The Influence of Soil Decompaction and Amendments on Soil Quality

Introduction

Soil compaction within the urban landscape is frequently caused by anthropogenic activity (grey infrastructure installation and maintenance, urban development, pedestrian and vehicular traffic) and is negatively associated with soil quality (Day and Bassuk 1994; Day et al. 1995; Smiley 2001; Sax et al. 2017; Rahman et al. 2019). Construction activities occurring during urban development and building tasks may intentionally compact soil in order to make grade alterations or construct roads. Soil compaction, however, directly reduces soil pore space and pore continuity, resulting in reduced gas exchange between the atmosphere and soil surface. If soil bulk density exceeds the root limiting bulk density value for the respective soil texture, root elongation decreases, and root branching and radial thickening increases, further limiting the spread of the root system and decreasing the uptake of essential soil nutrients (Gliński and Lipiec 1990; Day and Bassuk 1994). Trees growing in compacted soils also tend to experience winter waterlogging due to slowed water infiltration and percolation, summer drought stress due to reduced water-holding capacity, and increased proportion of hygroscopic water. Furthermore, soil compaction results in detrimental effects on soil flora and fauna, increasing a tree’s disposition to root-borne diseases such as Phytophthora and Armillaria. Consequently, soil compaction is recognised as a major constraint to urban tree survival (Hascher and Wells 2007; Scharenbroch and Watson 2014; Watson et al. 2014).

To remediate soil compaction, a few management strategies can be adopted, such as air spading, vertical mulching, soil application of biochar, and/or a woodchip mulch layer across the compacted soil surface. Air spading is functionally similar to conventional mechanical tilling but uses a hand-held air excavation tool, which transforms pressured air generated from a compressor into a jet of supersonic air. The supersonic air fractures compacted soil into smaller aggregates and has been shown to be highly effective in decompacting soils, increasing soil aeration, and promoting root growth while inflicting limited damage to existing tree root systems (Smiley 2005; Day and Harris 2008; Day et al. 2009).

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 performed with either an air excavation tool or an earth auger.

Biochar is the solid co-product of biomass pyrolysis: 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 woodchip 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; Scharenbroch 2009).

Few if any studies have examined the long-term benefits of these systems individually and in combination 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 woodchip mulch) and a biochar soil incorporation individually 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

Results

Discussion

Throughout the 5-year study, in most cases the greatest treatment effects on soil quality in terms of bulk density, soil organic matter, vegetation ground cover, cotton strip degradation, root dry mass, and earthworm counts were the result of air spading in combination with biochar and woodchip mulch applied across the treated surface area. Few published studies evaluating the short- and long-term effects of combining 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 singly 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 incorporation of fertilizer and a woodchip mulch layer treatment 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 combination with biochar and a woodchip mulch layer was the most effective but time consuming and expensive treatment in this study. In contrast, application of woodchip mulch alone, which was the most inexpensive 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 documented in the literature and include fertilization, organic matter enhancement, maintenance of soil temperature and moisture, and weed suppression (Chalker-Scott 2007; Scharenbroch 2009). Few studies have, however, evaluated the long-term (3 to 5 year) impacts of a woodchip mulch layer on soil quality 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 Betula nigra in a compacted urban soil. Results here support 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 density, increased active carbon, and potentially mineralized 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 recognized as a useful proxy of soil health and biological activity (Riches et al. 2013; AHDB 2018). For example, application of earthworms to compacted agricultural 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 provides a standardized measure of organic matter decomposition (Reid and Cox 2005). Higher rates of cotton strip degradation in woodchip-mulch-treated plots in this study indicate enhanced biological/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 compacted (Morris et al. 2009). Even the addition of the soil amendment biochar in combination with vertical mulching had little influence on enhancing soil quality, 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 woodchip 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 amendments, 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 gradual increase in bulk density values from year 1 to year 5, possibly as a result of long-term soil settling combined with light foot traffic caused by members of the public (Morris et al. 2009). Similarly, air spading alone or in combination with biochar soil amendment and a woodchip mulch layer had the greatest effects on vegetation ground cover at the end of year 5. Ground cover is widely used as an indirect indicator of soil quality; a soil lacking a cover of living plants is almost certainly indicative of poor soil quality (Zuazo and Pleguezuelo 2009). A lack of vegetation ground cover or smaller stunted plants is also associated with compacted soils (Ben-Dor et al. 2009). By year 5, 100% vegetation ground cover was recorded following air spading in combination with biochar and a woodchip mulch layer compared to 5% in non-decompacted control soils, indicating a significant improvement in soil quality. Vegetation ground cover observed at year 5 consisted mainly of grassland species (Lolium perenne, Poa annua) and a few weed species, including white clover (Trifolium repens), dandelion (Taraxacum officinale), nettle (Urtica dioica), and dock (Rumex obtusifolius). The percent composition of each species within each treatment was not quantified in this trial.

The application of biochar as a soil amendment has been shown to enhance transplant survival of young trees, improve root growth and health of mature established trees, reduce soil compaction, and reduce the severity of root pathogens such as Armillaria and Phytophthora (Scharenbroch et al. 2013; Schaffert and Percival 2016; Blanco-Canqui 2017; Yoo et al. 2020; Schaffert et al. 2022). Results of this study, however, show little improvement in soil quality when biochar was added via vertical mulching. Similar results, i.e., little root growth, have been obtained when using other soil amendments such as perlite, an amorphous volcanic glass, in combination with vertical mulching (Kalisz et al. 1994). Indeed, the work of Kalisz et al. (1994) concluded that perlite-filled vertical mulch channels were actively avoided by tree roots of Acer saccharum. In another study, tree growth responded as well to empty holes as it did those filled with fertilizer following vertical mulching (Smith 1978). Watson et al. (1996) found that replacement of soil within the root zone of mature Quercus alba via radial trenching improved fine root density by 320% and rooting depth by 68%. Although application of biochar in combination with air spading resulted in a significant improvement in soil quality, these improvements were, in most instances, not significantly greater than air spading alone. Consequently, results of this study indicate that biochar addition in combination with air spading and vertical mulching had limited effects on improving soil quality compared to air spading and vertical mulching alone.

Conclusions

A combined treatment of air spading, biochar, and woodchip mulch proved optimal in improving a suite of soil physical, chemical, and biological properties in a heavily compacted soil over a 5-year period. This treatment was also the most expensive and time consuming. A woodchip mulch layer was the most effective of the individual treatments and the most cost effective. Vertical mulching alone had little influence on soil quality. Air spading alone proved effective in improving soil quality over a 5-year period, and the long-term effects of air spading could be improved by the addition of a woodchip mulch layer. In agreement with Fite et al. (2011), results demonstrate effective long-term soil decompaction measures exist that provide arborists with a useful means to improve compacted soils within urban landscapes.

Conflicts of Interest

The authors reported no conflicts of interest.