Are riparian buffer strips effective for nutrient retention under cold frozen conditions?

Abstract

Riparian buffer zones (RBZs) are commonly employed as a nature-based solution to mitigate the transport of nutrients from cultivated lands to freshwater bodies. However, effectively retaining nutrients in cold conditions presents a challenge. This is due to the fact that herbaceous vegetation wilts during winter, becoming covered by snow and ice, and unable to actively absorb pollutants. Additionally, frozen soils restrict or completely prevent infiltration. This study aimed to evaluate the performance of buffers in reducing nutrient levels in agricultural overland flow in a semi-arid region of northern China. Runoff monitoring was conducted using twelve steel flumes, established at both the inflow and outflow of the RBZs at each site along the predicted overland flow path. Furthermore, phosphorus (P) concentrations were estimated in RBZ soil samples taken along the runoff flow line to gain further insights into nutrient dynamics within various RBZs. Analysis of runoff samples from 35 events throughout the study demonstrated that, in most instances, buffers have minimal to no impact on reducing the levels of nitrogen (N) and P species under snowmelt-generated runoff. Although the influence of buffers on nutrients was limited during the snowmelt period, they performed better during summer than in the autumn, resulting in a moderate reduction of TN and TP levels in 83 % of the sites. Furthermore, the available P concentrations in the soil of 50 % of the studied buffers were considerably higher than in cropland soils. The subpar performance of buffers in retaining nutrient in the studied area can potentially be attributed to the fact that most of the overland flow is concentrated (rather than uniform sheet flow) via narrow fractions across buffers, and this occurs specifically during snowmelt when biogeochemical mechanisms (e.g., sorption, infiltration, plant and microbial assimilation) responsible for nutrient removal in buffers are limited. This study also revealed that the percentage of water extractable N and P released from various species subjected to 0, 3, or 6 freeze-thaw cycles varied from 6.13 to 68.90 % and 10.60–73.46 %, respectively. Future research efforts should focus on identifying alternative management measures that enhance nutrient removal during concentrated snowmelt runoff periods in these low-temperature areas.