Dissipation mechanisms of ecological ditch on agricultural non-point source pollution and their influencing factors

Agricultural Non-Point Source Pollution (ANSP) has posed a great challenge in modern agriculture worldwide, due mainly to the massive application of fertilizers and pesticides in fields. It is of great significance to develop ecological restoration technologies to mitigate the ANSP. Among them, an ecological ditch can function as a farmland drainage ditch and constructed wetland in recent years. Most previous studies have focused on the dissipation mechanisms of nitrogen (N) and phosphorus (P). However, it is still lacking in the pesticide removal in the ecological ditch. Meanwhile, it is a high demand for the multiple factors analysis rather than the commonly-used single factor analysis, because the dissipation mechanisms of the ecological ditch were influenced by the interaction of multiple factors. In this review, a literature survey was performed on the field experiments of ecological ditches using the ISI Web of Science and CNKI databases from 2001 to 2022. The dissipation mechanisms of ANSP included sediment sorption and plant resistance, as well as plant/microorganism absorption and degradation. The search criteria involved field experiment, ecological ditch or drainage ditch, and ANSP. A total of 559 groups of field experimental data were analyzed to explore the removal rates of total nitrogen, ammonia nitrogen, nitrate nitrogen, total phosphorus, orthophosphate, and pesticides. Four influencing factors were taken into account, i.e., the Initial Contaminant Concentration (IC), Hydraulic Retention Time (HRT), Plant Species (PS), and Plant Biomass (BM). Other influencing factors were also discussed, such as the temperature, hydraulic load, rainfall, and alternate wetting and drying. Both single and multiple factors analysis were carried out using correlation analysis and Multiple Linear Regression Model (MLR), respectively. The results indicated that both total nitrogen and total phosphorus increased as the IC, HRT and BM increased (P<0.01), while the PS had no significant correlation with total nitrogen and total phosphorus (P>0.05). The reason was that the sorption and uptake processes were stimulated by the higher IC, HRT and BM, whereas, the resuspension was inhibited by the higher HRT and BM in the ecological ditch. The MLR analysis demonstrated that the coefficient of determination (R2) was 0.80 and 0.92 for the total nitrogen and total phosphorus, respectively. Both the IC and BM showed a positive correlation with the total nitrogen and total phosphorus (P<0.05). Therefore, the high IC and BM greatly contributed to the high total nitrogen and total phosphorus. Particularly, the contribution ratio of IC was higher than that of BM in the MLR equations for the total nitrogen and total phosphorus. The high concentration gradient was attributed to the contribution of the ecological ditch. The diffusion of nutrients was promoted from the overlying water to pore water in the sediment. As such, more favorable absorption conditions were provided for the sediment microorganisms and plants. The single-factor analysis indicated that the pesticide declined as the IC raised (P<0.05), in terms of pesticides. The toxicity of pesticides might inhibit the pesticide-degradation ability of the microorganisms and plants in high IC condition, further weakening the metabolic activity of microorganisms and the absorption of plants. The pesticides raised significantly, as the HRT increased (P<0.05), indicating the longer microbial reaction for the higher pesticide removal rate. Besides, the MLR equation for the pesticides showed reliable statistical parameters in the training set with the R2=0.56. The pesticides in the MLR were negatively correlated with the IC (P<0.05), but positively correlated with the HRT (P<0.05), which were consistent with the single-factor analysis. Therefore, it is very necessary to supplement the experimental research on the interaction of multiple factors in the future. It is also of great significance to investigate the other hydrologic and environmental factors on the removal efficiencies of ecological ditches. Future research can also explore the effects of pesticide structures and microbiological mechanisms on pesticide removal in ecological ditches. © 2022 Chinese Society of Agricultural Engineering. All rights reserved.