Soil Nitrogen Transformation Pathways Shift Following Deep Tillage in Coastal Wetlands Invaded by <i>Spartina alterniflora</i>

<i>Spartina alterniflora</i> invasion has posed severe ecological threats to coastal wetlands. Deep tillage is considered an effective physical method for ecological restoration in such wetlands; however, its effects on sediment nitrogen transformation processes remain unclear. In this study, we investigated the impacts of deep tillage on soil physicochemical properties and key nitrogen transformation pathways, including nitrification, denitrification, anammox, and DNRA, across different soil depths (0–10, 10–20, 20–30, 30–50, and 50–100 cm) in <i>Spartina alterniflora</i>-invaded coastal wetlands. Deep tillage significantly restructured the distribution of soil moisture (<i>p</i> < 0.05), pH (<i>p</i> > 0.05), electrical conductivity (<i>p</i> < 0.05), and nutrients, promoting NO₃⁻-N accumulation in deeper layers while reducing NH₄⁺-N concentrations in surface soils (<i>p</i> < 0.05). It markedly enhanced denitrification and DNRA rates (<i>p</i> < 0.05), suppressed surface nitrification (<i>p</i> < 0.05), and altered the vertical distribution of anammox activity. Correlation analysis revealed that NH₄⁺-N and NO₃⁻-N concentrations were the primary drivers of nitrogen transformation, with pH and electrical conductivity playing secondary roles. Overall, deep tillage stimulated nitrogen removal processes and affected net ammonium changes. These findings reveal that deep tillage can stimulate nitrogen removal processes by alleviating soil compaction and altering nitrogen transformation pathways, thus supporting biogeochemical recovery mechanisms after deep tillage. These insights provide scientific guidance for the ecological restoration of Spartina alterniflora-invaded coastal wetlands.