Prediction of long-term sustainability of constructed urban soil: impact of high amounts of organic matter on soil physical properties and water transfer

Sustainability of urban soils lies in their ability to facilitate water and air permeabilities. Exogenous organic matter has been shown to have a positive impact on these properties. Under urban conditions, a large one-time input of an organic amendment was made to the reconstituted soil. Two organic materials, green-waste compost (gw) or cocompost from sewage sludge and wood chips (sw), were mixed with sandy loam soil (40% v/v) and placed in 600-L containers. Containers received a 29-cm thick layer of sandy loam soil-organic matter mix over a 28-cm thick layer without organic amendment. Volumetric water content, dry bulk density, hydraulic conductivity at saturation and water retention were measured over 5yrs in the soils and values for the mixes and a control compared. After this time, dry bulk density was greater (1.54g/cm3) in control than in gw or sw soils (1.31 and 1.11g/cm3, respectively), whereas hydraulic conductivity at saturation was smaller (4x10-7m/s) than in gw (3.4x10-6) or sw (3.7x10-6m/s). HYDRUS 1D water balance model indicated that below 27cm depth in the control after 5yrs, there was a high degree of anoxia, lasting >200days per year, compared with <40days in gw and sw. Amplification of the risk of anoxia below 27cm depth after 10yrs was 323, 151 and 100days in the control, gw and sw, respectively. Organic matter amendment could support sustainable urban soils for ten years after soil reconstitution.