Arboriculture & Urban Forestry 35(5): September 2009 227 als, but the effects of organic materials on urban soil organism activity, biomass, and diversity have not been adequately studied. It is probable that organic materials improve overall soil qual- Figure 5. Conceptual model for the effects of organic materials on soil quality, tree health and environmental health. Size of arrow indicates relative support in literature published in Arboriculture and Urban Forestry. sociated with soil available nutrients. A significant increase for potassium (Foshee et al. 1999) was detected, but Watson and Kupkowski (1991) detected a significant decrease in soil nitrate. Soil organic matter can chelate micronutrients making them more available for plant uptake (Evangelou 1998), and this is consis- tent with observed leaf Mn increase with organic materials (Fos- hee et al. 1999). The controls on the soil available nutrient pool (i.e., moisture, temperature, microbial activity) are erratic, thus a single temporal measurement of extractable nutrients is not a good indication of site fertility (Scharenbroch and Lloyd 2006). Studies in this meta-analysis show a decrease in soil pH with organic materials. Although the pH change was consistent, con- clusions drawn from this meta-analysis regarding soil pH should be tempered as the data is based on only two responses. One would expect that soil pH may be more impacted if the study were performed in a container rather than in a field setting, but the observed pH decreases with organic materials were ob- served for both conditions (Himelick and Watson 1990; Foshee et al. 1999). Studies not included in this meta-analysis suggest that acidification is generally beneficial as many urban soils are too alkaline for optimal plant nutrient availability (Kelsey and Hootmann 1988; Craul 1999). Soil organic matter is a soil acidification source via H+ dissociation from carbonic acid and other acid functional groups (e.g., malic, carboxylic, and cit- ric acids) (Evangelou 1998). Soil organic matter forms soluble complexes with nonacid cations, and as these cations leach, pH decreases (Sikora et al. 1996). Soil organic matter fuels micro- bial-mediated processes such as nitrification and sulfur oxida- tion, and through H+ Soil organic matter is the C and energy source for many soil organisms (Bardgett 2005). Consequently, activity and bio- mass tend to increase with increasing SOM (Sikora et al. 1996). Himelick and Watson (1990) found increased mycorrhizae in- fection and density with organic materials. Scharenbroch and Lloyd (2006) reported significantly greater labile substrate (i.e., particulate organic matter) and potential C mineralization (i.e., microbial respiration) with organic materials. This meta-analysis suggests positive impacts on soil organisms with organic materi- ity (e.g., decreased root resistance, increased aeration, water and nutrient availability, etc.), and these improvements would likely lead to increased resource acquisition. This meta-analysis did confirm an association between organic materials and increased shoot and root growth. There is much evidence in the literature showing that soil resources do impact physiological function (i.e., photosynthesis), C allocation patterns, and ultimately tree health (Matson and Waring 1984; Lorio 1986; Christiansen et al. 1987; Herms and Mattson 1992; Herms 2002; Glynn et al. 2003). Results from this meta-analysis generally support this explana- tion, but significant data demonstrating that organic materials directly improve soil properties, increase resource acquisition, increase photosynthesis, impact C allocation, and improve tree health were not available from any study in the meta-analysis. Improvements in soil quality from organic materials can im- pact environmental health via numerous mechanisms (Figure 5). Organic materials protect soil and decrease losses with runoff and erosion (Lal et al. 2003). The end-products of humification (i.e., humus) are stable colloids with large exchange capacities, thus are very effective at nutrient, water, and toxin retention (Sikora et al. 1996). If organic materials are able to improve the water and nutrient status of soils, then reliance on inorganic fertilization and irrigation should decrease (Rechcigl 1995). Organic materi- als are proposed to create more diverse soil food webs (Coleman et al. 2004) and decrease weeds (Stinson et al. 1990), thus in- creasing competitive pressure on disease-causing organisms and weeds and our reliance on pesticides and herbicides. Globally, twice as much C is stored in the soil, 3340 Pg (3.68 * 1012 as in the vegetation, 550 Pg (6.06 * 1011 tons) and atmosphere, 760 Pg (8.38 * 1011 tons) combined (Batjes 1996; Soloman et al. tons) 2007); yet, benefits of soil C sequestration with urban organic materials were not quantified in this meta-analysis or elsewhere. The assumption that a meta-analysis uniformly represents production, they acidify soils (Paul 2007). the final and accurate viewpoint of an area of research is not warranted. A meta-analysis has a number of areas with the po- tential for bias, such as the inclusion or exclusion criteria used to select the studies for the meta-analysis. A particular bias in this study is that the responses were compiled solely from AUF research, and these responses may be a product of the research interests or perhaps the associated ease of measurement. On the other hand, no or low responses may suggest specific needs in AUF literature for identifying the impacts organic materi- als have on certain parameters (e.g., soil chemical, soil bio- logical, and environmental properties). Attempts were made to limit bias by applying relatively stringent criteria (significant results on at least half of reported data for a given response) and including a robust suite of parameters in the meta-analysis (any and all parameters measured by any study published in AUF relating to organic materials). CONCLUSION At the coarse scale, positive responses for organic materials were detected for all attribute categories (Figure 2). Divergences in positive response occur when the type and mode of organic ma- terials are considered separately (Figure 3; Figure 4). Studies in Arboriculture & Urban Forestry reported more responses for ©2009 International Society of Arboriculture
September 2009
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