Earthworms reduce soil nitrous oxide emissions during drying and rewetting cycles

Abstract

Nitrous oxide (N2O) is a greenhouse gas that is released from both nitrification and denitrification processes. Soil moisture content is a key controller of the biochemical pathways leading to N2O emission, causing a switch between nitrification and denitrification processes. Earthworms are reported to increase N2O emissions from soil under aerobic and anaerobic conditions, but how earthworm-induced N2O emissions are affected by soil drying and rewetting cycles is unknown. The objectives of this study were to (1) evaluate earthworm-induced N2O emissions from soils with aerobic, anaerobic, and fluctuating soil moisture conditions; and (2) determine the earthworm effects on soil denitrifiers responsible for N2O fluxes. Soils were kept in mesocosms (polyvinyl chloride plastic tubes, 10 cm diameter, filled with soil to 15 cm depth) at constant 33% water-filled pore space (WFPS), constant 97% WFPS or underwent three wetting-drying cycles (WD). Each soil moisture treatment had 2 earthworm treatments, including (1) a mixture of endogeic Aporrectodea turgida and anecic Lumbricus terrestris and (2) no earthworm treatment. These gave a total of 6 treatments in this study, with 5 replicates for each treatment. The N2O fluxes were quantified every one to three days, and the soil denitrifier activities were measured after 69 days, when the experiment ended. Soil moisture significantly affected N2O emissions and the WD treatment had the highest cumulative N2O emissions. Earthworms increased N2O emissions by 50% in the 33% WFPS treatment but decreased N2O emissions by 34% in the 97% WFPS treatment, probably due to more complete reduction of N2O to N2. Earthworms strongly reduced N2O emission rate in WD treatment, and they significantly reduced cumulative N2O emissions by 82%. Soil denitrification enzyme activity (DEA) increased significantly when earthworms were present. Abundance of 16S rRNA, nirS, and nosZ genes was affected significantly by the earthworm × soil moisture interaction, with the highest 16S rRNA and nosZ abundance in soil from the WD treatments. We conclude that the decrease in cumulative N2O emissions from soil at 97% WFPS and the WD treatment by earthworms was due to an alteration of the denitrifying bacterial community composition.