The Role of Biological Factors in Nitrogen Attenuation in Ditch: An Innovative Approach Through Quantification of Biological Factors

Abstract

Agricultural nonpoint source pollution has been a major cause for surface water degradation. Furthermore, the transport and transformation of nitrogen within ditches are recently found to be critical processes in determining agricultural non-point source pollution. Currently, research on the quantification of nitrogen attenuation in ditches primarily focuses on hydrological factors. Although the role of biological factors is gaining recognition, effective methods to quantify their impacts are still lacking. Additionally, the understanding of biological effects at larger scales, such as the watershed scale, is also limited. In this study, a Biodiversity (B) index was developed to quantitatively assess the biological status in ditches. Furthermore, a case study with a 15N isotope tracing experiment was performed to investigate nitrogen attenuation among different ditches. Our results provide new insights into the role of biological factors in nitrogen attenuation progress. We observed the Biodiversity (B) index in unhardened ditches (1.10) was higher than in vegetated hardened ditches (0.67) and minimally vegetated hardened ditches (0.05). This indicated that the biological status in unhardened ditches is significantly healthier. Moreover, our findings revealed that nitrogen attenuation of vegetated ditches is higher than in non-vegetated ditches. Specifically, the cumulative attenuation coefficient and optimal attenuation distance of NO3- in D1 were 1.7-fold and 92.9% higher than that in D2. Both hydrological and biological factors collectively influenced the N attenuation coefficient. By examining the relationship between the nitrogen attenuation coefficient and these influencing factors, we found that 94% of the variation in nitrogen attenuation was explained by three major factors, including distance, Biodiversity (B) index, and discharge. Overall, the biological factors assessment framework we constructed offers a novel approach to quantifying the role of biological factors in the nitrogen attenuation process in ditches.

Keywords: Ditch, Biological factors, Nitrogen, Attenuation process