Enhanced N2O Production Induced by Soil Salinity at a Specific Range

Abstract

Nitrous oxide (N2O) as a by-product of soil nitrogen (N) cylces, its production may be affected by soil salinity which have been proved to have significant negative effect on soil N transformation processes. The response of N2O production across a range of different soil salinities is poorly documented; accordingly, we conducted a laboratory incubation experiment using an array of soils bearing six different salinity levels ranging from 0.25 to 6.17 dS m−1. With ammonium-rich organic fertilizer as their N source, the soils were incubated at three soil moisture ( θ ) levels—50%, 75% and 100% of field capacity ( θ fc )—for six weeks. Both N2O fluxes and concentrations of ammonium, nitrite and nitrate (NH4+-N, NO2−-N and NO3−-N) were measured throughout the incubation period. The rates of NH4+-N consumption and NO3−-N accumulation increased with increasing soil moisture and decreased with increasing soil salinity, while the accumulation of NO2−-N increased first then decreased with increasing soil salinity. N2O emissions were significantly promoted by greater soil moisture. As soil salinity increased from 0.25 to 6.17 dS m−1, N2O emissions from soil first increased then decreased at all three soil moisture levels, with N2O emissions peaking at electric conductivity (EC) values of 1.01 and 2.02 dS m−1. N2O emissions form saline soil were found significantly positively correlated to soil NO2−-N accumulation. The present results suggest that greater soil salinity inhibits both steps of nitrification, but that its inhibition of nitrite oxidation is stronger than that on ammonia oxidation, which leads to higher NO2−-N accumulation and enhanced N2O emissions in soil with a specific salinity range.

Greenhouse Gas Emissions from Salt-Affected Soils: Mechanistic Understanding of Interplay Factors and Reclamation Approaches

“…Soil moisture has a significant effect on N 2 O emissions from saline soil. For instance, 4.7-37.6 and 5.0-15.3 times higher N 2 O emissions at 100% soil moisture level than at 50 and 75% soil moisture levels, respectively, was reported by Li et al [85], and it was mainly due to an increased rate of nitrification with increasing soil moisture [85]. Thapa et al [86] reported that increasing salinity from 0.81 to 4.65 dS m −1 increased N 2 O flux at 90% of Water-filled pore space (WFPS) but reduced at 60% of WFPS in sulfate-dominated saline soil and it was due to that denitrifying bacteria performed more efficiently even at higher salinity levels because water addition reduces the adverse effect of salinity [86].…”

“…N 2 O emissions from low salinity wetland soils (0.060 mg kg −1 ) were reported higher as compared to high salinity wetland soils [26]. It is reported that the addition of salts up to a certain threshold may enhance the N 2 O emissions and decrease thereafter [85]. The lower salinity level inhibits both the steps of nitrification (conversion of NH 4…”

“…+ to NO 2 − and NO 2 − to NO 3 − ), however, the inhibition of NO 2 − to NO 3 − conversion is stronger than that of NH 4 + to NO 2 − causing higher NO 2 − accumulation and enhanced N 2 O emissions (Figure 3b) [85]. Li et al [25] reported 110% higher N 2 O emissions in slightly saline soil (1.0 dS m −1 ) but 20% lower in moderate salinity (5 dS m −1 ) soil as compared to non-saline soil (0.3 dS m −1 ) under urea and ammonium sulfate fertilizer application.…”