Methane and nitrous oxide have separated production zones and distinct emission pathways in freshwater aquaculture ponds

Aquaculture systems receive intensive carbon (C) and nitrogen (N) loadings, and are therefore recognized as major anthropogenic sources of methane (CH4) and nitrous oxide (N2O) emissions. However, the extensively managed aquaculture ponds were identified as a hotspot of CH4 emission but just a weak N2O source. Here, we investigate annual CH4 and N2O fluxes from three earthen ponds used for crab culture, of different sizes, in southeast China. Our purposes are to identify the spatiotemporal variations of CH4 and N2O emissions and their components among ponds and to evaluate the zone for CH4 and N2O production. Static chamber-measured CH4 flux ranged from 0.03 to 64.7 mg CH4 m‒2 h‒1 (average: 9.02‒14.3 mg CH4 m‒2 h‒1), and temperature, followed by dissolved organic C (DOC) concentration, and redox potential, were the primary drivers of seasonal CH4 flux patterns. Annual mean diffusive CH4 flux was 1.80‒2.34 mg CH4 m‒2 h‒1, and that by ebullition was up to 7.20‒12.0 mg CH4 m‒2 h‒1 (79.1‒83.5% of the total CH4 flux). Annual CH4 emission was positively correlated with sediment DOC concentration but negatively (P < 0.05) correlated with water depth across ponds, with the highest CH4 emission occurred in a pond with low water depth and high DOC concentration. The calculated diffusive N2O flux by the gas transfer velocity was 0.32‒0.60 times greater than the measured N2O emission, suggesting that N2O in water column can not only evade as water-air fluxes but diffuse downwards and to be consumed in anaerobic sediments. This also indicates that N2O was primarily produced in water column. The highly reduced condition and depletion of NO3‒-N in sediments, can limit N2O production from both nitrification and denitrification but favor N2O consumption, leading the ponds to become a weak source of N2O annually and even a sink of N2O in summer. Our results highlight that the current global CH4 budget for inland waters is probably underestimated due to a lack of data and underestimation of the contribution of ebullitive CH4 flux in small lentic waters. The downwards N2O diffusion from the water column into sediment also indicates that the extensively-used model approach based on gas transfer velocity potentially overestimates N2O fluxes, especially in small eutrophic aquatic ecosystems.