Improving degraded and compacted urban soils to support tree establishment and growth

© 2019 Peter Damien SomervilleThe creation and expansion of human habitats alters the natural properties of urban soils, not only through direct anthropogenic soil disturbance, but also indirectly through impacts from the anthroposphere. This creates soils that are novel and different to those found in non-urbanised settings. These urban soils are often compacted and degraded and not be suitable for the establishment and growth of deep-rooted tree species that make urban habitats liveable. In this thesis, I have studied methods to rehabilitate compacted and degraded urban soil to improve the successful establishment and growth of trees planted into those soils by improving key physical and biological properties through tillage and the incorporation of OM amendments. My objectives for the study were: 1. To investigate the effects that the incorporation of organic amendments and tillage have on improving the soil properties of degraded and compacted urban soils and their ability to support improved tree performance. 2. To develop an understanding of the interaction between organic amendments and soil type when improving degraded urban soils. 3. To understand the interaction between tree water strategies in determining tree growth response and levels of water stress in degraded urban soils amended with different organic amendments. For the first objective, I found that the use of a ‘scoop and dump’ tillage technique, with the incorporation of organic matter (compost and/or biochar), was effective at improving key physical and biological properties of compacted and degraded urban soils. However, the successful growth of trees in remediated soils was partly determined by the initial site conditions found at each site, and improved tree growth was not always apparent. In both sandy and clay soils, improvements to physical and biological properties, achieved with the use of tillage, were maintained for longer, when organic matter was also incorporated into soils during the tillage. The addition of organic matter improved the biological activity of tilled sand and clay soils, and this may have been a result of improved soil water availability. In addition, I found that there were very few differences in soil and tree growth outcomes between the compost and biochar used in either soil type. Although the incorporation of biochar was effective in the remediation of urban soils, it was no more effective than the incorporation of compost or of compost and biochar combinations. There were no synergistic tree growth benefits from using a combination of compost and biochar. The second objective of this project was to investigate the interaction between organic amendments and soil types when improving degraded urban soils. The water holding properties of sandy soils were improved with the addition of organic matter where the plant available water content was increased in both a greenhouse pot study and a field study. Alternatively, the plant available water content was decreased in a clay soil amended with organic matter in a greenhouse experiment and unchanged in the clay soils of a field experiment. Although the water characteristics were not changed in the clay soil, tree growth was nevertheless improved due to improvements in other soil hydrological properties achieved with the addition of organic matter. These changes allowed for faster infiltration and decreased the bulk density of the clay soils. The third objective of this thesis evaluated the interaction between tree water use strategies, soil types and the incorporation of different types of organic matter. Compared to an unamended control, a xeric tree species accumulated more biomass when grown in a sandy soil amended with compost, biochar and a combination of compost and biochar, under both well-watered and water deficit irrigation regimes. There were no biomass changes in the clay soil. There were no differences measured in the growth of the mesic species in either the sand or clay soil or with the compost, biochar or the combination of compost and biochar. This thesis suggests that while urban soils can be successfully remediated with the application of tillage, the incorporation of organic matter will prolong the benefits achieved by the tillage. However, successful remediation of compacted and degraded urban soils will ultimately depend on applying solutions based on an understanding of existing soil properties and the tree resource acquisition strategies required for a particular site. Management practices utilising the incorporation of organic matter should assess the cost/benefits of different types of organic matter amendments as the outcomes of using different organic matter types may not differ