Invasive Spartina alterniflora accelerates the increase in microbial nitrogen fixation over nitrogen removal in coastal wetlands of China

Abstract

Salt marsh plants play a vital role in mediating nitrogen (N) biogeochemical cycle in estuarine and coastal ecosystems. However, the effects of invasive <i>Spartina alterniflora</i> on N fixation and removal, as well as how these two processes balance to determine the N budget, remain unclear. Here, simultaneous quantifications of N fixation and removal via ¹⁵N tracing experiment with native <i>Phragmites australis</i>, invasive <i>S. alterniflora</i>, and bare flats as well as corresponding functional gene abundance by qPCR were carried out to explore the response of N dynamics to <i>S. alterniflora</i> invasion. Our results showed that N fixation and removal rates ranged from 0.77 ± 0.08 to 16.12 ± 1.13 nmol/(g·h) and from 1.42 ± 0.14 to 16.35 ± 1.10 nmol/(g·h), respectively, and invasive <i>S. alterniflora</i> generally facilitated the two processes rates. Based on the difference between N removal and fixation rates, net N₂ fluxes were estimated in the range of -0.39 ± 0.14 to 8.24 ± 2.23 nmol/(g·h). Estimated net N₂ fluxes in <i>S. alterniflora</i> stands were lower than those in bare flats and <i>P. australis</i> stands, indicating that the increase in N removal caused by <i>S. alterniflora</i> invasion may be more than offset by N fixation process. Random forest analysis revealed that functional microorganisms were the most important factor associated with the corresponding N transformation process. Overall, our results highlight the importance of N fixation in evaluating N budget of estuarine and coastal wetlands, providing valuable insights into the ecological effect of <i>S. alterniflora</i> invasion.