226 Scharenbroch: A Meta-analysis of Studies Relating to Organic Materials AUF did not provide specific chemical information (e.g., C/N ra- tios) for the organic materials. Twigs, wood chips, dead leaves, and residues of dead plants are rich in C and low in N and these materials often have C/N ratios exceeding 200/1 (Stratton et al. 1995). As substrates are decomposed (i.e., composted), C is lost via respiration and N is gained through immobilization, thus substrate C/N will decrease with time, and C/N of compost is commonly observed to be 20/1 to 35/1 (Stratton et al. 1995). Microbial decomposition and mineralization kinetics of organic materials are controlled by substrate quality (e.g., C/N ratio, lignin, and polyphenol content) and environmental conditions (e.g., tem- perature, water, oxygen, and pH) (Bardgett 2005). Nitrogen immo- bilization occurs when the C/N ratio of the substrate exceeds ap- proximately 20/1 to 25/1 (Sylvia et al. 1999). Lignin contents greater than 20% and polyphenol contents greater than 3% are suspected to slow decomposition (Melillo et al. 1982; Northup et al. 1995). Relative to C/N ratios, even less study has been directed to lignin and polyphenol content of urban landscape organic materials. Du- ryea et al. (1999) studied the biochemical composition of mulch as it impacts lettuce seed germination, but controlled experimentation is required for impacts on trees, soils, and environmental quality. The meta-analysis of AUF literature supports others whom have found that nutrient-rich, fine-textured compost favors mineralization and is an excellent nutrient source (Lloyd et al. 2002). It is worthwhile to note that mulches made from dis- eased plant materials potentially contain those pathogens, thus high-temperature composting may be preferential in cases where disease may be an issue. Although compost is a fertile base, it is also a potential seed bank for weed establishment and growth (Chalker-Scott 2007). Compost is not effective at weed suppression relative to coarse mulch (Maynard 1998). This meta-analysis found greater improvements in soil physi- cal properties with compost incorporated into the soil com- pared to surface-applied mulch; but, others have suggested coarser mulches may be better at water retention (Chalker-Scott 2007) and temperature buffering (Tilander and Bonzi 1997). Tree attribute responses tended to be greater for compost relative to mulch, but both compost and mulch were associ- ated with positive root growth, shoot growth, tree physiologi- cal responses. The negative responses associated with mixed compost-mulch studies are derived from observations of de- creased germination of lettuce seeds (Duryea et al. 1999) and short-term decreases in shoot growth of containerized seed- lings grown in a variety of composted biosolids (Roberts 2006). Schulte and Whitcomb (1975) observed a decrease in tree height of young silver maples with an increase in pine bark mulch, and they attribute this decrease to a “tie-up” of nitro- gen by soil microorganisms (i.e., N immobilization). Hen- sley et al. (1988) also observed a decrease in tree height with organic materials, but only during the first year, and the trend was reversed after 22 months, likely as C/N decreased. Long- term N immobilization or growth suppression is not likely to occur with mulch (Greenly and Rakow 1995; Pickering and Shepherd 2000). Experimental research has found increased soil and/or foliage nutrient levels with mulch (Arthur and Wang 1999; Foshee et al. 1999; Szwedo and Maszczyk 2000). Mode of Organic Material Research in AUF suggests backfill organic amendments im- proved soil physical properties relative to surface applica- ©2009 International Society of Arboriculture tions. Particle density of organic matter is 1.0 g cm-3 lb ft-3), which is less than mineral soil, 2.65 g cm-3 (62.4 (165.4 lb ft-3) (Rühlmann et al. 2006); thus, direct incorporation of or- ganic material in planting holes will reduce soil bulk density. The AUF meta-analysis shows that surface applications tend- ed to improve shoot growth, root growth, and physiological re- sponse relative to backfill amendments. It has been proposed that when backfill soil differs from the site soil, roots may have dif- ficulty crossing the interface (Pellet 1971; Schulte and Whitcomb 1975); but, Watson et al. (1992) did not observe root confinement to planting holes with organic backfill. The interface created in the planting hole between the organic and mineral soil is likely to impact soil water movement; but, to my knowledge, this has yet to be conclusively demonstrated in experimental study. The nega- tive responses associated with backfill applications in this meta- analysis are from decreases in shoot growth and physiological properties, reported with containerized seedlings (Roberts 2006). This meta-analysis found that normal surface applications of 0 to 10 cm tended to have more positive impacts on shoot growth and physiological attributes compared to deep surface applica- tions. Arnold et al. (2005) reported negative impacts of decreased water penetration, increase soil tension, decreased shoot growth and increased plant stress with > 15 cm (6 in) of mulch. Con- versely, Watson and Kupkowski (1991) did not observe detri- mental effects on root density, temperature, moisture, or aeration from 45 cm (18 in) of mulch. Thicker layers of mulch may be better able to resist compaction and be beneficial as better tem- perature buffers and weed suppressors (Chalker-Scott 2007). Impacts of Organic Materials on Trees, Soil, and Environment This meta-analysis showed that AUF research found that organic materials had generally positive impacts on tree, soil, and envi- ronmental parameters (Figure 2). The strongest positive respons- es were observed for soil physical (n = 18; Δ = 36.8; d = 2.1), root growth (n = 9; Δ = 113.7; d = 1.1), shoot growth (n = 18; Δ = 34.7; d = 0.5), and physiological (n = 20; Δ = 25.2; d = 0.3) attributes. Percent changes were highest for soil biological attributes (n = 4; Δ = 160.0; d = 0.0), but the low sample sizes suppressed d values. Soil chemical properties had positive, but variable responses with low repetition (n = 5; Δ = 10.7; d = 1.1). The detectable response for environmental attributes was minimal, likely due to low num- ber of studies reporting these values (n = 5; Δ = 7.2; d = 0.0). Responses to organic materials observed in the literature sum- mary and meta-analysis are compiled in a conceptual model (Figure 5). Changes in soil physical properties associated with increased organic materials include temperature buffering, reduced evapora- tion, increased infiltration, increased retention, increased drainage, reduced splashing, decreased density, and increased porosity. This meta-analysis did provide substantial evidence that organic mate- rials are associated with buffering of soil temperature, improving soil moisture status, decreasing density, and increasing porosity. There are many proposed soil biochemical improvements from organic materials (Figure 5). However, this meta-analysis did not identify many studies in AUF literature with data show- ing significant improvements in chemical or biological proper- ties. It is logical to expect that organic materials would increase SOM, but only one AUF study provided significant data dem- onstrating this relationship (Scharenbroch and Lloyd 2006). Due to low sample sizes, we were unable to identify any trends as-
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