What makes ditches and ponds more efficient in nitrogen control?

Ditches and ponds are widely used ecological engineering measures to control agricultural non-point source pollution. Numerous site-level studies have examined the removal rate of total nitrogen (TN) in ditches and ponds, but these results vary greatly. In this work, a statistical analysis of the TN removal rate in ditches and ponds was performed by summarizing 398 data from 92 studies. Through meta-analysis, we obtained the overall effect was a 38.7% reduction in TN. And the influence of different factors (including temperature, vegetation, ditches structure, location, and influent concentration) on the TN removal rate of ditches and ponds are analyzed in this paper. In the correlation analysis, the temperature and influent concentration showed a significant positive correlation with the TN removal rate, while water depth and ditch length have little effect on TN removal rate. Natural plants have similar functions as selected plants in water purification. For full-concrete ditches, their primary function is drainage and almost do not have water purification ability. However, there was no significant difference between part-concrete and non-concrete ditch, indicating that partial concrete of the ditch is acceptable. Temperature is a vital constraint factor for nitrogen retention. Ditches and ponds do not perform well at low temperatures, and may even become sources. Our results demonstrated the great potential of TN removal in ditches and ponds and provided information for better design of ecological ditches and ponds to treat agricultural runoff.