Earthworms mediate the effects of nitrogen addition on nitrous oxide and carbon dioxide emissions

Background/Question/Methods: Increased nitrogen (N) via deposition and chemical fertilization affects the pattern of greenhouse gas (GHG) emissions, such as stimulating nitrous oxide (N₂O) while restraining carbon dioxide (CO₂) emissions. Moreover, current models of soil N₂O and CO₂ emissions in responses to N enrichment have been developed so far across various scales, but the roles of soil biota such as soil fauna in mediating the effects of N additions remain unclear. Through a 2 × 5 full-factorial experiment manipulating earthworm (with and without) across five N input levels (0, 60, 120, 240 and 480 kg N ha^-1), the dynamics of N₂O and CO₂ emissions and soil abiotic/biotic properties were determined twice during the whole incubation experiment (i.e. 5 and 13 weeks after earthworm introduced).

Results/Conclusions: Results showed that earthworms aggravated the positive effects of N additions on N₂O emissions regardless of N doses and sampling time. The effects of N additions on CO₂ emissions changed from a positive to a neutral with the experiment duration extended, earthworms can reverse the N addition effects on CO₂ emissions from neutral to positive when N additions beyond 240N. Structural equation models indicated that the earthworm increased the extent of N effect on N₂O was mainly attributed to their indirect effects on abundance of denitrifiers. However, the earthworm-mediated CO₂ emission pattern was due to their effects on microbial biomass and community composition. Furthermore, earthworms may modulate the effects of N additions on N₂O and CO₂ emissions through shifting soil C and N availability and abundance and activities of microbes and strategy for C acquisition of microorganisms (C-degrading enzymes). In conclusion, it is essential to incorporate soil fauna into a biogeochemical framework via systematic understanding the process through which N inputs impacts on soil C and N cycling in ecosystems under future climate change scenarios.