Effects of Earthworm Density on Greenhouse Gas Emissions from Constructed Wetlands with Different Plant Configurations

Abstract

Plant configuration and earthworms play an important role in water purification and greenhouse gas emissions in constructed wetlands (CWs). However, the impact of earthworm density on greenhouse gas emissions across different plant configurations has not been explored. In this study, four wetland plant species, Canna indica, Lythrum salicaria, Oenanthe javanica, and Typha orientalis, were selected for monocultures. Under each monoculture, three earthworm densities (control, low, and high densities) were conducted to explore the effects of earthworm density on greenhouse gas emissions in CWs with different plant configurations. The results showed that: (1) in systems without earthworms, the CO₂ emission from O. javanica monoculture was 69.9% lower than that from C. indica monoculture; the CH₄ emission decreased with the increasing earthworm density across all plant configurations, with high earthworm density resulting in negative CH₄ emission. (2) In systems with low and high-density earthworms, C. indica exhibited the highest biomass among four monocultures. However, earthworm density did not significantly affect plant biomass under the same plant configuration. (3) In systems without earthworms, the substrate organic carbon (SOC) of O. javanica monoculture was 18.94% and 4.93% lower than that in T. orientalis and C. indica monocultures, respectively; For L. salicaria monoculture, the SOC was 35.69% and 40.59% lower in systems without earthworms compared to those with low and high-density earthworms, respectively. (4) In systems without earthworms, the global warming potential (GWP) value, including GWP_{CH4+CO2+N2O+SOC}, GWP_{non-CO2+AGB+SOC}, and GWP_{CH4+CO2+N2O+AGB+SOC} were lowest in L. salicaria monoculture among four monocultures. Moreover, in L. salicaria monoculture, the GWP_non-CO2+SOC of systems without earthworms was 36% and 40.7% lower than in systems with low and high-density earthworms by, respectively. These results indicate that adding high-density earthworms can reduce CH₄ emissions in constructed wetlands with different plant configurations. L. salicaria monoculture without adding earthworms demonstrated a low global warming potential.