Confluence/diversion catch-drainage layout measures decreasing phosphorus loss from sloping lands in Three Gorges Reservoir Areas

Catch-drainage layout measures have been widely applied as a benefitial soil and water conservation measure on the sloping land. However, little information is availiable on the effect of catch-drainage measures on soil phosphorus loss on the sloping lands. Optimizing the catch-drainage layout measure can effectively control the nutrient loss on the sloping lands, which is of great significance for preventing and controlling agricultural non-point source pollution and ensuring the safety of downstream water environment. Soil samples were collected from typical sloping lands of three land use types (slope farmland, grassland and forest land) for simulated rainfall with an rainfall intensity of 106 mm/h to study the effects of three catch-drainage layout measures on phosphorus loss on sloping land. The PVC plastic plate was inserted into the soil to form three catch-drainage layouts, including T1-Confluence, T2-Divergence, CK-Control check. Runoff and sediment samples were collected during the artificial simulated rainfall to determine the concentration of total phosphorus, dissolved phosphorus and particulate phosphorus. The results showed that T1 and T2 decreased the average runoff rates by 40.33%-44.33% and 42.57%-54.52% and the average sediment yield rates by 68.14%-71.15% and 60.55%-72.57% at each land use type compared with CK, repsectively. It was because that T1 and T2 showed segmented retention effects on runoff, which promoted water infiltration, reduced runoff velocity, and effectively weaken the scouring effect and sediment carrying capacity of runoff, resulting in a decrease in runoff volume and sediment yield. Under each catch-drainage layout measure, the average sediment yield rate under forest land was significantly lower than that under grassland and slope farmland. The variation range and average concentration of total phosphorus, dissolved phosphorus and particulate phosphorus under different land use types showed T1 < T2 < CK. Dissolved phosphorus was the main form of phosphorus loss in this study, which was different from previous research results. The reason may be that the T1 and T2 showed better reduction effects on sediment than that on runoff, and then led to the larger loss amount of dissolved phosphorus. Compared with CK, T1 and T2 significantly reduced the loss amounts of total phosphorus, dissolved phosphorus and particulate phosphorus (P < 0.05). Compared with CK, the average reduction rate of T1 and T2 on total phosphorus, dissolved phosphorus and particulate phosphorus showed T2 (70.34%) > T1 (66.47%) under slope farmland, while these reduction rate of T1 and T2 showed T1 (65.19%) > T2 (59.16%) under grassland and forest land. The results indicated that both confluence and divergence catch-drainage layouts could effectively reduce the loss of phosphorus on the sloping land, and the confluence was more suitable for water systems on the grassland and forestland, while the divergence was more suitable for water systems on the slope farmland. Our study could provide theoretical support for optimizing the catch-drainage layout in the slope water system, and provide effective meaures to prevent and control the non-point source pollution in the Three Gorges Reservoir area.