262 Wiseman et al.: Organic Amendment Effects in the Root Zone Arboriculture & Urban Forestry 2012. 38(6): 262–276 Organic Amendment Effects on Soil Carbon and Microbial Biomass in the Root Zone of Three Landscape Tree Species P. Eric Wiseman, Susan D. Day, and J. Roger Harris Abstract. There is increasing interest in amending degraded soils with organic matter to improve soil quality, especially in urban areas where reha- bilitation of damaged soils may enhance tree growth and provision of ecosystem services. To assess the potential of such organic amendments for producing a sustained alteration in soil biological characteristics, researchers studied the effects of three organic amendments incorporated into the root zone of three tree species on root development, soil carbon dynamics, and soil microbial biomass over one year beginning 20 months after amend- ment application. Soil amendment with leaf-based, and to a lesser extent, biosolids-based composts increased root length within the amended root zone of red maple (Acer rubrum), but not of pin oak (Quercus palustris) or chestnut oak (Q. montana). There was a concomitant increase in mi- crobial biomass carbon for red maple. Across all species, sphagnum peat moss amendment reduced microbial biomass carbon by 47% compared to unamended root zones and suppressed maximum seasonal soil respiration relative to composts. In contrast, leaf-based compost increased microbial biomass carbon by 12% (P = 0.0989) compared to unamended root zones. Carbon/nitrogen ratios remained stable throughout most of the year ex- cept in the root zones of chestnut oak and pin oak amended with peat, where it declined 44%–85%. Total soil carbon was stable in all treatments, although unamended soils averaged about 40% lower than amended soils. Across all species and treatments, cumulative fine root length explained 19% of the variation in microbial biomass carbon. The study authors conclude that soil microbial activity can be increased by compost amend- ment of the root zone and that this increase is mediated to some degree by tree roots. In addition, stable C/N ratios suggest this alteration in the root zone may be sustainable. Further research may clarify whether compost amendment combined with tree planting can accelerate soil restoration. Key Words. Acer rubrum; Quercus montana; Quercus palustris; Soil Food Web; Soil Rehabilitation; Soil Respiration; Tree Roots; Urban Soil. Land-use changes and management practices have long-term effects on soil ecology and consequently on the provision of ecosystem services. Urbanization in particular results in highly altered soils, which has profound repercussions for urban eco- systems. Urban soils typically have disrupted or absent hori- zons, are compacted, have poor structure, and possess low or- ganic matter (Jim 1998; Pouyat et al. 2007). These soils may also have diminished microbial populations, particularly on re- cently disturbed sites (Scharenbroch et al. 2005). Soil microor- ganisms are integral to the soil food web because of their role in the decomposition of organic matter (Wardle 1999; Scheu 2002). As such, they are critical drivers of nutrient cycling, ni- trogen fixation, nitrification, and the aggregation of clay par- ticles (Lynch and Bragg 1985; Lee and Pankhurst 1992; Te- jada et al. 2009). In addition, soil microbial communities can exert significant control on soil carbon dynamics (Grandy et al. 2009) and thus on the global carbon cycle (Doran 2002). Organic amendments, particularly compost, are receiving re- newed attention in the context of restoring disturbed urban soils to address environmental issues as well as improve tree growth. For example, improved vegetative growth and water infiltration resulting from soil rehabilitation can improve water quality by mitigating stormwater (Cogger 2005). Organic amendments (e.g. peat and compost) in urbanized landscapes are generally targeted at improving soil physical properties in the short term to aid in transplant establishment (e.g., Day et al. 1995). Less familiar ©2012 International Society of Arboriculture benefits ascribed to organic amendments include suppression of root pests (Hoitink et al. 1997; Widmer et al. 1998; Forge et al. 2008) and elicitation of enzymatic or hormonal growth respons- es by plant roots (Chen et al. 1994; Raviv 1998). However, few studies have examined whether organic amendments can affect soil health through their influence on the soil microbial com- munity. Cheng and Grewal (2009) found that amending subsoil with compost prior to lawn establishment enriched the nema- tode food web (i.e., content of macronutrients, organic matter, and microbial biomass), but had little effect on nematode food web structure (i.e., abundance and diversity of nematode genera). Disturbance and subsequent management of urban soil is quite different from the more frequently studied agricultural and forest- ed systems. In urban settings, drastic soil alterations during land development may be followed by relatively stable management regimes—especially where trees are planted—potentially creat- ing opportunities to restore functional soil microbial communi- ties that can promote nutrient cycling and plant health. Although trees play a prominent role in urban ecosystem function (Nowak and Crane 2002; McPherson et al. 2005; Nowak 2006; McDonald et al. 2007), no studies, to the knowledge of the authors, have investigated the effects of organic matter amendment on the soil microbial community in the context of landscape tree planting. Microbial biomass has been reported to be most often limited by the availability of soil carbon rather than nitrogen (Ekblad and Nordgren 2002), although the role of nitrogen may be influenced
November 2012
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