Context-dependent responses of soil microbes and nematodes to grazing: Evidence from a global meta-analysis

Livestock grazing is one of the most important land-use activities in
grassland ecosystems, imposing significant impacts on soil community
abundance, composition, and biodiversity. Soil microbiota, such as
microbes and nematodes, are key components of soil micro-food webs and
contribute substantially to ecosystem functioning. However, it remains
unknown how grazing affects soil microbiota on a global scale. In this
study, we conducted a meta-analysis based on 146 published studies
encompassing 1,560 paired observations across various environments to gain
comprehensive insights into the influences of grazing on the abundance and
diversity of soil microbes and nematodes. We found that grazing
significantly decreased soil microbial biomass, total nematode abundance,
and the abundance of all nematode trophic groups except for
fungal-feeders, particularly in temperate grasslands. Such effects were
stronger under continuous, heavy-intensity, and long-term grazing than
under rotational, light or short-term grazing regimes. Sheep grazing, as
opposed to cattle grazing or mixed grazing by both sheep and cattle,
reduced the abundance of soil microbes and nematodes. While the overall
effects of grazing on microbial and nematode diversity are limited,
significant declines were observed under heavy, continuous, or long-term
grazing compared to light, rotational, or short-term grazing. Synthesis
and applications: Our study indicates that intense (e.g., heavy or
continuous) grazing, long-term grazing or grazing by sheep substantially
suppresses the abundance and diversity of soil microbes and nematodes.
These declines in abundance and diversity would undermine the roles of the
soil food webs, thereby impairing ecosystem processes such as organic
matter decomposition and nutrient cycling. Therefore, maintaining light to
moderate grazing intensity, implementing rotational grazing or extended
rest periods, and prioritizing cattle or mixed-livestock grazing are
essential to protect soil food webs and maintain grassland ecosystem
functions under increasing human pressures.