Optimization of the contour ridge system for controlling nitrogen and phosphorus losses under seepage condition

Contour ridging can enhance the occurrence of deep seepage because more rainwater concentrates in the low areas along furrows when soil is saturated. Greater seepage could significantly increase nonpoint source pollution. To optimize the contour ridge system to effectively control nutrient losses under seepage condition, 23 treatments with three variables (row grade, field slope and ridge height) in five levels were arranged in a quadratic orthogonal rotation combination design. Results showed that field slope and interactions between the three factors did not significantly influence NO₃‐N and PO₄‐P losses. The dominant factor that controlled NO₃‐N loss was ridge height, followed by row grade. The smallest NO₃‐N loss was obtained at a ridge height of 8.72 cm and a row grade of 7.05°. The dominant factor that controlled PO₄‐P loss was ridge height. The optimal ridge height for efficiently controlling PO₄‐P loss was 9.79 cm. For simultaneously maximally controlling NO₃‐N and PO₄‐P losses, a ridge height of 9.26 cm, a row grade of 7.05° and a field slope of 5° were optimum. This study provides guidance for implementing the contour ridge system.