Effects of poplar-amaranth intercropping system on the soil nitrogen loss under different nitrogen applying levels

Characteristics of soil nitrogen loss were investigated based on field experiments in two types of poplar-amaranth intercropping systems (spacing: L₁ 2 m×5 m, L₂ 2 m×15 m) with four N application rates, i.e., 0 (N₁), 91 (N₂), 137 (N₃) and 183 (N₄) kg·hm^-2. The regulation effects on the soil surface runoff, leaching loss and soil erosion were different among the different types of intercropping systems: L₁ > L₂ > L₃ ( amaranth monocropping). Compared with the amaranth monocropping, the soil surface runoff rates of L₁ and L₂ decreased by 65.1% and 55.9%, the soil leaching rates of L₁ and L₂ with a distance of 0.5 m from the poplar tree row decreased by 30.0% and 28.9%, the rates with a distance of 1.5 m decreased by 25.6% and 21.9%, and the soil erosion rates decreased by 65.0% and 55.1%, respectively. The control effects of two intercropping systems on TN, NO₃^--N and NH₄⁺-N in soil runoff and leaching loss were in the order of L₁ >L₂ >L₃. Compared with the amaranth monocropping, TN, NO₃^--N and NH₄⁺-N loss rates in soil runoff of L1 decreased by 62.9%, 45.1% and 69.2%, while the loss rates of L₂ decreased by 23.4%, 6.9% and 46.2% under N₁(91kg·hm^-2), respectively. Higher tree-planting density and closer positions to the polar tree row were more effective on controlling the loss rates of NO^3--N and NH₄⁺-N caused by soil leaching. The loss proportion of NO₃^--N in soil runoff decreased with the increasing nitrogen rate under the same tree-planting density, while that of NH₄⁺-N increased. Leaching loss of NO₃^--N had a similar trend with that of NH₄⁺-N, i.e., N₃ >N₂>N₁ >N⁰.