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

Highlights

• Grazing effects were only observed at bare soil i.e. ca. 1% of the grazed area.

• Extractable OC and available N were higher under bare soil conditions.

• Short-term re-grazing did not affect the microbial abundance.

• Concentrations of DOC and nitrate in the draining creek remained very low.

• The studied alpine soils show an initial resistance to extensive re-grazing.

Abstract

Grazed alpine pastures have shaped landscapes of the European Alps for millennia, but have partially been abandoned since the 1950s. Re-grazing of abandoned pastures could preserve this cultural landscape with its high species diversity, but also alter soil carbon and nitrogen cycles, as well as microbial communities, potentially affecting ecosystem services (e.g., water purification, carbon and nitrogen storage). However, there is a lack of information on the resistance of soil characteristics to re-grazing effects. After characterising the distribution of vegetation types of an abandoned pasture in the German Alps, we investigated short-term effects of re-grazing on soil organic carbon and nitrogen biochemistry, soil microbial communities, and water quality along a gradient of grazing intensity. The abandoned grassland studied presented a remarkably high diversity of species and habitats even 60 years after abandonment. It was also found to be particularly rich in terms of microbial biomass, as well as in soil carbon and nitrogen. 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 (i.e. concentration of cows with excreta inputs), which insignificantly contributed to the overall area. Re-grazing did not affect the microbial abundance, whatever the grazing intensity, but induced a community shift towards a smaller proportion of fungi compared to bacteria and an increase of ammonia oxidizers (archaea/bacteria) under bare soil conditions. 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 resistance of the studied alpine soils to extensive re-grazing. Our results indicate how to develop sustainable management strategies that preserve alpine pastures from degradation.

Introduction

Traditionally, grazed pastures (called “Alm” or “Alp” in the Alpine region of central Europe) have been shaping big parts of the landscapes of the European Alps for millennia (Ringler, 2010; Bätzing, 2015). The European Alps are recognized as one of the six key biogeographical zones for plant and animal species, as well as habitats in the European Habitats and Species Directive (92/43/EEC) (MacDonald et al., 2000). Grasslands of the Alps also play an essential role in providing ecosystem services such as carbon (C) and nitrogen (N) storage, water purification, as well as aesthetic and recreational values (Lamarque et al., 2011; Schirpke et al., 2016; Vigl et al., 2016; Kohler et al., 2017; Kühnel et al., 2019). However, alpine pastures, especially those on steep slopes, have largely been abandoned since the 19th century (abandonment rate >60%) (Zimmermann et al., 2010) mainly due to the intensification of agricultural practices in the valleys and for economic reasons (Gellrich et al., 2007). Without grazing, the highly diverse plant communities of pastures are replaced by tall grass swards and ultimately by secondary forest. These changes in plant communities often lead to a decline in biodiversity and conservation value (Tasser and Tappeiner, 2002; Maurer et al., 2006; Spiegelberger et al., 2006; Kampmann et al., 2008; Peco et al., 2012), although the magnitude of decline is highly variable depending on the landscape context (Zimmermann et al., 2010).

The impacts of abandonment and re-grazing on soil functions, and especially soil organic matter (SOM) quality, storage, and turnover are still uncertain and vary according to grassland type, time since land use change and management intensity (Meyer et al., 2012; Rumpel et al., 2015). Grazing cessation in grasslands of the Alps tends to increase aboveground plant biomass (Gamper et al., 2007; Tappeiner et al., 2008; Puissant et al., 2017). The resulting litter accumulates due to its low decomposability (Zeller et al., 2001; Gamper et al., 2007) and less labile C is transferred to the rhizosphere (Karlowsky et al., 2018). The low quality of the litter (e.g., high C:N ratio and lignin content) in abandoned grasslands generally leads to a decrease of microbial biomass (Bardgett et al., 1997) and an increase of the fungal-to-bacterial ratio (Donnison et al., 2000; Zeller et al., 2001; Sjögersten et al., 2011). Furthermore, abandonment favoured spontaneous reforestation of high elevation pastures in recent decades, a trend that is accelerated by climate change (Tasser et al., 2007; Schirpke et al., 2017). Reforestation of mountain pastures would lead to a decrease of the forage quantity, the overall plant and animal biodiversity including habitat loss of endangered animal species such as grouse, as well as potential decrease of touristic values (Sitzia et al., 2010; Schirpke et al., 2017). Although spontaneous reforestation could possibly help to mitigate climate change via C storage mainly in the above-ground biomass, it can induce a critical loss of biodiversity, as highlighted in the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) report (Scholes et al., 2018).

The above-mentioned studies mostly reported on changes occurring after abandonment. However, very few papers addressed the resistance of ecosystems, i.e. the capacity of “staying essentially unchanged despite the presence of disturbances” (Grimm and Wissel, 1997), under future land use change scenarios (Pykälä, 2003; Schirpke et al., 2017; Tasser et al., 2017). Kohler et al. (2017) highlighted the development of extensive pastures by re-grazing as a plausible land-use option to preserve the high species diversity of the Stubai Valley in Austria. In addition, grazing with robust and slow growing cattle can also induce an increase of plant species richness (Pauler et al., 2019). While current knowledge of effects of extensive grazing is largely derived from abandonment studies, there is a lack of information on the effects of extensive re-grazing of abandoned mountain pastures on vegetation and soil characteristics, biogeochemical cycles, and microbial communities. Grazing of formerly abandoned grasslands can alter the physical properties of soil via trampling, generally leading to a compaction of soil (Steffens et al., 2008; Wen et al., 2016). Further changes in nutrient cycling and thus soil fertility are expected due to grazing. The effects of grazing on soil fertility are either direct via dung- and urine-borne nutrient inputs and nutrient redistribution (Peco et al., 2006, 2017), which increase the amounts of rather labile organic matter and N in soil (Bardgett and Wardle, 2003; Abdalla et al., 2018), or indirect via the modification of plant species composition, richness, and functional structure (Bardgett and Wardle, 2003; Valencia et al., 2015). Soil fertility also affects soil microbial communities whereby animal excreta inputs tend to increase microbial biomass and activity (e.g., Bardgett et al., 1997). Consequently gross N mineralisation and nitrification, and thus mineral N availability as well as N losses along hydrological and gaseous pathways, can increase (Bardgett et al., 1997; Mikola et al., 2009; Cai et al., 2017; Peco et al., 2017). However, depending on management, soil and climate, the grazing-induced increase of labile organic matter and nutrients does not necessarily lead to nutrient leaching at the expense of the water quality (Tamminga, 2003; O’reagain et al., 2005; Wu et al., 2011) or to increased soil nitrous oxide emissions (Wolf et al., 2010). While spatio-temporal variations (e.g. soil temperature and moisture, soil parameters) tend to have an overall greater effect on soil microbial biomass compared to grazing (Bardgett et al., 1997, 1999; Zeller et al., 2001), grazing or aboveground biomass harvest can significantly alter the microbial community structure and decrease the fungal-to-bacterial ratio (Bardgett et al., 1996; Zeller et al., 2001; Fuchslueger et al., 2019). The aforementioned studies exposed the impact of grazed vs abandoned pastures on soil and water characteristics but did not address how re-grazing of abandoned alpine grassland would affect soils.

Our study investigated short-term effects of extensive re-grazing of a long-term (> 60 years) abandoned mountain pasture in the German Alps (Brunnenkopfalm: 1500−1700 m.a.s.l.) on multiple soil characteristics. After characterising the distribution of vegetation types and the spatial heterogeneity of grazing, we examined re-grazing effects on soil microbial communities, C and N turnover and contents, as well as water quality. We hypothesised that re-grazing will (1) enhance the amount of extractable organic matter due to animal excreta inputs and trampling, (2) increase the abundance and alter the structure of microbial communities, resulting in (3) functional changes of the N cycle with increased gross N mineralisation and nitrification, leading to an increased mineral N availability in the soil. This will eventually result in increasing dissolved organic C (DOC) and nitrate (NO₃⁻) loads in the creek water draining the grazed area. 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 C (OC), gross N turnover rates, soil mineral N availability), as well as the abundance and characteristics of microbial communities (microbial biomass, phospholipid-derived fatty acids (PLFA) analysis, abundance of N-related microbial communities).