Shifts in size,genetic structure and activity of the soil denitrifier community by nematode grazing

Bacterial-feeding nematodes represent an important driver of the soil microbial activity and diversity. This study aimed at characterizing the impact of nematode grazing on a model functional bacterial guild involved in N-cycling, the denitrifiers. Bacterial-feeding nematodes (Cephalobus pseudoparvus) were inoculated into soil microcosms whose indigenous nematofauna had previously been removed. The size, genetic structure and activity of the soil denitrifier community were characterized 15 and 45 days after nematodes inoculation using quantitative PCR of the nirK, nirS and nosZ denitrification genes, fingerprinting of the nirK and nirS genes and denitrification enzyme activity measurements, respectively. A significant impact of C. pseudoparvus was observed on genetic structure of the nirK community, mainly due to shifts in the relative abundance of the dominant populations, but not on the nirS community. The grazing pressure also tended to decrease the density of all denitrification genes as well as that of 16S rRNA genes. Despite being non-significant, the extent of this decline in gene copy numbers ranged between 60 and 80% of the control microcosm genes densities. Finally, compared to non-inoculated microcosms, denitrification activity significantly decreased by 8% in response to the nematodes inoculation. The herewith data showed that predation by a single species of bacterial-feeding nematode can affect the soil denitrifier community.