of C and N cycles in old-growth Hyrcanian forests top soils

Abstract

Microbial activity of top soils is directly linked with carbon (C) and nitrogen (N) turnover in terrestrial ecosystems, and thus is an indicator to identify hot spot areas of C and N cycling in forest habitats. The Hyrcanian forests are one of the last remnants of natural deciduous forests in the world and Oriental Beech (Fagus orientalis Lipsky) is one of the most abundant tree species of these forests. It has not been resolved which forest properties of Hyrcanian forests facilitate hotspots with high microbial process rates. We studied those primary causes (i.e. forest properties) of changes on soil characters and processes within old-growth Vaz forests, Northern Iran, and separately sampled different landform positions (i.e. catena, pit-mound and gilgai), single-trees, deadwoods and canopy gaps. Principal component analysis (PCA) of the 31 soil parameters resulted in two principal components that explained more than 60% of the total variance. The first component (PC1) explained the highest variance (48.13%) in the results; while second component (PC2) accounted for 13.68% of the variance. The obtained PCA results indicate a clear differentiation in selected soil properties among forest properties and thus, three groups of drivers for microbial activities were identified. Group 1 (foot and toe slope of V-shape catena, toe slope ofΛ-shape catena, pit structure of hornbeam, decay class 4 of beech deadwoods, small and medium canopy gaps) enhanced soil microbial process and enzyme activities. Group 2 (pit structure of beech, control or non-gilgai position, beech and hornbeam single-trees, decay class 3 and 4 of hornbeam deadwoods and large canopy gap) enhanced soil microbial biomass and greenhouse gas fluxes. Group 1 and 2 drivers were associated with high values of soil water content, organic C, total N, available nutrients, organic matter fractions, earthworm and nematode activities. Group 3 drivers (summit and shoulder slope of V- and Λ-shape catena, back slope of V-and Λ-shape catena, foot slope ofΛ -shape catena, mound of beech and hornbeam, position in a gilgai, decay class 2 and 3 of beech deadwoods, decay class 1 and 2 of hornbeam deadwoods and very large canopy gap) correlated with lowest microbial activities and was associated with high values of soil bulk density, pH, electrical conductivity and C/N ratio. Thus, distribution of microbial hotspot areas in the Hyrcanian forests can be considered as mosaic of pedons characterized by specific sets of key parameters, which depend on landform, especially the toe slope of V-shape catena, and stand characteristics (e.g. tree species or canopy gap).