Arboriculture & Urban Forestry 38(6): November 2012 269 Table 3. Microbial biomass measured in the root zone of bare root tree seedlings 33 months after transplant into field soil amend- ed with three organic amendments. For each species × amendment combination, n = 8 (standard error in parentheses). Within a column, lowercase letters denote significant differences (P < 0.05) in amendments. Pooled values are marginal means (and standard error) for treatment factors. Uppercase letters denote significant differences in marginal means. Control Peat moss Leaf-based compost Biosolids-based compost Pooled value Soil microbial biomass carbon (mg kg-1 Chestnut oak 289.8 (23.8) ab 252.5 (19.9) b 371.9 (38.7) a 370.0 (30.8) a 319.5 (16.6) B ) Red maple 344.2 (13.1) ab 250.9 (37.2) b 389.5 (18.0) a 382.1 (35.7) a 341.6 (16.1) B and root activity were both responding to amendment type. The complex relations among roots, root exudates, microbial activ- ity, microbial exudates, and soil structure could be expected to be reflected in soil respiration. Indeed, PM-treated trees gener- ally had lower soil respiration levels during the growing sea- son, although only occasionally could these differences be reli- ably attributed to treatment (Figure 6). Carbon/nitrogen ratio remained stable during the course of measurements for all soil treatments except PM (Figure 4). MBC may have in part been suppressed in PM because of low nitrogen content. Fungi and bacteria are reported to be nitrogen limited at substrate C/N ra- tios above 30 (Kaye and Hart 1997). However, the relationship between C/N ratio and plant-microbial competition for nitrogen has been questioned. Månsson et al. (2009) found such competi- tion to be uncoupled from C/N ratios of 20, 31, and 34 – simi- lar to the ranges of C/N for PM during the current measurement period. In this study, regression analysis suggested that both C/N ratio and nitrogen were drivers of MBC, but only PM had C/N ratios of 15 or greater, so it is not possible to determine if higher C/N ratios suppressed MBC, or another attribute of PM. Below 14, C/N ratio was not limiting to microbial biomass. Were changes mediated in part by increased root prolifera- tion in the enhanced backfill soil, which in turn sustained organic matter deposition to the root zone? Tree roots play a direct role in soil aggregate formation through mechanical alteration of the soil and an indirect role through root exudates and turnover, which create opportunities for the nucle- ation of mineral particles around these plant residues to form ag- gregates (Kay 1998). In addition, long-term studies with birch trees (Betula spp.) indicate that tree species can have a pronounced cascading effect on soil ecology, especially the microbial com- munity (Mitchell et al. 2007; Mitchell et al. 2010). In this study, chestnut oak had lower root zone microbial biomass carbon accu- mulation than pin oak overall (see Table 3). Chestnut oak also had distinctively less root length than either red maple or pin oak (Fig- ure 1). Root length is important because this produces a greater soil/root contact surface. Soil in the traditional rhizosphere (soil in immediate contact with the root surface) has greater microbial activity than other soil (Wardle 1992). This effect can vary among both tree species and soil types (Priha et al. 1999; Priha et al. 2001). In the current study, pin oak and red maple root zones had greater root length overall within the core samples than chestnut oak. It should be noted that the root sampling procedure for this study was not intended to characterize overall tree growth or per- formance and the extent of root growth or total root growth. In- stead, the study aimed to assess the relations between root length and soil biological response. Thus, pin oak also had greater MBC Pin oak 489.6 (23.4) a 269.4 (21.2) b 501.3 (47.9) a 463.3 (68.5) a 429.8 (26.9) A Pooled value 380.6 (19.6) A 258.9 (11.6) B 427.2 (22.0) A 408.9 (24.8) A when compared to chestnut oak root zones, indicating a possible association between root growth and MBC in the oak species. Within each species, MBC responded similarly to the various or- ganic amendments (i.e., there was no interaction), yet there was also a pronounced species effect on MBC that may be related to degree of root proliferation. It is possible that other species characteristics may have resulted in greater MBC in pin oak. In addition, although red maple had greater root length than chest- nut oak, differences in MBC could not be attributed to species. Nonetheless, when MBC was regressed with cumulative fine root length, there was a significant relationship and fine root length ex- plained 19% of the variation in MBC, suggesting that MBC was influenced to some degree by fine root proliferation (Figure 5). The present study only included amendment of a small region of soil immediately surrounding the planted tree. Larger areas of soil amendment might be expected to have greater effect on tree growth overall, and thus a concomitant increase in the role of roots in the ecology of the soil. Incorporating organic matter into back- fill soil in urbanized or degraded sites is thought to create a more favorable rooting environment for transplanted trees by reducing soil strength, improving water retention, and enhancing nutrient content (Ferrini et al. 2005; Roberts 2006; Cogger et al. 2008; Price et al. 2009). This practice is widely believed to improve tree survival and growth, which are key determinants of successful tree plantings. The researchers amended individual planting holes to assure root exploration in the study zone during the time of the experiment. However, in practice, the proportion of the soil area explored for the expanding root system would rapidly increase since roots of similar sized transplants can grow approximate- ly 1 m per year in this climate (Richardson-Calfee et al. 2004). Nonetheless, although organic amendment incorporation had no effect on root length of chestnut oak or pin oak, red maple root length was clearly increased by leaf-based compost (Figure 1). Other researchers have found red maple root development to be responsive to organic amendment (Smalley and Wood 1995; Kelting et al. 1998; Roberts 2006). While differential species re- sponses to amendments are not surprising when comparing exper- iments that assumedly differ in a multitude of influential factors, the current experiment has shown that some tree species may re- spond differently even under identical environmental conditions. Greater root response by red maple may reflect the species’ high physiological plasticity (Kelly et al. 2000; Bauerle et al. 2003), which permits red maples to thrive in a broader range of habitats than either pin oak or chestnut oak. In addition, composts contain plant nutrients, such as P and N, which have the potential to im- prove plant growth. Conversely, amendment-induced root growth may seem counterintuitive given that increased soil resource lev- els generally result in decreased root growth (as demonstrated for ©2012 International Society of Arboriculture
November 2012
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