Earthworms increase soil carbon dioxide emissions through changing microbial community structure and activity under high nitrogen addition

Nitrogen (N) enrichment has been shown to increase, decrease or have no effect on soil carbon dioxide (CO2) emissions via changing microbial activity and community composition. Earthworms are known to substantially influence soil microbes, and to thus CO2 emissions, potentially fueling an interactive effect with N enrichment. Yet, we have a limited understanding of how earthworms interact with N enrichment to influence CO2 emissions from arable soil. Here, we investigated how manipulations of N enrichment and earthworms influenced CO2 emissions over 91 days by using a mesocosm experiment. Our results showed that N enrichment gradient changed the effects of earthworms on soil CO2 emissions. Specifically, earthworms did not influence soil CO2 emissions under lower N addition rates, however, earthworm presence did increase soil CO2 emissions at levels of 240 kg N ha-1 and higher, mainly by their top-down controls on microbes, i.e., reducing fungi:bacteria ratio, and enhancing microbial C-acquiring activities, as evidenced by a positive relationship between soil CO2 emissions and glycosidase activity. Together, we suggest that N enrichment and key soil fauna-microbe interactions can determine the degree to which microorganisms regulate soil C dynamics. Furthermore, our findings potentially advocate for the specific soil fauna effects on soil CO2 emissions mediated by N enrichment as a mean to reduce the unexplained variation in large-scale biogeochemical models.