270 red maple by Canham et al. 1996) as plants efficiently reallocate carbon to other sinks. However, amendment effects on soil nutri- ent content may be confounded by effects on soil physical proper- ties, which may exert great control on root responsiveness, partic- ularly in a compacted or poorly structured native soil. Of primary interest here, however, is whether organic matter used as soil amendments can create a more sustainable root zone by enhanc- ing the intimate relations between roots, microbial communities, and soil mineral particles that have the potential to result in en- hanced soil aggregation and sustained carbon stores (Kay 1998). Tree root growth responses to organic soil amendments are varied and may reflect changes to soil physical, chemical, or biological characteristics. Past work has demonstrated amend- ments can increase tree root growth (Smalley and Wood 1995; Prince et al. 2000), as was found in red maple, or have no ef- fect (Kelting et al. 1998; Gilman 2004), as was found with both oak species. This study was conducted in an agricultural soil of adequate quality for producing nursery trees. In degraded ur- ban soils where the microbial community may be more impov- erished (Scharenbroch et al. 2005) and soil physical properties may inhibit root growth (Daddow and Warrington 1983; Day et al. 2000), rooting may be restricted and increases in microbial activity connected with amendments could be less pronounced. CONCLUSION This study demonstrated that incorporating organic amendments into the backfill soil of transplanted deciduous trees can affect soil microbial biomass carbon and soil carbon levels as long as 33 months after a single application. Changes in the root-zone soil were both species- and amendment-specific. Red maple, a physiologically plastic tree species, showed greater root length response to organic amendments than the more specialized spe- cies, pin oak and chestnut oak. In addition, peat moss amend- ment suppressed microbial biomass and respiration compared to leaf-based and biosolids-based compost. The very high ini- tial C/N ratio of the peat moss amendment remained higher than other treatments during the measurement period. Greater fine and total root length in red maple corresponded with in- creases in microbial biomass, suggesting that microbial activity was mediated to some extent by root activity. These changes in root zone ecology could have long-term consequences for tree performance or soil health – aspects that deserve attention in future research, although this study did not address overall tree growth or performance or root exploration outside of the back- filled area. In the face of global climate change and rapid ur- banization, society must find sustainable uses for urban organic resources. Because soils in disturbed urban environments often lack suitable properties for sustainable tree development, amend- ing impoverished soils with composts may hold promise for re- storing soil function and enhancing urban canopy development. Acknowledgments. This work was sup- ported in part by a Hyland R. Johns grant from the Tree Research and Education Endowment Fund. The technical sup- port of John James, Velva Groover, David Mitchem, John Peterson, and Mike Tyree at Virginia Tech is greatly appreciated. We also thank Guillermo Ramirez in the North Carolina State University Department of Soil Science and Greg Evany- lo in the Virginia Tech Department of Crop and Soil Environmental Sciences. ©2012 International Society of Arboriculture Wiseman et al.: Organic Amendment Effects in the Root Zone LITERATURE CITED Annabi, M., S. Houot, C. Francou, M. 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