©2023 International Society of Arboriculture Arboriculture & Urban Forestry 49(4): July 2023 185 treatment in this study. In contrast, application of woodchip mulch alone, which was the most inexpen- sive and least time-consuming treatment, also proved very effective in increasing vegetation ground cover, cotton strip degradation, root dry mass, earthworm count, and soil organic matter content. The benefits of a woodchip mulch layer on soil quality are well doc- umented in the literature and include fertilization, organic matter enhancement, maintenance of soil temperature and moisture, and weed suppression (Chalker-Scott 2007; Scharenbroch 2009). Few stud- ies have, however, evaluated the long-term (3 to 5 year) impacts of a woodchip mulch layer on soil qual- ity and fertility. Of that available, Scharenbroch and Watson (2014) concluded that a woodchip mulch provided a cost-efficient and effective treatment for alleviating soil compaction, improving soil quality, and stimulating tree growth of Acer rubrum and Bet- ula nigra in a compacted urban soil. Results here sup- port the findings of Scharenbroch and Watson (2014) and Fite et al. (2011) in that a woodchip mulch layer provides a simple and relatively inexpensive method of long-term soil structure improvement, creating a soil environment optimal for root growth. In a related study, Sax et al. (2017) evaluated a Scoop & Dump (S&D) system, i.e., physical fracturing of compacted soils in combination with application of locally sourced compost and annual mulching. Benefits on soil quality over 12 years included reduced bulk den- sity, increased active carbon, and potentially mineral- ized nitrogen, with the conclusion that the S&D system has potential for improving long-term quality of compacted soils using relatively straightforward methodology. One of the functions of a woodchip mulch layer is to suppress competing vegetation growth. The increase in vegetation ground cover at year 5 in this study can be explained by the fact that by the end of year 3, most of the woodchip layer had decomposed, and the subsequent increase in soil fertility allowed for rapid establishment of vegetation (Watson 1988; Fite et al. 2011). Likewise, the increase in vegetation ground cover recorded at year 5 involving woodchip mulch (treatments 2, 4, 6, and 9) may be due to enhanced soil biological activity caused by mulch decomposition (Scharenbroch and Watson 2014). In support of this, a significantly higher worm count was recorded in woodchip-mulch-treated soils compared to a non-decompacted soil (control). Earthworms, through their burrowing activity, play a major role in the structural and functional heterogeneity of soils (AHDB 2019). Earthworm counts are also recog- nized as a useful proxy of soil health and biological activity (Riches et al. 2013; AHDB 2018). For exam- ple, application of earthworms to compacted agricul- tural soils has been shown to alleviate compacted soil and improve structure, resulting in enhanced crop yields (Riches et al. 2013; Thomsen et al. 2019). Likewise, the cotton strip assay is a quantitative assay for biological activity and is consequently used to assess ecosystem integrity, because this method pro- vides a standardized measure of organic mat- ter decomposition (Reid and Cox 2005). Higher rates of cotton strip degradation in woodchip-mulch- treated plots in this study indicate enhanced biologi- cal/microbial activity below ground (Reid and Cox 2005). Although vertical mulching is recognized as a means of decompacting soils, results of this study found few significant effects on soil quality over a 5-year period following this treatment. One of the disadvantages of vertical mulching is that although this process ameliorates compaction in each hole, the bulk soil between the holes remains largely com- pacted (Morris et al. 2009). Even the addition of the soil amendment biochar in combination with vertical mulching had little influence on enhancing soil qual- ity, as effects on soil quality were not, in many cases, significantly different from non-decompacted control soils at year 5 after treatment. Application of a wood- chip mulch after vertical mulching and biochar soil amelioration, however, resulted in a significant increase in all soil quality measurements, again emphasizing the effectiveness of a woodchip-mulch treatment in ameliorating long-term soil compaction. Other researchers indicate vertical mulching to be more effective as a means of decompacting soil and improving soil microbial activity and plant quality when combined with fertilizer-based soil amend- ments, which was not undertaken in this study (Kuncheva 2015; Nandhini et al. 2021). In agreement with other findings, air spading alone or in combination with biochar soil amendment and a woodchip mulch layer had the most immediate effect on significantly reducing bulk density (Morris et al. 2009; Fite et al. 2011). Differences in bulk density were also, in most cases, still significantly lower than the non-decompacted control soil by the end of the fifth year after treatment. There was, however, a grad- ual increase in bulk density values from year 1 to year
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