Discussion
Our results show strikingly consistent patterns in the response of
ɑ-diversity of soil microbial and soil fauna groups to cessation of
grazing. A number of key soil fauna and microbial groups, with the
exception of soil bacteria, showed a marked decrease in ɑ-diversity
following grazer exclusion, which also coincided with a marked decline
in local plant species richness. The decline in plant species richness
with grazer exclusion is consistent with previous studies showing that
extensive grazing generally has a positive effect on local plant
diversity (Olff & Ritchie 1998; Bakker et al. 2006; Epeldeet al. 2017). However, we found no evidence that plant species
richness itself was a prominent proximate driver of changes in
belowground α-diversity. Rather, our analysis showed that other factors,
especially soil pH, carbon concentration, soil moisture, bulk density,
litter biomass and aboveground biomass, were the most important
determinants of grazer exclusion-induced changes in belowground
α-diversity, largely consistent with previous studies showing that
habitat characteristics are important determinants for the effect of
grazer exclusion on soil communities in grasslands (e.g. (Bardgettet al. 1993; Bardgett et al. 1997; Bardgett et al.2001; Epelde et al. 2017; Oggioni et al. 2020).
Nevertheless, we speculate that, although plant communities were not
identified as a direct driver of soil fauna communities, they may be one
of the ultimate drivers underlying these patterns in α-diversity. For
example, we observed a shift towards fern- or dwarf shrub-dominated
vegetation in many of the abandoned plots on acid soils, which is often
associated with reduced soil pH and increased litter mass
(Johnson-Maynard et al. 1998). Such vegetation-induced changes in
soil biotic and abiotic conditions would then be the proximate driver of
the observed changes in soil communities. This suggests that, rather
than changes in plant species diversity per se , shifts in plant
species composition ultimately drive the observed patterns in local
belowground richness.
At the local scale, effects of cessation of grazing on belowground
species were even more pronounced for relatively rare than for
relatively widespread and common species. These results are consistent
with McKinney & Lockwood’s (1999) original idea of ‘biotic
homogenization’ and (scenario A & F in Figure 1): relatively rare
species suffer more from land use change than relatively common or
widespread species. We speculate that a mix of drivers may be
responsible for this pattern. First, an important driver may be the loss
of certain (rare) plant species from the areas where grazing was halted.
Different plant species are known to selectively influence community
composition in their rhizosphere (Bezemer et al. 2010; Leffet al. 2018). For example, most short grass species (e.g.,Cynosurus cristatus, Agrostis stolonifera ), legumes (e.g.,Trifolium repens ) and short herbs (e.g., Bunium
bulbocastanum ) decreased strongly or disappeared altogether after
grazing exclusion. In addition, the selective loss of relatively rare
species might result from a decrease in local heterogeneity after
grazing exclusion as a result of a lack of small scale trampling
(Sørensen et al. 2009) or a lack of local defecation (Augustine
& Frank 2001). Because of a lack of ecological information about
specific plant-microbe and plant-fauna relationships and changes in
patterns of local heterogeneity, it is impossible to provide conclusive
evidence for each of the two hypotheses. As both processes typically
coincide with removal of grazing (Adler et al. 2001; Pykälä
2005), it is likely that the resulting pattern can be generalized to
other systems: cessation of grazing results in the loss of belowground
species richness through the combined effect of a loss of local
heterogeneity and local plant species richness.