Drainage ditches are significant sources of indirect N2O emissions regulated by available carbon to nitrogen substrates in salt-affected farmlands.

• Drainage ditches were critical sources of N 2 O emissions in salt-affected farmlands. • Precipitation events decreased the dissolved N 2 O concentrations and indirect N 2 O emissions. • High available carbon relative to nitrogen reduced N 2 O emissions at low DOC:DIN ratio levels by inhibiting nitrification. • DOC:DIN ratio regulated the spatiotemporal variations in EF 5. • The DOC:DIN ratio could enhance the predictions of EF 5 for drainage ditches in salt-affected farmlands. Agriculture is a main source of nitrous oxide (N 2 O) emissions. In agricultural systems, direct N 2 O emissions from nitrogen (N) addition to soils have been widely investigated, whereas indirect emissions from aquatic ecosystems such as ditches are poorly known, with insufficient data available to refine the IPCC emission factor. In this contribution, in situ N 2 O emissions from two ditch water‒air interfaces based on a diffusion model were investigated (almost once per month) from June 2021 to December 2022 in an intensive arable catchment with high N inputs and salt-affected conditions in the Qingtongxia Irrigation District, northwestern China. Our results implied that agricultural ditches (mean 148 μg N m−2 h−1) were significant sources for N 2 O emissions, and were approximately 2.1 times greater than those of the Yellow River directly connected to ditches. Agronomic management strategies increased N 2 O fluxes in summer, while precipitation events decreased N 2 O fluxes. Agronomic management strategies, including fertilization (294–-540 kg N hm−2) and irrigation on farmland, resulted in enhanced diffuse N loads in drain water, whereas precipitation diluted the dissolved N 2 O concentration in ditches and accelerated the ditch flow rate, leading to changes in the residence time of N-containing substances in water. The spatial analysis showed that N 2 O fluxes (202–233 μg N m−2 h−1) in the headstream and upstream regions of ditches due to livestock and aquaculture pollution sources were relatively high compared to those in the midstream and downstream regions (100–114 μg N m−2 h−1). Furthermore, high available carbon (C) relative to N reduced N 2 O fluxes at low DOC:DIN ratio levels by inhibiting nitrification. Spatiotemporal variations in the N 2 O emission factor (EF 5) across ditches with higher N resulted in lower EF 5 and a large coefficient of variation (CV) range. EF 5 was 0.0011 for the ditches in this region, while the EF 5 (0.0025) currently adopted by the IPCC is relatively high. The EF 5 variation was strongly controlled by the DOC:DIN ratio, TN, and NO 3 −-N, while salinity was also a nonnegligible factor regulating the EF 5 variation. The regression model incorporating NO 3 −-N and the DOC:DIN ratio could greatly enhance the predictions of EF 5 for agricultural ditches. Our study filled a key knowledge gap regarding EF 5 from agricultural ditches in salt-affected farmland and offered a field investigation for refining the EF 5 currently used by the IPCC. [Display omitted] [ABSTRACT FROM AUTHOR]