Favour street tree root development with high additions of organic matter induces changes in urban soil properties.
Résumé
Physical properties of urban soils often limit tree development. Urban soils are compacted resulting in reduced water infiltration and air. The available soil volume for root colonisation is usually limited by the underground infrastructures. Exogenous organic matter has been shown to have a positive impact on the physical properties of agricultural soils. In some urban situations, the wide-scale availability of organic matter and the need for important soil restructuring have led to the addition of very large quantities of organic matter. Our objectives were to measure in soil reconstituted with high levels of organic matter: (1) the effects of organic matter decomposition on the evolution of physical properties, and on street tree root and shoot development; (2) the changes induced by tree root development on the physical properties of bulk soil.
We monitored over a 60-month period, physical properties – dry bulk density, structural stability and permeability – of reconstituted soils placed in 600-L containers under natural conditions. The top layer was a sandy loam amended with 40% by volume of sphagnum peat or organic compost – a sewage sludge and wood chip compost or a green waste compost – lying on a layer of sandy loam. The control container was composed of two layers of sandy loam. Bare root, Ostrya carpinifolia Scop were planted into half of the containers and we monitored root biomass and distribution and, shoot development.
Organic matter had significant effects on the soil physical properties compared to the control with a significant decrease of the soil bulk density and a significant increase of structural stability and infiltration rate. Root system significantly responded to organic matter content which increased the root mass and density. Root system development followed the vertical gradient of soil bulk density and, the underlying layer presented a small root number and was unsuitable for root growth. Similarly to the roots, branching number and order were favoured by organic matter compared to the control.
After 2 years, the root development in the containers was assessed but did not affect the bulk soil physical properties which were similar for all treatments. After 5 years, statistical analysis showed that tree root density had significant effects on structural stability and infiltration rates comparing containers with and without tree.
Great level of compost in a constructed soil increased physical properties and tree above and below ground development. We assessed that, five years after planting, the tree root system improved urban soil structure.