Seasonal variations in indirect N2O emissions from an agricultural headwater ditch

Agricultural headwater ditches are an important source of indirect agricultural nitrous oxide (N 2 O) emissions, but their contribution is difficult to quantify. In the present study, the static chamber-gas chromatography technique was used for measurement of N 2 O emissions from vegetated (V, the whole ditch ecosystem) and non-vegetated (NV, the sediment-water interface only) zones in an agricultural headwater ditch in the Central Sichuan Basin in Southwestern China during 2014–2015. Annual N 2 O emissions from the agricultural headwater ditch were similar to direct N 2 O emissions from an adjacent N-fertilized purple soil cropland, suggesting nitrogen (N)-enriched ditches are important anthropogenic N 2 O sources. Mean cumulative N 2 O emissions during summer and autumn were higher than those in spring and winter. Overlying water nitrate (NO 3 − -N) concentration and sediment-water interface temperature were primary factors affecting seasonal N 2 O emissions. Heavy precipitation transported NO 3 − -N from cropland and increase NO 3 − -N in the agricultural headwater ditch water, and subsequently stimulate N 2 O emissions. A literature review of EF 5r (the indirect N 2 O emission factor for rivers) revealed a mean value of 0.23%, similar to our values (0.27%), and also the default value (0.25%) proposed by the Intergovernmental Panel on Climate Change. The number of studies on indirect N 2 O emissions remains limited, and more in situ measurements are needed to have more accurate values of EF 5r .