Salinity-induced concomitant increases in soil ammonia volatilization and nitrous oxide emission

Abstract

Environmental concerns about the release of reactive nitrogen (N) from intensively fertilized farmland are growing, especially regarding gaseous N losses in the form of ammonia (NH3) and nitrous oxide (N2O). While saline soils are widely distributed and gradually expanding worldwide, the effect of soil salinity on soil NH3 and N2O emissions is poorly understood. To assess the effect of soil salinity levels on NH3 volatilization and N2O emissions, a field experiment was conducted with three soil salinity levels (S0 = non-saline, S1 = 1.0 dS m−1 and S2 = 5.0 dS m−1) and two N fertilizer types (urea and ammonium sulphate). Compared to the non-saline soil, S1 and S2 salinity levels both led to significant increases in cumulative NH3 volatilization losses by 40.0%–65.2% and 89.0%–92.2%, respectively (p < 0.05). Following the application of either urea and ammonium sulphate, N2O emissions at the S1 salinity level were significantly greater than those at both S0 and S2 salinity levels (p < 0.05). The correlation showed that the NH3 flux was an exponential function with respect to soil ammonium (NH4+-N) concentration (R2 = 0.766, p < 0.05), and the N2O flux displayed a linear relationship with soil nitrite (NO2−-N) concentration (R2 = 0.816, p < 0.05) which was accumulated to a much greater degree under the S1 salinity level. Saline soil’s promotion of NH3 volatilization was a result from urea hydrolysis, nitrification and soil NH4+-N adsorption capacity being suppressed under elevated soil salinity. Increased N2O emissions were ascribed to the accumulation of soil NO2−-N caused by the imbalance of salinity’s inhibition on nitrite oxidation and ammonia oxidation. This study indicates that saline soils have a considerable potential to simultaneously increase NH3 and N2O emissions, so mitigation options should be explored when cultivating saline soils.