Root production of hybrid poplars and nitrogen mineralization improve following mounding of boreal Podzols

Abstract

Successful establishment of fast-growing trees could depend on early root development and the access to belowground resources. Boreal podzolic soils present a distinctive vertical zonation wherein nutrient availability and the presence of plant roots decline sharply with depth. Mechanical soil preparation that modifies the vertical arrangement of soil layers creates microsites with improved physical conditions but potentially lower nutrient availability. We compared the vertical distribution of proximal roots of young hybrid poplars in soil layers of mechanically prepared (by mounding) and unprepared microsites. We also evaluated the relationship between root distribution and the availability and mineralization of soil nitrogen. Hybrid poplar roots were less abundant in the surface organic layer of unprepared soils, whereas they proliferated in the buried organic layer of mounds. Total mineralized N was highest in the upper mineral layer of mounds, whereas it was similar between the buried organic layer of mounds and the unprepared organic layer. Altogether, mounding created conditions conducive to greater soil N mineralization and greater production and vertical distribution of proximal roots. This possibly provided access to a larger soil volume and greater soil nutrient pools, which may explain the success of mounding in terms of aboveground growth of hybrid poplars.