Soil fauna alter the responses of greenhouse gas emissions to changes in water and nitrogen availability

Fertilization and drought are two of the most important global change drivers that impacting greenhouse gas (GHG) emissions. Soil organisms are among the fundamental biotic drivers of biogeochemical cycles and can play critical roles in mitigating global change. However, the contributions of soil macrofauna in explaining the responses of GHG emissions to fertilization and drought remain poorly understood. Here, we designed a three-factor microcosm experiment to examine how soil macrofauna (no fauna, earthworms, and millipedes) alter the responses of CO2, N2O, and CH4 emissions, as well as the C and N contents in response to contrasting levels of N (N0: without N addition, N+: N addition) and available soil water (40% and 60% of soil water holding capacity). We show that soil fauna were significant regulators of CO2 and N2O emissions in response to changes in water and N availability, as supported by multiple identified statistical interactions. Millipedes were observed to reduce the positive influence of soil water availability on soil CO2 emissions in response to the addition of N. Similarly, earthworms weakened the effects of elevated N and water availability on soil N2O emissions. Moreover, CH4 emissions occurred only when millipedes were present. The structural equation models revealed that earthworms and millipedes modified soil CO2 and N2O emissions through their influences on soil total dissolved nitrogen and microbial biomass carbon. Overall, this study demonstrated that soil macrofauna can notably mediate the responses of GHG emissions and soil biogeochemical cycles to global environmental changes.