The effect of soil salt content and ionic composition on nitrification in a Fluvisol of the Yellow River Delta

Abstract

Soil salinization is a common cause of land degradation and a major constraint on global food production. However, the effects that different salts have on soil biogeochemical processes such as nitrification are not fully understood. An incubation experiment was conducted with two different salt treatments applied at varying concentrations (sodium chloride addition, NaCl1: 2.06, NaCl2: 4.12, NaCl3: 5.00 g kg−1; sodium sulphate addition, NaS1: 5.00, NaS2: 6.07, NaS3:12.14 g kg−1). A Fluvisol soil was used as incubation medium and urea was supplied as the nitrogen source. Results showed that urease activities were reduced (28–76%) in salt treated soils on day 1, but recovered over time and finally surpassed that of the control on day 56. Moreover, the average urease activity of Na2SO4 treatments was 1.26 times higher than that of NaCl treatments on day 56. In addition, increasing soil salinity levels simultaneously reduced hydrolysis and nitrification rates, which led to the temporary accumulation of ammonium (NH4+-N) in soils for up to 14 days. The maximum contents of NH4+-N accumulation in NaCl3 and NaS3 were 1.64 and 1.48 times higher than those of NaCl1 and NaS1, respectively. Nitrification rates were inhibited more severely in soils dominated by Cl-. Low soil salt contents (less than 7.79 g kg−1) only delayed nitrification without reducing the total amount of nitrate (NO3--N) produced during the incubation. However, high salt contents (NaS3) significantly decreased the total NO3--N production. Our results indicate that nitrite oxidation process may also limit the speed of nitrification in salt-affected soils. Additionally, a new model is introduced to better simulate the dynamics of nitrification in salt-affected soils. In conclusion, soil salt content is the primary factor that inhibits nitrification. Anions affect the inhibitory strength of salinity on nitrification in soils with similar salinity levels, where Cl- is more suppressive than SO42-.