Water and sediment regulation eluting and washland planting lead to nitrogen increase in the lower reaches of the Yellow River

The Yellow River is an important agricultural production base in China, plays a key role in terrestrial sea transport and nitrogen transformation. However, the reason for the transient nitrogen increase in the lower Yellow River remains unclear. This study explored the contributions to transient nitrogen elevation from the water column, suspended particulate matter, surface sediments in the lower Yellow River, and washland soils along it throughout the water and sediment regulation event in 2023. Results indicated that the average dissolved nitrate concentrations in the lower Yellow River were 1.38 and 1.12 times higher before and during water and sediment regulation than after, because of excess reactive nitrogen elution from the beach by the water and sediment regulation. The nitrogen release risk was low in suspended particulate matter and surface sediments (ion exchangeable form nitrogen content was 0.007–0.033 mg·g⁻¹) but high in soil (average ion exchangeable form nitrogen content was 0.092 mg·g⁻¹). Leaching results indicated that nitrate concentration in the water was not significantly influenced by the reduction in suspended particulate matter or surface sediments. In contrast, in soil S-13 (ion exchangeable form nitrogen content was 0.371 mg⋅g⁻¹), the estimated leaching rate of nitrate averaged 14.74 %, and ion exchangeable form nitrogen accounted for 19.25 % of total nitrogen, with 76.56 % of ion exchangeable form nitrogen leached. Therefore, the water and sediment regulation elution and the continuous leaching of nitrogen from washland soils around the lower Yellow River notably increased the nitrogen concentration in the lower Yellow River.