High resilience of soils to re-grazing in a long-term abandoned alpine pasture

Grazed alpine pastures have shaped landscapes of the European Alps for millennia. However, especially steep alpine areas have largely been abandoned since the 1950s, resulting in a fast re-forestation of mountain pastures in the last decades, which is accelerated by climate change. Re-grazing of abandoned pastures could preserve the cultural landscape of the European Alps with its high species diversity, but there is a lack of information on the response of the soil system to re-grazing. We investigated short-term effects of re-grazing of an abandoned pasture in the German Alps on soil organic carbon and nitrogen biochemistry, soil microbial communities, and water quality. In May 2018, we set up a pilot grazing experiment at Brunnenkopfalm (1500-1700 m a.s.l.), abandoned since 1955. Four ha were fenced and a herd of rustic, local and endangered breeds (ca 1/ha) was introduced. Two and five months after the beginning of grazing, we investigated the short-term re-grazing effects, considering grazing-induced heterogeneity, as well as the distribution of vegetation types. In order to gain a functional understanding of soil responses to re-grazing, we used a wide array of techniques to characterize soil biogeochemical properties (salt-extractable and total organic carbon, gross nitrogen turnover rates, soil mineral nitrogen availability), as well as the abundance and characteristics of microbial communities (microbial biomass, phospholipid-derived fatty acids analysis, abundance of nitrogen-related microbial communities). A few months after re-grazing started, extractable organic carbon, gross nitrogen mineralisation rates and inorganic nitrogen concentrations were increased only in intensively grazing-affected areas with bare soil. Bare soils represented a small fraction of the study area (~ 1 %), and the grazing effects on these areas could at least partially also be driven by the initial site heterogeneity (soil and vegetation) rather than solely by recent grazing activities. Re-grazing did not affect the microbial abundance, but induced a community shift towards a smaller proportion of fungi compared to bacteria and an increase of ammonia oxidizers (archaea/bacteria). Concentrations of dissolved organic carbon and nitrate in the draining creek remained very low. Overall, re-grazing of pastures in the first season had very limited effects on microbial communities and associated carbon and nitrogen turnover and concentrations, highlighting the initial resilience of alpine soils to extensive re-grazing. However, a slight increase in nitrifier abundances at bare soil spots, as well as the low organic carbon:nitrogen ratios of soils suggest that a future increase in inorganic nitrogen accumulation is possible at least at bare soil areas. This could possibly endanger some biodiverse grassland biotopes via eutrophication and result in environmental nitrogen losses along hydrological or gaseous pathways. Thus, long-term studies are needed to verify whether soils are also resilient to re-grazing in the long-term. On the short-term, undesired re-grazing effects can be avoided by extensive, guided grazing with adapted cattle breeds targeted to avoid trampling-induced bare soil areas.