Agricultural non-point nitrogen pollution control function of different vegetation types in riparian wetlands: A case study in the Yellow River wetland in China

Riparian wetland is the major transition zone of matter, energy and information transfer between aquatic and terrestrial ecosystems and has important functions of water purification and non-point pollution control. Using the field experiment method and an isotope tracing technique, the agricultural non-point nitrogen pollution control function of different vegetation types in riparian wetland was studied in the Kouma Section of the Yellow River. The results showed that the retention of agricultural non-point nitrogen pollution by riparian wetland soil occurs mainly in top 0-10 cm layer. The amount of nitrogen retained by surface soils associated with three types of vegetation are 0.045 mg/g for Phragmites communis Trin Linn, 0.036 mg/g for Scirpus triqueter Linn, and 0.032 mg/g for Typha angustifolia Linn, which account for 59.21%, 56.25%, and 56.14% of the total nitrogen interception, respectively. Exogenous nitrogen in 0-10 cm soil layer changes more quickly than in other layers. One month after adding (KNO3)-N-15 to the tested vegetation, nitrogen content was 77.78% for R communis Trin, 68.75% for T angustifolia, and 8.33% for S. triqueter in the surface soil. After three months, nitrogen content was 93.33% for P. communis Trin, 72.22% for S. triqueter, and 37.50% for T Angustifolia. There are large differences among vegetation communities respecting to purification of agricultural non-point nitrogen pollution. The nitrogen uptake amount decreases in the sequence: new shoots of P. communis Trin (9.731 mg/g) > old P. communis Trin (4.939 mg/g) > S. triqueter (0.620 mg/g) > T angustifolia (0.186 mg/g). Observations indicated that the presence of riparian wetlands as buffers on and adjacent to stream banks could be recommended to control agricultural non-point pollution.