Effects of poplar intercropping systems on reducing the concentration of different forms of inorganic nitrogen in the soil profile

This study focused on understanding how different spacing arrangements of poplar (Populus deltoides)-wheat (Triticum aestivum) agroforestry systems and the processes of the tree and wheat roots in these systems impact the elimination efficiency of inorganic nitrogen from soil. Three planting systems were tested: a monoculture wheat system (control), a widely spaced wheat-poplar system (PWW) and a narrowly spaced wheat-poplar system (PWN). Soil and root samples were collected in four different soil layers (0-20, 20-40, 40-60, and 60-100 cm) at four different sampling sites (PWN: 0, 1.5, 2.5, and 3.5 m; PWW: 0, 1.5, 3.5, and 7 m from tree row, respectively). The nitrate-nitrogen (NO3--N), ammonium-nitrogen (NH4+-N), and nitrite-nitrogen (NO2--N) in the soil samples were analyzed to determine the elimination efficiency of the different systems. The results indicated that the concentration and soil accumulation content reduction of inorganic nitrogen in the two intercropping systems, especially in the PWW system, were encouraging compared to those in the control system. The NO3--N and NO2--N concentration reductions and poplar fine root length density (FRLD) were higher at the sampling point near the poplar row, especially in surface soil (p < 0.05), but the distribution of the wheat FRLD decreased with soil depth and proximity to the poplar trunk. Furthermore, the poplar and wheat FRLD had significant positive relationships with the concentration reduction of inorganic nitrogen. In conclusion, poplar-agroforestry systems should be considered as an important method for controlling nonpoint pollution from farmland.