©2023 International Society of Arboriculture Arboriculture & Urban Forestry 49(4): July 2023 181 samples were collected as outlined in the Natural England technical information note TIN035: 3 soil cores taken at 1-m intervals in a “V” pattern were amalgamated into one sample and oven dried for 72 h (Tytherleigh et al. 2008). Soil pH was determined by calibrating a pH meter (SevenExcellence™ pH meter S400-Std-Kit, Mettler-Toledo LLC, Columbus, Ohio, USA) over a pH range of 4 to 7 using standard buf- fers. Then 5 g of sieved (4-mm), air-dried soil was placed into a 100-mL centrifuge tube, and 10 mL deionized water was added to achieve a soil:water ratio of 1:2. The soil sample and deionized water were stirred vigorously for 15 seconds and left to stand for 30 minutes to equilibrate with atmospheric CO2 and warm to room temperature. A pH electrode was placed in the soil/distilled water solution, swirled carefully, and the pH value taken to the nearest 0.01. Between samples, the electrode was rinsed with deionized water. Soil organic matter was estimated using the loss on ignition method, i.e., combustion of 100 g of air-dried, sieved (2-mm) soil in an oven at 360 °C for 6 hours followed by measuring weight loss. Earthworm Count Earthworm population size is a popular metric for overall soil health due to their role in nutrient cycling, soil aggregation, soil aeration, and water infiltration and percolation. For each plot, 3 soil pits (20 × 20 × 20 cm) were dug in a “V” pattern, and the excavated soil was placed on a mat. The soil was broken up by hand, and the number of earthworms per pit were counted (AHDB 2018). Cotton Strip Assay The cotton strip assay is a field test used to assess the biological component of the soil. For each plot, 3 strips (20 × 15 cm) of unbleached calico cotton were buried 20 cm below the soil surface for 3 weeks; they were then gently lifted and washed in water to remove the soil. The amount of decomposition on the strips was determined on a visual percentage basis (Reid and Cox 2005). Root Dry Mass At year 5 (February 2021), when the mulch layer had degraded, 5 soil cores 5 cm in diameter and 20 cm deep (393 cm3) were removed per plot, and any leaf litter, organic matter, and vegetation were removed from the top of the core. Within each plot, cores were pruned from trees located within Hatfield Forest and chipped with a commercial brush chipper to produce 4- to 6-cm long chips. Each mulch (50% silver birch, 50% English oak) was applied to the respective treat- ment plots immediately after chipping. The mulch was made when trees were fully dormant (February 2016) when no foliage was present on the tree. The biochar used in this experiment was derived solely from English oak (Q. robur L.) and produced in a SuperChar 100 Mk I kiln (Carbon Gold, Cleve- don, United Kingdom) at 600 °C for 2 hours, which, when cooled, was crushed to pass through a 5-mm grade sieve to ensure all particles were less than 5-mm diameter prior to use. Each trial site was then protected from possible animal and/or human ingress by installing 2 × 3 m (height and length, respectively) wooden fencing around the peripheral site edge of each experimental site, and each Ride was closed to public access by installing 2 × 3 m (height and length, respectively) wooden fencing at the entrance and end of each Ride. During the 5-year experimental period, no man- agement interventions (irrigation, fertilization, etc.) of any site occurred. Bulk Density (Soil Compaction) Bulk density measurements were taken prior to treat- ment application (February 2016) and annually throughout the study (2017–2021). Bulk density was calculated for each plot from 3 soil cores based on a 1-m “V” pattern, as stipulated under United Kingdom soil sampling procedures, to account for soil variation (Tytherleigh et al. 2008). Soil cores were taken with a slide hammer and a corer head which measured 20 cm in length and had a 5-cm cutting edge diameter. Any mulch and the top 2 cm of the soil core were dis- carded, and the remaining core was trimmed to 7.5 cm in length. The cores were transferred to aluminum trays and dried at 65 °C for 7 days or until the mass remained unchanged. The dry mass was then used to calculate bulk density (g/cm3). Soil Organic Matter and pH Measurements Soil organic matter content and soil pH were mea- sured for each plot annually to assess the chemical component of compaction. These analyses were under- taken by a United Kingdom Accreditation Service (UKAS) accredited laboratory: Cawood Scientific Ltd. T/A NRM, Bracknell, United Kingdom. Soil
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