©2023 International Society of Arboriculture 184 mulch (treatment 6); air spading in combination with woodchip mulch (treatment 4); air spading in combi- nation with biochar (treatment 5); air spading alone (treatment 3); vertical mulching with biochar amended backfill and woodchip mulch (treatment 9); wood- chip mulch alone (treatment 2); vertical mulching with biochar amended backfill (treatment 8); vertical mulching with native soil backfill (treatment 7); control. Root Dry Mass (Table 3) The treatments that had the greatest effect on root dry mass as measured in year 5 were: woodchip mulch alone (treatment 2), air spading in combination with woodchip mulch (treatment 4), and air spading in combination with biochar and woodchip mulch (treatment 6). These treatments resulted in an increase in root dry mass over the non-decompacted control soil that ranged from 77% to 112%. Vertical mulch- ing with native soil backfill (treatment 7) had the low- est effect on root dry mass, with a non-significant increase of 26% over the control, and vertical mulch- ing with biochar amended backfill (treatment 8) resulted in a slightly higher root dry mass (33% increase over the control), which was also non-significant. The only treatment which showed a significant increase in root dry mass when compared to the non-decompacted control soil was air spading in combination with bio- char and woodchip mulch (treatment 6). Earthworm Count (Table 4) Air spading combination treatments (treatments 4 to 6), woodchip mulch alone (treatment 2), and vertical mulching with biochar amended backfill and wood- chip mulch (treatment 9) had the largest positive effect on earthworm counts when counts were taken in years 3 and 5 (2019 and 2021, respectively). These treatments had an earthworm count that was 150% to 250% higher than the control in year 3 and 33% to 133% higher in year 5. These differences were signif- icant (P < 0.05) with the exception of air spading in combination with woodchip mulch (treatment 4) in year 3, and woodchip mulch alone (treatment 2) and air spading in combination with biochar (treatment 5) in year 5. Although earthworm counts were higher than the control by 150%, 33%, and 67%, respec- tively, these differences were not significant. Air spading (treatment 3), vertical mulching with native soil backfill (treatment 7), and vertical mulching with biochar amended backfill (treatment 8) had no significant effect on worm counts compared to the non-decompacted control soil at years 3 and 5. DISCUSSION Throughout the 5-year study, in most cases the great- est treatment effects on soil quality in terms of bulk density, soil organic matter, vegetation ground cover, cotton strip degradation, root dry mass, and earth- worm counts were the result of air spading in combi- nation with biochar and woodchip mulch applied across the treated surface area. Few published studies evaluating the short- and long-term effects of com- bining air spading, biochar, and woodchip mulch on soil quality exist. Of the research available, Fite et al. (2011) investigated the effects of air spading using an AirSpade® 2000, fertilizer, and mulch application sin- gly and in combination at 4 urban sites located in Pennsylvania, South Carolina, Massachusetts, and Boston. Soil quality parameters measured included: bulk density, soil strength, soil organic matter, soil nutrient levels, and soil volumetric water content. They concluded that the air spading plus the incorpo- ration of fertilizer and a woodchip mulch layer treat- ment was the most effective in improving soil quality, and that a multi-pronged approach to soil remediation provides arborists with an effective means to improve compacted soils under established trees. Similarly to the study of Fite et al. (2011), air spading in combina- tion with biochar and a woodchip mulch layer was the most effective but time consuming and expensive Percival et al: The Influence of Soil Decompaction and Amendments on Soil Quality Table 4. The influence of air spading (AS) and vertical mulching (VM) alone and in combination with a layer of woodchip mulch (WCM) and/or biochar (B) soil amendment on earthworm counts at years 3 and 5 after treatment. Treatment Year 3 Year 5 1. Control 1.7a 2.5a 2. WCM 5.6c 4.2ab 3. AS 2.6ab 2.3a 4. AS + WCM 4.8bc 6.6b 5. AS + B 6.0c 4.9ab 6. AS + B + WCM 6.7c 6.1b 7. VM 2.7ab 2.9a 8. VM + B 2.0a 2.9a 9. VM + B + WCM 6.5c 6.6b Numbers within a column followed by a common letter are not signifi- cantly different according to Tukey’s honest significance test (P = 0.05).
July 2023
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