Soil microbial structure and enzymatic activity along a plant cover in Victoria Land (continental Antarctica).

In continental Antarctica, autotrophs are exclusively represented by cyanobacteria, algae, lichens and mosses.
Consequently, Antarctic soil communities are expected to be rather simple and primarily dominated by microorganisms.
Recently, a change in abundance of mosses and lichens has been observed in continental
Antarctica in response to an increase of the active permafrost layer, but the implication of this change to soil
micro-organisms remains little known. Here we aim to clarify to what extent the abundance of mosses and
lichens affects soil biogeochemistry in Victoria Land, with a particular focus on soil microbial abundance and
associated soil enzymatic activity. To achieve this aim, we assessed the structure of soil microbiome and the
activity of hydrolytic C, N, and P enzymes along a gradient in soil physico-chemical conditions and plant cover.
Moss cover strongly relates to the amount of soil organic carbon (SOC), soil water and nutrient content. Soils
with higher content of organic carbon were characterized by higher microbial biomass and showed a relatively
higher abundance of fungi as compared to bacteria. More specifically, PLFAs biomarkers for Actinomycetes and
Gram-positive bacteria were mainly associated to soils with lower SOC. In order to sustain a higher microbial
biomass, total activity of hydrolytic enzymes increased with increasing SOC content. Eco-enzymatic stoichiometry,
based on C to P and C to N ratios, indicates a higher investment in N- and P-hydrolytic enzymes
(ratio < 1), particularly at low SOC content. Oppositely, an increase in C-hydrolytic enzyme activity (ratio≈1)
was observed with increasing accumulation of organic carbon. Such a result seems to indicate a stronger role of
soil pH at low SOC on enzymatic stoichiometry (abiotic control) whereas with increasing accumulation of organic
matter the enzymatic stoichiometry is more affected by microbial metabolism (biotic control).