Effectiveness of grass planting in drainage ditches to intercept nitrogen and phosphorus pollution from farmland into rivers

Objectives The coupling effect of excessive fertilization with rainfall runoff causes a large amount of nitrogen and phosphorus loss from farmland into river channels, resulting in water pollution of rivers and lakes. This effect is very severe in the intensive agricultural sloping lands with shallow soil layer and poor water storage of tropics and subtropics. Under rainfall events, the fertilizer will be more easily lost from slopping land and transported to rivers and lakes through ditches, resulting in water pollution downstream. How to control the transport of this non-point source pollutants derived from farmland erosion to rivers and lakes has attracted more and more attention. Therefore, a plant interception system was constructed in the downstream ditches of watershed, and the effectiveness of this system was evaluated in term of reduction of nitrogen and phosphorus pollutants passed through this plant interception system. It is expected that this study can provide a theoretical basis for the control of water erosion-induced agricultural non-point source pollutants in the watershed.

Methods In this study, a 500 meters long ditch in the downstream was selected in Nala small watershed, located in the water source area of Kelan Reservoir, Guangxi. This ditch collects the water of the upper watershed and directly transport to the reservoir. The dominant plants, Cynodon dactylon (L.) Pers and Pennisetum purpureum, and the introduced plant Vetiveria zizanioides (L.) Nash, transplanting from a Nanning Market, Guangxi, were planted spacing out in the ditch. Water samples were collected from inlet and outlet of ditch, respectively, for testing at 13 rainfall events, and the benefits of planted grasses in controlling non-point source pollution ware investigated.

Results Compared with the upstream inlet of watershed, the concentration of dissolved total nitrogen (TDN) in the downstream outlet of the vegetated ditches decreased from 17.55 mg/L to 12.43 mg/L, and NH₄⁺-N decreased from 1.06 mg/L to 0.73 mg/L. NO₃⁻-N concentration decreased from 15.10 mg/L to 10.92 mg/L, and dissolved total phosphorus (TDP) decreased from 0.031 mg/L to 0.021 mg/L. The average removal rates of TDN, NH₄⁺-N, NO₃⁻-N and TDP from watershed input ditches under rainfall events by plants were 31.90%, 27.92%, 29.80% and 31.02%, respectively.

Conclusions The results showed that in the intensive multi-runoff agricultural watershed of tropical and subtropical regions, grass planting in ditches linking upstream slopes with downstream rivers or lakes can effectively intercept and remove nitrogen and phosphorus runoff. This measure is a simple, feasible and effective approach for preventing and controlling agricultural non-point source pollution in small watershed, and is worthy of popularization and application.