Salt stress alters the short-term responses of nitrous oxide emissions to the nitrogen addition in salt-affected coastal soils

Increasing nitrogen deposition has become one of major environmental concerns in coastal wetlands. However, little is known about the response of soil nitrous oxide (N2O) emissions, a powerful greenhouse gas, to the different levels and forms of nitrogen addition, in salt-affected coastal soils. To close the knowledge gap, a laboratory factorial incubation experiment with four nitrogen addition levels (0, no N addition; low-N, 45; medium-N, 90; high-N, 270 mg N kg-1), three forms (NO3- (KN); NH4+ (NH); DN, NH4NO3 (DN)), and three levels of salt addition (0, no salt addition; 12 ppt, low-salt; 35 ppt, high-salt) was carried out in two coastal soils with different initial salinity levels (LW: lower salinity wetlands; HW: higher salinity wetlands) in the Yellow River Delta, China. Results showed that, in no salt addition treatments, the cumulative N2O emissions were linearly related to nitrogen addition, and high nitrogen addition significantly promoted N2O emissions by 213% in LW soils and 848% in HW soils (p < 0.05), indicating that the mitigate effects of nitrogen addition to the deleterious salt stress were stronger in HW soils. Meanwhile, the promoting effects of DN and KN treatments were more obvious than NH treatments, suggesting that denitrification was responsible for the N2O emission. However, with salt addition, the nonlinear response pattern and reduced response sizes were observed for KN and DN treatments (p < 0.05), suggesting the alteration in responses of N2O emission to nitrogen addition by salt stress. In addition, the reduction and modification of response pattern was more obvious in soil with lower initial salinity. The findings of this work suggest the uniqueness and complexity of N2O emission responses to nitrogen inputs related to the salinity levels in coastal wetlands.