Effects Of Earthworms On Greenhouse-gas Emissions In The Sanjiang Plain,Northeastern China

Wetland soil is an important source and sink, as well, of greenhouse gases （GHGs）. Earthworms are a major component of the soil fauna, and are significant players of soil matter recycling. As a soil animal the highest in biomass in the soil, earthworms directly or indirectly affect the generation and emission of GHGs （CO₂, N₂O and CH₄） in the soil. The earthworm density, biomass and species of circled depressions in Sanjiang Plain were studied by quadrate investigation in September 2014 and May 2015, respectively represents the season of autumn and spring. And the relationships between earthworm distribution and environmental factors were analyzed as well. Furthermore, microcosm experiments were conducted to investigate the effects of earthworms on GHGs emissions from wetland soil under oversaturated circumstances, drying-rewetting cycles and warming.We found in total 3 earthworm species in the marshes of the circled depressions in Sanjiang Plain, including Eisenia fetida, E. nordenskioldi nordenskioldi, and E. harbinensis. All of the these species are epigeic, belonging to Lumbricidae Claus （family）, Eisenia Malm （genus）, and the earthworm density in the 3 sample areas ranges from 9 to 90 ind./m2. The density, biomass as well as richness of earthworms in circled depressions showed much higher values compared to those in the reclaimed soybean fields. It is clear that the earthworm density and biomass are higher in spring than in autumn. E. fetida is the dominant species in natural wetlands, and as the soil water content increase from the edge to the center of circled depressions, earthworm density decreases along with wetland type series "island forest marsh- shrub marsh- Calamagrostis angustifolia meadow marsh". Furthermore, soil water content is the fundamental factor for earthworm distribution, while the earthworm density positively correlates to TOC and pH, the biomass positively correlates to TOC and TN of soil, flooding and recession drove the earthworms to migrate.The emission of CO₂ in wetland soil presented a process "decreasing, increasing, decreasing, and keep stable". Overall, earthworms accelerated the CO₂ emission, the effects mainly appeared in the intermediate phase during the incubation as increasing the emission rate of CO₂ at most by 87.06%, and the effect of earthworms was in positive correlation to inoculation density. The emission of CH₄ in wetland soil increased during the incubation, earthworms accelerated the CH₄ emission rate by 207%, and the effect of earthworms was in positive correlation to incubation time and inoculation density. The emission of N.2O in wetland soil increased at first and decreased afterwards, in the earlier and intermediate stage of the incubation earthworms increased the N.2O emission rate by 4 times, while as time went on the increasing effect of earthworms weakened and even decreased the N₂O emission.Soil water content was an important factor of GHGs emissions from wetland soil. CO₂ emission increased as soil water content; the net CH₄ release happened under oversaturated circumstances, while net CH₄ uptake occurred in 45%WFPS; the middle soil water content which was near saturation was most beneficial to N.2O emission. The effect of earthworms were most significant in 90%WFPS for CO₂ and N₂O, and in 135%WFPS for CH₄. Drying and rewetting cycle could significantly increase CO₂ emission, but decrease N₂O emission by 31.83-49.18% and transformed wetland soil from source to sink of CH₄. Interactions of drying-rewetting, especially rewetting process and earthworms increased N₂O release.Warming as well as earthworms could significantly facilitate CO₂ and CH₄ emission from wetland soil, however, earthworms increased N₂O emission in 20 ℃ while oppositely in 25 ℃. Under worming circumstances, earthworms respectively increased accumulative CO₂ and CH₄ emission by 36.72% and 12.14 times, but reduced accumulative N₂O emission by 24.42%. In the environment of changing temperature, soil water content and earthworm activity, temperature change was the key factor for CO₂ emission, the effect of earthworms was quite enhanced under the condition of warming. Soil water content was the controlling factor for CH₄ and N₂O emission, but the difference was that the most obviously acceleration of earthworm on CH₄ emission appeared in 135%WFPS while N₂O emission in 90%WFPS.