Divergent impacts of animal bioturbation on methane and nitrous oxide emissions from mariculture ponds

Aquaculture systems are of increasing concern as an important source of atmospheric methane (CH₄) and nitrous oxide (N₂O). However, the role of animals in regulating CH₄ and N₂O emissions from aquaculture systems remains unclear. Here, we established mesocosm trials to investigate impacts of bioturbation of different aquaculture species (i.e., clam, shrimp, and crab) on CH₄ and N₂O fluxes in a mariculture pond. Across the initial, middle, and final culturing stages, mean CH₄ flux in mesocosm without animals was 4.81 ± 0.09 µg CH₄ m^‒2 h^‒1, while the existence of clam, shrimp, and crab significantly increased CH₄ flux by 35.3 %, 80.6 %, and 138 %, respectively. Bioturbation significantly decreased dissolved oxygen (DO) concentration by 5.19‒44.8 % but increased porewater CH₄ concentration by 14.1‒59.9 %, indicating that lowered DO caused by animal respiration promoted CH₄ production in sediment. Moreover, bioturbation of animals significantly increased ebullitive CH₄ fluxes by 41.0‒216 %, contributing 57.4‒77.2 % of the increased CH₄ emission in mesocosms with animals. However, shrimp and crab significantly reduced N₂O flux by 30.3 % and 42.5 %, respectively, primarily due to lowered DO conditions suppressing nitrification and limiting NO₃^‒ supply for denitrification. By contrast, clam significantly increased N₂O emission by 181 % because its filter-feeding behavior excreted more NH₄⁺ and NO₃^‒ into overlying water and thereby facilitating N₂O production. The N₂O concentration in overlying water was 1.72‒2.83-fold of that in porewater, and the calculated diffusive N₂O flux was 1.80‒37.5 % greater than chamber-measured N₂O efflux. This implied that N₂O might be primarily produced in overlying water rather than sediments, and the produced N₂O can either evade as water-air fluxes or diffuse downwards into sediments to be consumed. Overall, our study advocates that aquaculture-related climate mitigation strategies should place more attention on the divergent impacts of animal bioturbation on CH₄ and N₂O emissions.