The relevance of grazing exclusion in different ecological contexts for SOM and soil microbial biodiversity

In SE Spain, an ecological laboratory spanning a national park and adjacent managed pasture site has offered an opportunity for understanding soil properties and biodiversity responses to ungulate grazers, thanks to grazing exclusion areas. Since grazing has the potential to impact soil processes, including SOM dynamics indirectly through modulation of the soil microbial community, we have monitored changes in the soil microbial community through fatty acid methyl ester (FAME) profiling and genomic diversity through extraction and sequencing of fungal and bacterial DNA.

Analyses show that SOC is higher in the forested landscape, but the modulation of SOC depends on the presence of grazers, increasing by up to 0.8 % in the pasture area, and decreasing by up to 1.3 % in the forested park. Soil chemical and biochemical properties are better explained by context (forested or pasture landscapes), not grazing exclusion. Moreover, total microbial biomass, evaluated using FAMEs, shows that, compared with the baseline of 1032 nmol g^-1 soil, grazing tends to increase microbial fatty acid (FA) concentrations by 137 nmol, explained by total bacterial FA (72 nmol) and gram-negative FA (38 nmol). Though not significant, there is also a tendency towards greater fungal:bacterial ratios in the forested area (p=0.08).  Variance partitioning analyses of 𝛼-diversity show that soil variables and context explain more variability in fungal and bacterial diversity than grazing exclusion. Further analyses will explore potential pathways of influence and relevance of specific fungal guilds for SOM dynamics.