Above-ground grazing affects floristic composition and modifies soil trophic interactions

Abstract There are few data on the functional inter-relationships between above- and below-ground components of soil ecosystems. Here, we report changes in below-ground soil invertebrate trophic relationships (manifested as alterations in stable isotope natural abundances, δ 13 C and δ 15 N) that arose in association with the removal of sheep grazing and from the resulting changes in above-ground floristic composition. Consequent to grazing removal, Lolium perenne L. (perennial rye-grass) was replaced as the dominant plant species in ungrazed treatments by Ranunculus repens L. (creeping buttercup), a species with more 13 C-enriched foliage. Consequently, all invertebrate functional groups studied, but not whole soil, were more 13 C-enriched in ungrazed treatments. Earthworms (detritivore) from grazed treatments were significantly 15 N-enriched compared with earthworms from ungrazed treatments. In contrast, slug (herbivore) δ 15 N exhibited no treatment effect. Reasons for this are unclear but may be related to the effects of above-ground grazing on the composition of below-ground microbial/microfaunal communities. Omnivores/carnivores (beetles and spiders), were more 15 N-enriched than primary producers in the grazed than in the ungrazed treatments (6 vs. 4‰) suggesting a longer below-ground foodchain in the grazed plots. The cessation of fertilizer application had no comparable effects on below-ground trophic relationships.

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