Earthworm invasion is associated with reduced soil nitrification potential and a decline of active nitrifiers identified by DNA-stable isotope probing

Invasion by exotic earthworms into formerly glaciated, earthworm-free soils has huge impacts on forest ecosystems. Recent studies have revealed that invasive earthworms could alter the soil microbiome and reduce the abundance of nitrification-related genes. However, gene data alone cannot confirm this decrease, as nitrifiers detected by microbiome analysis are not necessarily active or responsible for soil nitrification. Here, by combining field microbiome analysis, nitrification potential assays, and DNA stable isotope probing (SIP), we examined how earthworm invasion influences nitrification in a hardwood forest in Minnesota, USA. Soils heavily invaded by earthworms had markedly lower nitrification potential, particularly in the surface layer, than minimally invaded soils. In the field soils, archaeal amoA abundance was substantially lower in heavily invaded than in minimally invaded soils, whereas bacterial amoA and nxrB abundances showed no significant differences. Within the ammonia-oxidizing archaeal community, the relative abundance of “ Candidatus Nitrosocosmicus” was lower in heavily invaded than in minimally invaded soils. However, based on the 13 C-DNA SIP, “ Ca. Nitrosocosmicus” was identified as an active nitrifier in both heavily and minimally invaded soils. These results suggest that the low nitrification potential in heavily invaded soils is likely due to the lower abundance of “ Ca. Nitrosocosmicus” than in minimally invaded soils. Overall, our findings indicate that the differences in exotic earthworm invasion between the two sites were associated with soil nitrification potential and nitrifier community structure, highlighting belowground microbial functions as a potentially overlooked dimension of invasive-earthworm impacts.