Impact of grass traits on the transport path and retention efficiency of nitrate nitrogen in vegetation filter strips

Abstract

Vegetation filter strips (VFS) have been shown to effectively intercept water flow and remove nitrogen. Studies of the vegetation effects on water flow and nitrogen transport are typically studied based on qualitative analyses with species, layouts, growth stage, seasonal changes, and other vegetation conditions and did not considered quantification based on plant traits. In this study, the transport path and retention efficiency of nitrate nitrogen (NO3-N) in VFSs are investigated from the view of the plant specific trait effects by simulating runoff experiments using three grassed VFSs (centipede grass, tall fescue, and vetiver grass), as well as a bare VFS under different slope gradients (2%, 7%, and 12% slope gradients). The primary grass traits (stem spacing, root depth, root length density, and nitrogen total uptake) were measured, and the responses of water flow and NO3-N loss above and below ground to those were quantitatively assessed using the grey correlation analysis method. Results indicated that grasses and slope gradient significantly influenced the water flow and NO3-N loss in general, except the NO3-N loss concentration of the surface flow, in which the tall fescue VFS showed the highest NO3-N total retention rates under each slope condition. Considering the impact of specific grass traits, stem density and root length density has the greatest effects on the surface flow and the subsurface flow respectively, while the effect of roots was lower than that of stems on NO3-N loss below ground. Additionally, the NO3-N loss mass in the surface and subsurface flow were mostly related to the water flow volume and the NO3-N loss concentration respectively. This indicated that grass traits that reduce water flow should be considered more to control the NO3-N loss mass above ground, whereas the grass traits reducing NO3-N loss concentration should be considered more to control the NO3-N loss mass below ground.