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

Abstract

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.