STREET TREE ROOT DEVELOPMENT IN TOPSOIL AMENDED WITH HIGH LEVELS OF COMPOST

Abstract

The supply of high levels of organic matter can considerably improve the physical properties of urban soil and tree root growth. In return, tree root development can induce changes in the physical properties of bulk soil. In October 2004, 49 big box containers (600 L) were filled with two layers of substrate. The underlying layer (25-40 cm in depth) consisted of sandy loam. The topsoil layer (0-25 cm) was enriched with 40% v/v organic matter. Three organic products were tested: sewage sludge and woodchip compost (SW), green waste compost (GW) and sphagnum peat (SP). The control container was composed of two layers of sandy loam. Ostrya carpinifolia Scop were planted in 21 containers. Two years after planting, dry bulk density, surface permeability and root development (total root dry mass, root number and distribution) were measured in containers with and without trees. Organic matter had a significant impact on the physical properties compared to the control, with a decrease of the soil bulk density (SW=SP>GW>control) and an increase of the surface hydraulic conductivity (SW>GW=SS>control). At the same time, organic matter improved the total root dry mass and the development of thin roots (diameter SW>SP=control). The underlying layer was unsuitable for root growth with a small number of roots. In contrast, tree root development had almost no impact on the physical properties of the bulk soil: for each organic product, statistical analysis did not reveal significant differences in bulk density and surface permeability values when comparing containers with and without trees. Globally, large quantities of compost enhanced physical properties and thin root development; however, after two years, soil physical property measurements did not seem to be influenced by roots.