Soil nematode assemblage responds weakly to grazer exclusion on a nutrient-rich seabird island

The effects of aboveground herbivores on plant-soil interactions are highly context dependent and a key underlying factor controlling this is thought to be nutrient availability. Here, we tested whether the effects of vertebrate grazing on the soil food web varied with nutrient availability and hypothesised that soil food web structure would be driven more by the exclusion of vertebrate grazers than by nutrient enrichment. An 8-year long grazer exclusion experiment was performed in grasslands on a small Scottish island near soil nutrient-enriching seabird colonies at the coast and in less fertile conditions inland. We investigated the trophic structure of the soil nematode assemblage as a proxy for soil food web structure. Across all eight study sites the bacterial energy channel was predominant over the fungal channel. Grazer exclusion strongly enhanced plant biomass accumulation and although this tended to be associated with a somewhat lower abundance of bacterial-feeders, this effect was non-significant and surprisingly weak given the observed changes aboveground. Indeed, plant species identity, diversity and dominance were, just as any other vegetation descriptor, weak predictors of nematode trophic structure. Instead, site specific conditions were important, despite the small island area and apparently homogenous sampling conditions.