Ammonia Monooxygenase Activity Connects Nitrification Rate with Dominant Edaphic Properties Under Salinity Stress in Coastal Fluvo-aquic Soil

Nitrification is a key player in biogeochemical cycling of nitrogen, and there is a need to regulate the nitrification for nitrogen loss reduction in agroecosystems. The current study aimed to identify the intrinsic connection between edaphic attributes and nitrification for the salt-affected cultivation soils. Based on the coastal saline fluvo-aquic soil samples under different vegetation patterns, this study used classical statistics and structural equation model to investigate the statistical characteristics and the causal relationships among soil nitrification rate ( N r ), physico-chemical properties, and key enzyme activity under salinity stress. The results indicated that soil N r and ammonia monooxygenase (AMO) activity were unaffected by salinity level (EC e ) below 4 mS cm −1 , suppressed by salinity level greater than 4 mS cm −1 , and the inhibition increased with the increase of salinity levels. Hydroxylamine oxidoreductase (HAO) and nitrite oxidoreductase (NXR) activity was insusceptible to salinity levels. With a total of 73.8% variance explained, N r was well explained by AMO, cation exchange capacity (CEC), pH, Na + , and microbial biomass nitrogen (MBN). Soil CEC and Na + had direct positive and negative effects on N r , respectively, and soil CEC, pH, and Na + exerted indirect influence on N r through the pathway of AMO. It was concluded that AMO activity played a key role in connecting N r with dominant edaphic properties, including soil CEC, pH, and Na + , and the AMO-catalyzed step in the ammonia oxidation process, i.e., from ammonium nitrogen to hydroxylamine, was the rate-limiting step of nitrification in coastal salt-affected fluvo-aquic soil.