Dissolved Nitrous Oxide in Shallow-Water Ecosystems under Saline-Alkali Environment

Abstract

The problem of global warming is becoming more and more serious. N2O is a potent greenhouse gas. Most current studies on dissolved N2O concentration have focused on inland freshwater and seawater while paying less attention to coastal agricultural catchment areas. The coastal agricultural catchment area is the link between the farmland ecosystem and the aquatic ecosystem, which is shallow in water depth. Moreover, due to the high salt content and obvious periodic change, it is highly sensitive to environmental changes and human activities and has strong potential for N2O emission. Therefore, it is of great significance to understand the characteristics of the changes in the dissolved N2O concentration in the shallow-water ecosystem under the saline-alkali environment of the coastal reclamation area and to identify the main controlling factors. The soil of Yudong reclamation area in Rudong County, Jiangsu Province was collected to carry out the submerged cultivation experiment. In order to simulate the saline-alkali situation of the coastal reclamation area, four salt gradients (S1–S4), four alkali gradients (A1–A4), and three levels of exogenous nitrogen concentration (N1–N3). In addition, the experiment set a control treatment (CK) without salt and alkali addition. After 2 weeks of cultivation in a shallow water layer of about 5 cm, the dissolved N2O concentration and its influencing factors were measured and analyzed by collecting the overlying water sample and sediment after 24 h of fertilization. The results showed that changes in the saline-alkali environment in shallow-water ecosystems significantly affected the changes in dissolved N2O concentration. The saline-alkali indicators (EC and pH of the overlying water and sediment), DO of the overlying water, and the microbial genes nirS, nirK, and nosZ were the key influencing factors of N2O production in shallow-water systems. The correlation between nirS gene abundance and the dissolved N2O concentration was the highest. The BP neural network model can be used to simulate and predict the dissolved N2O concentration in overlying water under saline-alkali environment. Based on the experimental results, this study can provide a scientific basis for understanding the nitrogen cycling process in shallow-water ecosystems in the coastal reclamation area, improving the absorption of non-point-source nitrogen and reducing N2O emissions in shallow-water wetlands.