Are soil carbon stocks in mountain grasslands compromised by land-use changes?

Mountain grasslands are generally rich in soil organic C, but the typical high spatial variability of mountain environments, together with the different management systems, makes their soil C content particularly variable. Socio-economic changes of the past decades have caused a progressive abandonment of the traditional use for grazing of some areas, while grazing pressure at easily accessible grasslands have increased. Here, we analyse the effect of these land-use changes on the factors regulating the soil C accumulation and stocks. Overgrazing generally leads to a reduction above- and below-ground litter inputs and a decrease in soil C stocks, affecting some soil physicochemical and biological properties. Additionally, the labile C inputs coming from animal faeces may accelerate the mineralisation of organic matter. Grazing abandonment causes a reduction of aboveground productivity, but the lack of consumption causes a short-term accumulation of organic matter. Its effect on belowground biomass and productivity is less clear. At longer term, grazing abandonment causes a change in the plant community composition, having the shrub encroachment the strongest effect on C storage. The low biochemical quality of shrub litter delays its decomposition and allows higher organic matter accumulation in the topsoil. But the effect of shrub proliferation at the deeper soil is less clear. The low root turnover of shrubs compared to grasses may reduce the C inputs to the soil. But, at the same time, the reduction of the root exudates may also reduce the microbial activity and the organic matter mineralisation.This study summarises the work done in different research projects funded by the Ministerio de Economia y Competitividad, Spain (Carbopas, REN2002-04300-C02-02; VULCA, CGL2005-08133-CO3; GRACCIE Consolider Program, CSD2007-00067) and by the European Commission (GHG-Europe project, FP7-ENV-2009-1, project No. 244122). J.G.P. and P.C. are financially supported by the Spanish Ministerio de Economía y Competitividad, through Juan de la Cierva and Ramón y Cajal contracts, respectively