Indirect NO emissions with seasonal variations from an agricultural drainage ditch mainly receiving interflow water.

Nitrogen (N)-enriched leaching water may act as a source of indirect NO emission when it is discharged to agricultural drainage ditches. In this study, indirect NO emissions from an agricultural drainage ditch mainly receiving interflow water were measured using the static chamber-gas chromatography technique during 2012-2015 in the central Sichuan Basin in southwestern China. We found the drainage ditch was a source of indirect NO emissions contributing an inter-annual mean flux of 6.56 ± 1.12 μg N m h and a mean indirect NO emission factor (EF) value of 0.03 ± 0.003%. The mean EF value from literature review was 0.51%, which was higher than the default EF value (0.25%) proposed by the Intergovernmental Panel on Climate Change (IPCC) in 2006. Our study demonstrated that, more in situ observations of NO emissions as regards N leaching are required, to account for the large variation in EF values and to improve the accuracy and confidence of the default EF value. Indirect NO emissions varied with season, higher emissions occurred in summer and autumn. These seasonal variations were related to drainage water NO-N concentration, temperature, and precipitation. Our results showed that intensive precipitation increased NO-N concentrations and NO emissions, and when combined with warmer water temperatures, these may have increased the denitrification rate that led to the higher summer and autumn NO emissions in the studied agricultural drainage ditch.