Influence Of Soil Fauna On Greenhouse Gases （CO₂, N₂O） Emissions In Rice-Wheat Rotation Agro-ecosystem

Soil trace gas metabolism has been one of the important research subjects about interaction between the Earth’s spheres. Pedosphere, biosphere and atmosphere come into being interrelatively and mutually dependently in the cycling and dynamic process of soil C, N biogeochemistry. Soil biota drive the biogeochemical process by regulating trace gas metabolism, and further influence atmospheric environments and the global climate change on larger scale. Earthworms are one of the most important members of soil fauna in terrestrial ecosystem. However, little information of the impact of soil fauna especially earthworms on the generation of trace gases in agro-ecosystem is available so far.To identify the effects of soil fauna on greenhouse gases (CO2, N2O) emissions from agro-ecosystem, an outdoor pot experiment assisted with laboratory incubations were conducted under the same climate condition and agricultural management. The primary objective was to examine the effect of earthworm on greenhouse gases emissions with residues applied (incorporation or mulching), and to explore their processes and relevant mechanisms in agro-ecosystem. Also, laboratory incubations were carried out by inoculating different populations of nematodes and earthworm into soil samples to assess the influence of different populations of nematodes and earthworms on CO2 and N2O emission, and to make clear whether there exist synergies or antagonisms among soil faunal groups.(1) Short-term effects of actively earthworm (Eisenia foetida) with or without corn-straw residues on CO2 and N2O emissions were studied in a microcosm experiment throughout a 21 day incubation. Both residues application and earthworm presence resulted in significant differences in CO2 and N2O emissions relative to the control treatment. Cumulative emissions of CO2 and N2O in soil only with earthworm and soil only with residues were increased 60% and 1.35 fold for CO2,1.06 fold and 3.94 fold for N2O, respectively versus the soil treatment. The emission fluxes of CO2 and N2O in soil with residues and earthworm were 41% for CO2 and 45% for N2O higher than soil only with residues. Due to direct gases emissions of actively earthworms themselves are significant compared to fluxes measured from the whole systems, when emissions from earthworms themselves were deducted, cumulative amounts of CO2 and N2O emissions from the earthworm treatments are similar to that from the soil treatments. On the other hand, cumulative emissions of CO2 and N2O in treatment of earthworm and residues were 21% for CO2 and 25% for N2O higher than the treatment of residues (p<0.05). To supply easily available C, N as energy source for CO2 and N2O emissions, our experiment showed a much more positive earthworm effect on CO2 and N2O emissions in treatments with residues application. The value of MBC increased significantly by straw residues application meanwhile it decreased by earthworm inoculation, therefore the increase of CO2 emission cannot only attribute to MBC. The contents of NO3--N and NH4+-N increased significantly by earthworm inoculation, but not significantly by residues application, indicating that the relationship of N2O emission and NO3--N was more intimate than NH4+-N.(2) To determine the effects of soil fauna on greenhouse gases emissions, soil inoculated with different populations nematodes and earthworms (Eisenia foetida) were incubated for 15 days. Soil treated with greater populations of nematodes and earthworms enhanced CO2 and N2O emissions. Cumulative emissions of CO2 and N2O in the greater populations nematodes treatment and the greater populations nematodes with earthworm treatment increased by 4.4 times and 5.3 times for CO2, 1.8 times and 2.7 times for N2O, respectively versus the nematode-killed treatment. The effects of nematode abundance on CO2 and N2O emissions were apparent, nematodes play a important role in greenhouse gases metabolism. The greater populations of nematodes increased DOC value when compared with the lower populations of nematodes. Cumulative emissions of CO2 and N2O from the soils treated with different populations nematodes were positively correlated with DOC concentration measured at the start of gas sampling (p<0.05). Due to direct gases emission of earthworm activity were significant compared to amounts measured from whole systems, earthworm seem to be overestimated in greenhouse gases emissions. Earthworm does not contribute to higher CO2 release from soil instead of lead to increased production of the greenhouse gas nitrous oxide in Soil treated with greater populations of nematodes when gases derived from earthworm was deducted, which indicating that interaction between soil fauna and microorganism is also one of the determinant of greenhouse gases emissions.(3) An field experiment was performed to investigate the effects of residues and earthworm (Metaphire guillelmi) activities on CO2 emissions in a upland rice-wheat rotation system. The highest level of cumulative emissions and average rate of CO2 in upland rice season and wheat season were 1.60 kg·m-2 (596.2 mg·m-2·h-1) and 2.11 kg·m-2 (540.6 mg·m-2·h-1) respectively in the residues incorporated treatment, and the lowest level of cumulative emissions were both in the control treatment, the seasonal average rate of CO2 emissions from the control treatment were 417.2 mg·m-2·h-1 in upland rice season and 352.8 mg·m-2·h-1 in wheat season. Obviously, soil with straw residues application, relative to the control treatment, causing significant enhancement of CO2 emission. Two kinds of application of straw residues (incorporation or mulching) had different effect on CO2 emission, mulching decreased CO2 emission flux than incorporation. The presence of earthworm had complicated influence on CO2 emission. During the first 4 weeks both in upland-rice and wheat season, earthworms significantly (p<0.05) increased CO2 emissions. In contrast, significantly lower (p<0.05) CO2 emission amounts were measured at the rest of the entire annual cycle. The data suggested that earthworm activity was high during the first month due to the creation of burrows and incorporation of residues into the mineral soil. Both earthworm and straw residues application increased the SOC when compared with the control. Cumulative emissions of CO2 were positively correlated with the SOC and TN concentration measured at the end of the entire annual cycle. On the other hand, CO2 emissions were not positively correlated with MBC and DOC. (4) An field experiment was performed to further investigate the effects of residues and earthworm (Metaphire guillelmi) activities on N2O emissions. Both residues application (incorporation or mulching) and earthworm presence resulted in significant differences in N2O emissions. In the case of straw residues mulching, the cumulative emission amounts and average emission flux of N2O in the whole season significantly increased from 24.13 (362.9μg·m-2·h-1) to 29.20 kg-hm"2 (439.2μg·m-2·h-1) (p<0.01) in the presence of earthworm. However, if residues were incorporated into the soil, the earthworm effect disappeared except in the first stage of wheat season and emissions amounts were 29.11 kg·hm-2. Earthworm plays a more important role in N2O emission when residues mulched through mixing residue into the soil, switching residue decomposition to one with significant denitrification and N2O production. Soil mineral nitrogen (especially NO3--N content) was increased, and nitrogen mineralization was strengthened by earthworm activities. Earthworm inoculation significantly increased the contents of soil NO3--N by 20.1%(p<0.01) when straw residues mulched and up to 21.21 mg·kg-1, which means 11.7% increments when residues incorporated. Cumulative emissions of N2O were positively correlated with the NO3--N content measured at the end of the entire annual cycle.Both nematodes and earthworm presence resulted in significant differences in CO2 and N2O emissions accorrding to our outdoor pot experiment assisted with laboratory incubations. Short-term effects of actively earthworm (Eisenia foetida) and nematode abundance on CO2 and N2O emissions were apparent, nematodes play a important role in greenhouse gases metabolism. The presence of earthworm had complicated influence on CO2 emission which concerns the way of application of straw residues (incorporation or mulching). Earthworm plays a more important role in N2O emission when residues mulched both in field and microcosmic scales. It was concluded that the nutrient-enrichment processes (NEP) rather than the gut-associated processes (GAP) are responsible for the increased emissions of N2O in the presence of earthworms.