Drivers of belowground β-diversity
In grasslands, extensive grazing generally leads to spatial
heterogeneity in aboveground vegetation (Adler et al. 2001) where
different patches represent different phases on a successional gradient
(Olff et al. 1999). Cessation of grazing thereby becomes a
homogenizing factor that pushes patches towards a climax stage of
generally lower aboveground β-diversity (Olff & Ritchie 1998). As a
result, one might expect belowground β-diversity to exhibit a similar
decrease in response to cessation of grazing (i.e. greater
homogenization in community composition). However, in contrast to the
consistent negative responses for ɑ-diversity, our results show
remarkably mixed responses for β-diversity of different groups of
belowground biota. We observed a strong decline in β-diversity for
eukaryotic soil microbes (fungi, protists), no change in β-diversity of
prokaryotic soil microbes, and an in increase in β-diversity for
springtails, mites and nematodes. These differences in response between
larger bodied and smaller bodied soil organisms may result from
differences in the sensitivity of these groups of organisms to shifts in
plant species composition and/or changes in soil physical parameters
that happen as a result of grazer removal. A plethora of studies has
shown that soil microbial community composition is strongly related to
the composition of the plant species community (Grayston et al.1998; Kowalchuk et al. 2002; Berg & Smalla 2009; Leff et
al. 2018), as many microbial taxa are directly dependent on carbon
sources (e.g., exudates and litter) from plants. Soil animals are also
ultimately dependent on plant-derived carbon, but, due to their greater
mobility and size, may also be affected by other environmental factors
that change in response to the cessation of grazing. We therefore
propose that the changes in vegetation community and the accompanying
changes in root exudation patterns, litter quality, local changes in
pH-gradients (Fierer & Jackson 2006; Rousk et al. 2010) as well
as variation in litter recalcitrance (Freschet et al. 2012) may
explain the observed differences in β-diversity for soil microbial
groups, whereas the observed physical differences in soil properties,
soil organic matter and soil structure, and the spatial variation
therein might be more important for the spatial distribution larger
bodied species (Ettema & Wardle 2002; Quist et al. 2019).
A last remaining question is why β-diversity of soil fauna actuallyincreases in response to cessation of grazing. Here, we speculate
that this might be due to increased cover and patch size of mid-late
successional (clonal) plant species, and the observed coinciding
increase in air filled porosity. Indeed, clonal plant species
characteristic of mid to late successional stages (e.g., C.
vulgaris , Pteridium aquilium. Molinia caerulea , D.
cespitosa ) were more abundant in the abandoned plots. These species are
generally associated with more complex food webs and a greater abundance
of higher trophic levels as a result of larger belowground carbon inputs
(Morriën et al. 2017). Patches that consist of different
functional groups (heather species, legumes, grasses, other shrubs,
ferns, mosses) can differentially affect the diversity of organisms
through changes in resource supply and other abiotic properties (Wikberg
& Mucina 2002; Ward et al. 2015). Evidence for the idea that
such patches are associated with different soil communities comes from
vegetation removal experiments, which show that removal of entire
functional groups has major effects on belowground species composition
and functioning (Ward et al. 2009). In our study sites, this
increasing patch size is exemplified by the replacement of species rich
grasslands dominated by short grasses and herbs by clonal growth of
patch forming species such as ferns (e.g., P. aquilinum ), dwarf
shrubs (e.g., C. vulgaris, V. myrtillus, E. tetralix ) and tall
grasses (e.g. M. caerulea and D. cespitosa ). This
change in patchiness in the vegetation after cessation of grazing may
thus result in increased medium-large scale spatial heterogeneity. We
hypothesize that the resulting divergence in belowground communities may
start once the lack of grazing permits these clonal structures to become
locally dominant. Although more rigorous experiments are needed to test
the relative importance of these different possible mechanisms, we
conclude that β-diversity of soil fauna and soil microbial taxa respond
markedly differently to cessation of grazing and that changes in
vegetation properties likely underlie these patterns.