Relationships between soil physico-chemical properties and microbial activity along a restoration chronosequence of alpine grasslands following ski run construction

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Rapid establishment of herbaceous cover after ski run construction is fundamental to limit soil erosion and restore initial ecosystem functioning. This paper reports the disturbance and development of relationships between soil physico-chemical properties and microbial activities following ski run construction and along a chronosequence of restored alpine grasslands (1 month and 1, 4 and 13 years). Several measurements were made on vegetation (species composition, root biomass), soil (aggregate stability, particle size distribution, total carbon and nitrogen content, porosity) and soil microbial activity (heterotrophic microbial diversity and functional structure, nitrogenase activity). During the restoration chronosequence, effective porosity was found to be positively correlated with rock fragments (>2 mm) proportion ( r=0.96; P<0.05) while capillary porosity was positively correlated with clay content ( r=0.86; P<0.05). A specific microbial pattern of native alpine soil was evident with low non-symbiotic N2-fixation (6.61 E-11 mol C 2H 4 per gram), low heterotrophic functional diversity and low metabolic quotient (qCO 2). The increase in potential nitrogen fixation along the chronosequence was attributed to labile C input by root exudation and favourable soil moisture. High catabolic diversity in older restored soils (4 and 13 years old) was also sustained by the constant evolution of soil physico-chemical properties, especially heterogeneous C resource input. In accordance with the positive relationship between catabolic evenness and ecosystem function, our data suggests that the microbial community remained very unstable during the early stage of ski run restoration (<13 years old) because of the dominance of opportunist micro-organisms.