LAUSR

We conducted a microcosm experiment with soil being sterilized, reinoculated with native microbial community and subsequently manipulated the bacterivorous nematodes, including three treatments: without (CK) or with introducing one species of the two bacterivores characterized with different body size but similar c-p (colonizer-persister) value (Rhabditis intermedia and Protorhabditis oxyuroides, accounted for 6 and 59% of bacterivores in initially undisturbed soil, respectively). We monitored the N2O and CO2 emissions, soil properties, and especially quantified gross N transformation rates using 15N tracing technique after the 50 days incubation. No significant differences were observed on soil NH4+ and NO3− concentrations between the CK and two bacterivores, but this was not the case for gross N transformation rates. In comparison to CK, R. intermedia did not affect soil N transformation rates, while P. oxyuroides significantly increased the rates of mineralization of organic N to NH4+, oxidation of NH4+ to NO3−, immobilization of NO3− to organic N and dissimilatory NO3− reduction to NH4+. Furthermore, the mean residence time of NH4+ and NO3− pool was greatly lowered by P. oxyuroides, suggesting it stimulated soil N turnover. Such stimulatory effect was unrelated to the changes in abundance of bacteria and ammonia-oxidizing bacteria (AOB). In contrast to CK, only P. oxyuroides significantly promoted soil N2O and CO2 emissions. Noticeably, bacterivores increased the mineralization of recalcitrant organic N but decreased soil δ13C-TOC and δ15N-TN values, in particular for P. oxyuroides. Combining trait-based approach and isotope-based analysis showed high potential in moving forward to a mechanistic understanding of bacterivore-mediated N cycling.