An overlooked source of N2O: Insights from household farm ponds in a subtropical region of China

Farm ponds are traditional water conservancy facilities in subtropical China, where precipitation varies considerably across seasons. The farm ponds, though small in area, are significant yet often overlooked sources of nitrous oxide (N 2 O) due to their high numbers. This study combined seasonal in situ monitoring, isotopic tracing, and molecular analysis to quantify N 2 O dynamics and identify production pathways in farm ponds in a subtropical region of China. The type of farm ponds significantly affected the N 2 O emission, with aquaculture ponds (AP) exhibiting higher observed fluxes at 676.7 nmol m -2 h −1 in July during the mid-growing season compared to 193.2 nmol m -2 h −1 in domestic ponds (DP). N 2 O emissions were influenced by both natural seasonal patterns and agricultural practices. Sediment nutrient properties such as TN, TP, and NH 4 + -N had a greater impact on N 2 O emissions than biotic factors including denitrification rates and denitrifier gene abundances. Surface layers were identified as hotspots for N 2 O production based on sediment source-sink indices. During the highest measured emission period in July, isotopic analysis revealed that N 2 O production in AP was dominated by NH 4 + -derived pathways (∼76.5%), while the NO 3 – -derived pathway prevailed in DP (∼92.3%). We estimated that farm ponds in this watershed could emit between 3.7 and 5.0 kg of N 2 O each year. Farm ponds therefore represent significant but often overlooked sources of N 2 O, and it is imperative to incorporate them into greenhouse gas inventories.