Objectives To investigate the effects of exogenous ammonia nitrogen input on nitrogen transport and exchange fluxes at the surface sediment-water interface of the inflow area of Yunnan Plateau Lakes.

Methods The surface sediment of the Xingyun Lake Fishery Village River inflow area was selected as the research object and (NH₄)₂SO₄ was used as the exogenous ammonia nitrogen. Static sediment culture experiments were conducted to study the effects of different concentrations of ammonia nitrogen addition on nitrogen exchange flux at the sediment-water interface. The experiment was conducted with four gradient experimental groups (4, 8, 16, 32 mg/L) and one blank control group, with the mass concentration of nitrogen in the ammonium sulfate solution serving as the basis for the experimental design. The cultivation period was 30 days.

Results The results indicated that the average total nitrogen (TN) content in the surface sediment of the Xingyun Lake Fishery Village River inflow area was 2.95 g/kg, with an average single-factor pollution index of 5.36, reaching the severe pollution level in the single-factor pollution index evaluation criteria, posing a high potential ecological risk. The proportion of exchangeable nitrogen (EN) in TN averages 4.94%, indicating a strong nitrogen migration capability. Throughout the experimental period, the exchange flux F(NO₃⁻-N) of nitrate nitrogen (NO₃⁻-N) in the sediment was consistently positive, indicating a migration characteristic of NO₃⁻-N releasing from sediment to water, positively correlated with the concentration of exogenous input ammonia nitrogen (NH₄⁺-N). The higher the concentration of exogenous input NH₄⁺-N, the greater the release of NO₃⁻-N from sediment, and vice versa. On the other hand, the exchange flux F(NH₄⁺-N) of NH₄⁺-N gradually changes from positive to negative during the experimental period, ultimately approaching zero, indicating a migration characteristic of ammonia nitrogen being released initially and then precipitated, eventually reaching a dynamic equilibrium state.

Conclusions The rapid release of NO₃⁻-N from sediment in response to high concentrations of exogenous NH₄⁺-N input can result in the continuous accumulation of NO₃⁻-N in the water body. This accumulation can result in a deterioration of water quality and the emergence of environmental issues such as eutrophication and algal blooms. Consequently, in the future, it would be prudent to devote attention to the potential for endogenous nitrogen release from sediment in the study area. Further efforts should be made to reinforce the control and comprehensive management of lake runoff pollution to mitigate the water environmental problems caused by NH₄⁺-N input.