Soil substrates rather than gene abundance dominate DNRA capacity in the Spartina alterniflora ecotones of estuarine and intertidal wetlands

Background and aims: Dissimilatory nitrate reduction to ammonium (DNRA) plays an important role in keeping nitrate retention as a more bioavailable form (ammonium) in estuarine and intertidal environments. However, the effects of soil abiotic and biotic characteristics on DNRA in Spartina alterniflora ecotones of estuarine and intertidal wetlands remain unclear. Methods: In this study, we used nitrogen isotope tracing and molecular approaches to investigate DNRA activity, and abiotic and biotic factors of both rhizosphere and non-rhizosphere soils in Spartina alterniflora ecotones of the Yangtze estuarine and intertidal wetlands. Results: DNRA varied significantly throughout the sampling sites, with potential rates of 0.53–3.57 nmol N g ⁻¹  h ⁻¹ . The rates of DNRA were significantly higher in rhizosphere than non-rhizosphere soils at the oligohaline sites. Salinity had more influence on DNRA activity than Spartina alterniflora at the brackish sites. Total organic carbon, nitrate, Fe (II) and sulfide were significantly correlated with DNRA rates and nrfA gene abundance. Soil substrates strongly affected DNRA activity in rhizosphere soil, while nrfA gene abundance was the predominant factor mediating DNRA activity in non-rhizosphere soil. DNRA contributed more to the total nitrate reduction at the brackish than oligohaline sites, suggesting that DNRA plays an important role in nitrate reduction in estuarine and intertidal wetlands. Conclusions: This study suggests that soil substrates rather than nrfA gene dominate DNRA activity in estuarine and intertidal wetlands after Spartina alterniflora invasion. Our results are helpful to understand the importance of soil characteristics changes induced by the exotic plant invasion to nitrogen cycling in estuarine and intertidal wetlands.