Abstract: Triarrhena lutarioriparia, one of the most widespread dominant plant species in the Dongting Lake wetlands, has raised ecological concerns due to its large-scale abandonment. Based on field investigations and simulation experiments, this study comparatively analyzed the changes in soil nitrogen and phosphorus contents and their forms between harvested and abandoned areas, aiming to provide a scientific basis for the quantitative assessment of ecological risks following abandonment. Field investigation of plots abandoned for over five years revealed that abandonment increased the total nitrogen and ammonium nitrogen contents in the 0~20 cm soil layer, as well as the nitrate nitrogen content in the 0~60 cm soil layer. However, total phosphorus content in the 0~60 cm soil was not significantly affected, while aluminum-bound phosphorus increased and iron-bound phosphorus decreased. A 15-month continuous monitoring of simulated abandonment showed no significant changes in nitrogen or phosphorus contents in the 0~60 cm soil and pore water of soil. Random forest model analysis indicated that the contribution of aboveground biomass to changes in soil nitrogen and phosphorus increased significantly after abandonment, suggesting that plant residues and litter are major pathways for nutrient return to the soil. In conclusion, the impact of Triarrhena lutarioriparia abandonment on soil nitrogen and phosphorus in the Dongting Lake wetlands exhibits clear temporal dependency: short-term abandonment has limited effects, whereas long-term abandonment increases soil nitrogen and Al-P contents while decreasing Fe-P contents.