Effects of deep tillage and municipal green waste compost amendments on soil properties and tree growth in compacted urban soils

Large trees are often seen as a means of offsetting negative consequences of growing urban densification. To increase the tree canopy cover of dense urban landscapes, developers, planners and urban tree managers are often forced to plant into damaged and compacted sites. Compacted urban soils can hinder the establishment and growth of deep rooted, woody plants by: 1) impeding root exploration and development which is critical for water and nutrient acquisition; 2) reducing infiltration of water into the soil and the availability of water to plants; and 3) reducing gas exchange and the balance between anaerobic and aerobic conditions. At three sites in Melbourne, Australia with compacted and damaged soils, we established four soil re mediation treatments: 1 & 2) tillage to 0.25 m with and without 50% (v/v) municipal green waste compost (MGWC) additions, and 3 & 4) tillage to 0.5 m with and without 50% MGWC addition, plus a non-remediated control. Each treatment was replicated (n = 3), and one Corymbia maculata (spotted gum) tree was planted into the centre of each 2 x 2 m treatment plot (n = 15), at all three sites (n = 45). Bulk density and field-saturated hydraulic conductivity were improved by tillage, at least in the short-term. The use of MGWC may maintain these changes for longer. Depending on site soil conditions tree growth may be improved by tillage alone. At one site, we found that additions of MGWC lead to nitrogen immobilisation due to site soil conditions. At another site, deep tillage (with or without MGWC) led to significantly improved tree growth. Compacted and degraded urban soils may be improved through simple tillage and/or organic amendment strategies for the successful establishment of deep rooted woody plants. However, site soil conditions will dictate whether the addition of MGWC is beneficial or not, as one site showed no positive response to any tillage or MGWC. This research has examined a technique that can be used by landscape managers to improve soil physical characteristics and, in certain circumstances, can improve deep-rooted woody plant establishment and growth in challenging compacted urban soil conditions.