CO₂ and N₂ O emissions and microbial community structure from fields that include salt-affected soils

Although salinity and sodicity are worldwide problems, information on greenhouse gas (GHG) emissions from agricultural salt-affected soils is scarce. The CO₂ -C and N₂ O-N emissions were quantified from three zones intertwined within a single U.S. northern Great Plains field: a highly productive zone (electrical conductivity with 1:1 soil/water mass ratio [EC_1:1 ] = 0.4 dS m^-1 ; sodium adsorption ratio [SAR] = 1.8), a transition zone (moderately salt-affected; EC_1:1  = 1.6 dS m^-1 ; SAR = 4.99), and a saline/sodic zone (EC_1:1  = 3.9 dS m^-1 ; SAR = 22). In each zone, emissions were measured every 4 h for 7 d in four randomly placed chambers that were treated with two N rates (0 and 224 kg N ha^-1 ). The experiment was conducted in 2018 and 2019 during similar seasonal periods. Soil samples taken from treatments after GHG measurement were analyzed for soil inorganic N, and microbial biomass from different communities was quantified using phospholipid fatty acid analysis. Real-time polymerase chain reaction was used to quantify the number of copies of some specific denitrification functional genes. The productive zone had the highest CO₂ -C, the lowest N₂ O-N emissions, and the greatest microbial biomass, whereas the saline/sodic zone had the lowest CO₂ -C, the highest N₂ O-N emissions, and the lowest microbial biomass. Within a zone, urea application did not influence CO₂ -C emissions; however, N₂ O-N emissions from the urea-treated saline/sodic zone were 84 and 57% higher than from the urea-treated productive zone in 2018 and 2019, respectively. The copy number of the nitrite reductase gene, nirS, was 42-fold higher in the saline/sodic zone than in the productive soil, suggesting that the saline/sodic soil had a high potential for denitrification. These findings suggest N₂ O-N emissions could be reduced by not applying N to saline/sodic zones.