[Impact of Exogenous Nitrogen Import on Sediment Denitrification and N₂O Emissions in Ditches Under Different Land...

Farmland drainage ditch soil can consume part of the agricultural non-point nitrogen through nitrification-denitrification processes. Paddy fields, vegetable land, and orchards are the main types of land uses in the Taihu Lake region, and many drainage ditches are distributed across these lands. The way exogenous nitrogen is imported to drain ditches under different land uses differs significantly, which can directly affect the nitrogen consumption ability of the channels. A soil incubation experiment was conducted under laboratory conditions to study the denitrification loss and N₂O emissions of drainage ditch soil under different land uses. In this study, drainage ditch sediment was collected from orchards, paddy fields, and vegetable land in the Taihu Lake region. Five different NO₃^--N content import levels were set:0, 0.5, 1.0, 5.0, and 10 mg·L^-1, which were denoted as N₀, N₁, N₂, N₃, and N₄, respectively. The results showed that exogenous nitrogen input stimulated sediment denitrification in the drainage ditches. The sediment denitrification rates of the three types of channels increased significantly with the increase of input NO₃^--N concentration (P<0.05). There was a significant linear positive correlation between the cumulative denitrification loss and input NO₃^--N concentration (R²>0.75). Excluding for the vegetable land sediment, the N₂O emission rate and cumulative emissions did not increase significantly with the increase of input NO₃^--N concentration (P>0.05). There was no significant difference in the denitrification and N₂O emissions among the three kinds of channel sediment, with no or low exogenous nitrogen input (N₀and N₁) (P>0.05). As the input NO₃^--N concentration increased, especially under the condition of high exogenous nitrogen input (N₃and N₄), the nitrogen consumed via denitrification in orchard and paddy field sediment was significantly higher than that in vegetable land sediment (P<0.05), whereas the N₂O emissions of drainage ditch sediment from the vegetable land was significantly higher than that of the other two channel sediments (P<0.05). The mineralization rate of ditch soil organic carbon had a positive correlation with denitrification rate (n=15), and microbial mineralization (CO₂-C) promoted the nitrification and denitrification of the drainage ditch soils.