Effects of Stem Diameter on Nitrogen and Phosphorus Losses in Simulated Hedgerow-grass Ditch System on Purple Soil Sloping Lands

Hedgerow-grass ditch system, a comprehensive measure combining hedrgerow and ecological grass ditches, has been widely used on purple soil slopes. Stem diameter is an important variable that affects the plants' controlling effectiveness of nitrogen and phosphorus losses on slopes, but the effects of stem diameter on nitrogen and phosphorus losses on slopes with hedgerow-grass ditch system is not clear. In this study, we set up five kinds of plant hedge stem diameters, i.e., 0 mm (control, CK), 1 mm (T1), 2 mm (T2), 4 mm (T3), and 8 mm (T4), and investigated the characteristics of total nitrogen (TN) and total phosphorus (TP) loss of surface runoff from the plant hedge-grass ditch system through indoor simulated rainfall test, and evaluated the nitrogen and phosphorus loss reduction benefits of the different stem diameters of the plant hedge-grass ditch system in the purple soil slopes. The results show that (1) The sediment reduction rates of T1, T2, T3 and T4 were significantly higher than those of CK treatment with 39.95%, 59.86%, 83.60% and 94.42% (P<0.05), respectively; and the sediment interception benefits of T3 and T4 treatments were significantly higher than those of T1 (P<0.05). (2) Under each treatment, the changes of runoff TN and TP loss concentration with rainfall calendar time were 2.89-10.08 and 0.31-0.62 mg·L^-1, respectively; compared with CK, the average loss concentration of TN was significantly reduced in T2, T3 and T4 (P<0.05). The reduction of TN loss concentration in T2, T3, and T4 treatments was significantly higher than the reduction of T1 treatment (P<0.05). (3) Compared with CK, the TN loss and loss rate under T2 and T4 treatments were significantly reduced by 51.92%, 60.66% and 49.37%, 58.75%, respectively (P<0.05). (4) The concentration losses of TN and TP and loss amount of TP had significant negative exponential correlations with stem diameter (P<0.05). The results of this study provide theoretical basis for the optimization of agricultural non-point source pollution control measures in purple soil sloping lands.