Invasive Spartina alterniflora accelerates soil gross nitrogen transformations to optimize its nitrogen acquisition in an estuarine and coastal wetland of China

Saltmarsh plants are important components of estuarine and coastal wetlands because they regulate ecosystem nitrogen (N) dynamics. However, complex interactions between the N uptake of saltmarsh plants and soil N transformation remain unclear. Here, we conducted a series of ¹⁵N tracing experiments with native Phragmites australis, invasive Spartina alterniflora, and bulk sediment without plants to explore the effect of plants on soil N cycling. The results showed that the NH₄⁺ and NO₃⁻ uptake rates by the saltmarsh plants were 4.62–5.38 mg N kg⁻¹ d⁻¹ and 1.29–2.90 mg N kg⁻¹ d⁻¹, respectively, and the invasive S. alterniflora had a higher N uptake than the native P. australis. The presence of saltmarsh plants promoted N mineralization and dissimilatory NO₃⁻ reduction to NH₄⁺, increasing the available NH₄⁺ supply for the plants. Conversely, NH₄⁺ immobilization and autotrophic nitrification rates were drastically reduced in the presence of the saltmarsh plants, indicating that the plants were able to outcompete soil microorganisms in NH₄⁺ acquisition. Meanwhile, heterotrophic nitrification (organic N oxidation), which accounted for 66–82% of the total nitrification, was stimulated by the saltmarsh plants. Increased heterotrophic nitrification in the saltmarsh plants helped to provide NO₃⁻ substrates to meet the needs of the soil microorganisms and the plants. The regulatory effect of the invasive S. alterniflora on soil gross N transformation was more pronounced than that of the native P. australis due to the higher N requirements of the former. Microbial carbon sources and energy sources, relevant gene abundances and exoenzyme activities were the main factors by which the saltmarsh plants regulated gross N transformations. Overall, our results show that there are various interactions between soil microorganisms and saltmarsh plants and that S. alterniflora accelerates gross N transformations in the soil to meet its large demand for N. These findings provide valuable insights into the ecological management of invasive plants in estuarine and coastal ecosystems.