©2023 International Society of Arboriculture 180 Percival et al: The Influence of Soil Decompaction and Amendments on Soil Quality Vertical mulching involves creating a series of shallow holes 20 to 30 cm deep in and around the root zone and filling them with mixtures of high-quality topsoil, soil improvers such as compost, perlite, and/ or vermiculite, inorganic or organic fertilizers, and/or mycorrhizal inoculants (Morris et al. 2009; Kuncheva 2015). The removal of soil to create holes can be per- formed with either an air excavation tool or an earth auger. Biochar is the solid co-product of biomass pyroly- sis: a technique used for carbon-negative production of second-generation biofuels. Biochar can be applied as a soil amendment, where it permanently sequesters carbon from the atmosphere as well as improving soil structure, nutrient retention, and crop productivity (Blackwell et al. 2009; Elad et al. 2012; Schaffert et al. 2022). Indirect evidence exists that biochar will aid in the reduction of soil compaction by altering the physical nature of most soils through increasing the soil surface area and enhancing soil pore space (Blackwell et al. 2009). Woodchip mulches are regularly used within urban landscapes to improve soil quality. Woodchip mulches are applied to the surface of the soil from near the base of the tree trunk to at least the canopy edge at a depth of 5 to 10 cm. The beneficial effects of a wood- chip mulch are numerous and include improved soil moisture, mitigated temperature fluctuations, weed suppression, soil/root-borne disease suppression, and enhanced soil fertility (Chalker-Scott 2007; Scharen- broch 2009). Few if any studies have examined the long-term benefits of these systems individually and in combi- nation on soil quality and fertility (Smith 1978; Kalisz et al. 1994; Morris et al. 2009). The aim of this research was to evaluate 3 decompaction methods (air spading, vertical mulching, application of a wood- chip mulch) and a biochar soil incorporation individ- ually and in combination with each other. The evaluation was conducted by assessing alterations to the physical, chemical, and biological properties of a heavily compacted soil over a 5-year period. MATERIALS AND METHODS Experimental Site The study was conducted at Hatfield Forest, a 403.2- ha biological Site of Special Scientific Interest in Essex, United Kingdom (51°51'26", 0°13'46"). Hat- field Forest is also a National Nature Reserve and a Nature Conservation Review site. It is owned and managed by the National Trust. Soils throughout the forest have been identified as clay loam. Hatfield For- est can receive more than 10,000 visitors daily during the summer months. Consequently, multiple grass paths or “Rides” exist throughout the forest for visi- tors. Over time, soil compaction caused by human activity has become a serious problem. Prior to treat- ments, 4 Rides were identified. Bulk density of the Rides ranged between 1.73 and 1.80 g/cm3. Conse- quently, soil densities were higher than the growth-limiting bulk density (1.50 g/cm3) for a clay- loam texture, and root-growth impairment was highly likely. In support of this, waterlogging during wet weather was a frequent and annual problem, and a total lack of vegetation on the soil surface was observed at the 4 sites selected for study. Experimental Design and Treatment Application For each of the 4 heavily compacted Rides selected for study within Hatfield Forest, one experimental site per Ride was identified. Within each site, 9 treat- ments in 3 × 3 m plots with a 1-m spacing between plots were used as below: 1. Control: no soil decompaction work was undertaken. 2. WCM: addition of 5 cm of woodchip mulch over the compacted surface. 3. AS: the entire 3 × 3 m plot was air spaded (AirSpade® 2000, Concept Engineering Group, Verona, Pennsylvania, USA) to a depth of 25 to 30 cm. 4. AS + WCM. 5. AS + B: air spading and incorporation of biochar at 5% by volume. 6. AS + B + WCM. 7. VM: 30-cm deep, 7.5-cm wide holes were drilled for vertical mulching using a mechani- cal auger at 30 × 30 cm distances. The soils removed by the process were decompacted by hand and, where necessary, a wooden mallet was used to backfill the holes. 8. VM + B: only the decompacted soil was amended with biochar at 5% by volume. 9. VM + B + WCM. Woodchip mulch was created from 5- to 8-cm diameter branches of silver birch (Betula pendula Roth.) and English oak (Quercus robur L.) that were
July 2023
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