Earthworm effect on rhizosphere N-cycle microbial genes depends on soil properties

Earthworms are known to improve plant growth in a soil-dependent way, notably via modifications of the rhizosphere microbiota and its functions. We tested the hypothesis that earthworms influence the abundance of microbial genes involved in N cycle according to the type of soil. In three soils with contrasting texture, we quantified five N-cycling genes in different microsites (bulk, rhizosphere or earthworm casts) of microcosms containing (i) neither plants nor eathworms, (ii) plants, (iii) earthworms, (iv) both plant and earthworms. In the presence of earthworms, rhizophere was enriched in nifH (N₂ fixation) and depressed in nosZ or narG (denitrification) in sandy soil, suggesting a shift in N balance towards immobilization; rhizosphere was enriched in nifH but also nosZ and narG in loamy soil; no effect was detected in clayey soil. The pattern of gene abundance across the different soils and microsites suggests that earthworms could favor microorganisms with a potential beneficial effect on plants specifically in sandy soils.