Why soils are losing C after grazing abandonment? Effects of traditional pastoral activity on soil biochemical properties.

Alpine grasslands are the products of thousands of years of interaction between humanactivity and mountain harsh environmental conditions. Together, they inhibit development ofwoody vegetation, promoting meadow communities and short life cycles. Current trend in theland use in Italy is characterized by an intensification of the agricultural practices inthe lowlands and abandonment of traditional pastoral activities in the mountains.According to the scientific literature, the last results in the loss of C from soil at least inthe first decades after the pasture conversion to other land use types or completeabandonment. In this study we attempted to unravel the causes of soil C depletion with grazingabandonment. The study site, Brocon, is a mountain alpine grassland located 1700 m abovethe sea level in the eastern Alps (Cinte Tesino, Italy). The site is historically usedfor the summer cattle grazing, alpeggio. Here, the local farmers apply differentmanagement regimes: short and more intensive grazing; long and less intensivegrazing; and no grazing, where a portion of pasture has been fenced from cattle since2002. Soil and all plant species were sampled in the end of July in managed and abandonedplots. Diversity of plant species was assessed through Shannon and Simpson indices andspecies richness. Soil quality was evaluated by analyses of the following biochemicalparameters: enzymes involved in C, N, S and P cycles, microbial C and N, microbial basalrespiration and ecophysiological quotients: qmic, qCO2 and qN. Microbial functionaldiversity was assessed by means of Shannon index. Plant and soil samples were analyzed fortheir δ13C and δ15N in order to quantify changes in nutritional status, water and carbon useefficiency. Changes in soil structure were evaluated by means of aggregate stability and sizedistribution. The results demonstrated considerable changes in plant nutritional status and plantspecies richness with loss of numerous plant species in plots excluded from grazing. Changesin ecosystem C and N cycles were confirmed at the soil biochemical level. 15 years ofgrazing abandonment resulted in the deterioration of the soil chemical and physical propertieswith ongoing and future tendency to loss of C from the system. Lowering of the labile Cinputs combined with soil acidification affected negatively the microbial biomass pool inabandoned plots. On one hand, it slackened the biochemical cycles, on the other hand, itshifted microbial community to decomposition of older and more protected SOM.Impoverishment of the area and accumulation of the nitrogenous forms favored thedevelopment of nitrophilous and synanthropic species, not allowing for more demandingplant species to colonize the area. Prolonged and less intensive grazing appeared to be themost appropriate management practice for both, vegetation compartment and soil. [ABSTRACT FROM AUTHOR]