Abstract
There is increasing interest in amending degraded soils with organic matter to improve soil quality, especially in urban areas where rehabilitation 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 amendment 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 microbial 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 except 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 amendment 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.
- 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 ecosystems. Urban soils typically have disrupted or absent horizons, are compacted, have poor structure, and possess low organic matter (Jim 1998; Pouyat et al. 2007). These soils may also have diminished microbial populations, particularly on recently disturbed sites (Scharenbroch et al. 2005). Soil microorganisms 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, nitrogen fixation, nitrification, and the aggregation of clay particles (Lynch and Bragg 1985; Lee and Pankhurst 1992; Tejada 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 renewed 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 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 responses 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 community. Cheng and Grewal (2009) found that amending subsoil with compost prior to lawn establishment enriched the nematode 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 forested systems. In urban settings, drastic soil alterations during land development may be followed by relatively stable management regimes—especially where trees are planted—potentially creating opportunities to restore functional soil microbial communities 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 by soil C/N ratio in certain circumstances (Månsson et al. 2009). Besides the deliberate incorporation of organic matter, carbon enters soil from plant litter, carbon-rich rhizodeposition products (e.g., root exudates, dead cells, CO2), and dead roots (along with their associated mycorrhizae) (Grayston et al. 1997). As much as 40% of carbon assimilated through photosynthesis is returned to the environment through rhizodeposition (Lynch and Whipps 1990; van Veen et al. 1991), and nearly half of carbon allocated to fine roots and mycorrhizae can be deposited into soils as roots turn over (Fogel and Hunt 1983). Not only does soil carbon increase activity of microorganisms, but the presence of microorganisms can induce positive feedback responses that increase root exudation, further enhancing soil carbon content (Meharg and Killham 1991). Root carbon therefore exerts a significant influence on the soil microbial community (Brant et al. 2006).
The purpose of this study was to examine the effects of organic matter amendment on soil carbon dynamics and microbial activity within the root zone of trees planted into soil unoccupied by plants and to determine if these effects could be sustained over time. The soil surrounding the roots of newly transplanted trees was amended with organic matter and researchers measured the effects on soil carbon dynamics, microbial biomass, and tree growth up to 33 months after treatment. Researchers studied three deciduous tree species of contrasting cultural requirements, transplanted with three distinct types of organic matter incorporated into the backfill soil of their planting holes. The research objectives were to determine 1) if a single application of organic matter could alter carbon dynamics and increase soil microbial activity within the root zone; 2) whether the type of organic matter and tree species would influence these outcomes; and 3) whether these changes might be attributable to enhanced root production and rhizodeposition in the amended backfill soil.
MATERIALS AND METHODS
Study Site and Experimental Design
The study was conducted at the Virginia Tech Urban Horticulture Center in Blacksburg, Virginia, U.S. (USDA Hardiness Zone 6b) from March 2004 to December 2006. Soil at the center is a Groseclose silt loam (fine, mixed, semiactive, mesic Typic Hapludults); typical bulk density and pH of the Ap horizon (0–18 cm) is 1.05–1.35 g cm−3 and 6.1–6.7, respectively (Harris et al. 2008). Average annual precipitation is about 109 cm, and the growing season typically lasts from early May to mid-October.
The experiment consisted of three tree species and four backfill soil treatments combined factorially and replicated four times each in a completely randomized design, totaling 48 experimental units. The backfill soil treatments were unamended site soil (CON), and site soil amended with sphagnum peat moss (PM), leaf-based compost (LBC), or biosolids-based compost (BBC) as described in Table 1. See Bowden et al. (2007) for additional information concerning the preparation of LBC and BBC.
Soil amendments incorporated into backfill soil during transplanting of bare root, hardwood tree seedlings to a field site in Blacksburg, Virginia, U.S., in March 2004.
In March 2004, bare-root, seedling-grown pin oak (Quercus palustris Münchh.), red maple (Acer rubrum L.), and chestnut oak (Quercus montana Willd.) were randomly assigned to a backfill soil treatment and a planting location within five adjacent planting rows measuring 35 m long × 2 m wide (3.5 m spacing between trees). Planting rows had been uniformly prepared by killing the turfgrass with glyphosate (Roundup®, Monsanto Co., St. Louis, Missouri, U.S.) and tilling the topsoil to 15 cm depth at the end of the previous growing season. Average height of red maple, chestnut oak, and pin oak trees at planting were 2.9 m, 3.5 m, and 3.6 m, respectively.
Trees were placed in holes dug 45 cm deep with a 61 cm diameter mechanized auger, backfilled to grade with the assigned soil treatment, and thoroughly irrigated. Trees were then secured to a wire trellis spanning each planting row and mulched with a 5 cm deep × 2 m wide layer of uniform, shredded hardwood bark that was replenished seasonally for the duration of the study. The pre-emergent herbicide pendimethalin (Pendulum® Aquacap, BASF Corp., Florham Park, New Jersey, U.S.) was applied to planting rows every March, and weeds were spot-treated during the growing season with the post-emergent herbicide glyphosate. Trees were drip-irrigated once per week during the first growing season and then only during protracted drought in subsequent growing seasons.
Sampling and Measurements
In November 2005 (20 months after planting), researchers began a yearlong series of monthly soil and root measurements. Six measurement sub-plots were established equidistant around the perimeter of the amended root zone of each tree about 22 cm from its trunk. Each sub-plot was used for measurements during two consecutive months and then abandoned. Two weeks prior to each monthly measurement session, a PVC ring (10.1 cm inside diameter × 16 cm height) was placed at the center of the selected sub-plot and pressed through the mulch and into the soil to a 3 cm depth. Subsequently, soil respiration and temperature were concurrently measured using a LI-6400-09 Soil CO2 Flux Chamber coupled with a LI-6400 Portable Photosynthesis System (LI-COR Biosciences, Lincoln, Nebraska, U.S.). The flux chamber was affixed to the PVC ring, and the thermocouple was inserted 6 cm into the soil adjacent to the PVC ring. Volumetric soil moisture content was simultaneously measured adjacent to the PVC ring with a 6-cm dielectric probe (TH20 Soil Moisture Meter, Dynamax, Houston, Texas, U.S.). Equipment malfunction prevented measurements during April 2006; therefore, the experiment was extended through November 2006 to achieve twelve monthly sampling sessions.
Immediately following the second monthly sampling of respiration, temperature, and moisture at a measurement sub-plot, a 5 cm diameter soil core was extracted from the sub-plot center at 0–10 cm soil depth. Soil samples were first passed through a 6 mm sieve to collect coarse mineral fragments, coarse woody debris, and coarse tree roots. The coarsely-sieved soil was then passed through a 2 mm sieve to prepare soil sub-samples for carbon–nitrogen analysis. In addition, thin tree roots (2–6 mm diameter) were manually collected from the sieve and fine tree roots (<2 mm diameter) were manually collected from the sieved soil using methods described by Oliveira et al. (2000). Root fractions were then compiled for each core sample, washed, digitized using a flatbed computer scanner, and analyzed for physical dimensions using WinRHIZO Pro software (Regent Instruments, Quebec, Canada). Roots, mineral fragments, and woody debris were then oven dried at 65°C and weighed. Sub-samples of the finely-sieved soil were also oven dried at 65°C and then analyzed for total carbon and nitrogen content by dry combustion with an automated gas combustion analyzer (Vario MAX CNS elemental analyzer, Elementar Instrument, Mt. Laurel, New Jersey, U.S.).
In October 2006, soil samples were collected from the amended root zone of each tree to assay microbial biomass carbon (MBC). Three sub-samples per tree were collected with a 2.5 cm diameter corer at 0–7 cm soil depth and homogenized. All samples were immediately covered with a wet paper towel and quickly passed through a 2 mm sieve. Two, 25 g sub-samples were then collected from each sieved sample and each kept in a constant field-moist condition by placing in a sealed desiccator chamber along with 250 ml of distilled water for 24 hours. One of each pair of samples was then processed for MBC assay according to the chloroform fumigation extraction method of Horwath and Paul (1994). Fumigation destroys membranes and cell walls, allowing subsequent extraction of cell constituents with 0.5 M K2SO4. The extraction procedure was performed on both fumigated and non-fumigated samples and extracted constituents were shipped frozen to the Soil Analytical Service Laboratory at North Carolina State University (Raleigh, North Carolina, U.S.) for total organic carbon (TOC) analysis using a TOC-5050 analyzer fitted with an ASI-5000 autosampler (Shimadzu Corporation, Kyoto, Japan). Total MBC of the extractant was calculated as the difference between TOC in fumigated and non-fumigated samples using an extraction efficiency factor of 0.35 (Voroney et al. 1991). Soil MBC was then calculated by adjusting extractant volume for soil mass and correcting for gravimetric moisture content at the time of sampling.
Statistical Analysis
Measured values of response variables were first screened for normality and homogeneity of variance; violations of these assumptions were corrected through natural log transformation of the measured values prior to statistical analysis. Response variables were analyzed using repeated measures ANOVA with the PROC MIXED procedure of SAS ver. 9.1 (SAS Institute, Cary, North Carolina, U.S.). Where main effects of independent variables were significant, multiple comparisons of response means were conducted using the LSMEANS PDIFF option. Where there were significant interactions between independent variables, the LSMEANS SLICE and DIFF options were used to test their simple effects on response variables. Microbial biomass carbon was analyzed using a two-factor ANOVA with the PROC GLM procedure. Multiple comparisons of treatment factor levels were performed using Tukey’s HSD test. Regression models were created in JMP ver. 9.0.2 (SAS Institute, Cary, North Carolina, U.S.).
RESULTS
Root Growth
There was a marginally significant tree species × soil amendment interaction (P < 0.075) for both total and fine root length (Table 2). There was a significant species difference in root mass, but no interaction with soil amendment measured during the second year after treatment. Red maple had greater root mass than both chestnut oak and pin oak. Although the relationship varied among amendment × species for root length, both total and fine root length were always lower in chestnut oak than in pin oak and red maple (Figure 1; only fine roots depicted). On average, fine root length accounted for 91%–96% of total root length across species during the year.
Significance level of main effects and interactions of tree species (TS), soil amendment (SA), and measurement date (D) for response variables measured during the second year after tree transplant and soil treatment.
Fine root length (<1 mm diameter) of bare root tree seedlings 33 months after transplant into field soil amended with three organic amendments. Roots were sampled with a 5 cm diameter soil core at the 0–10 cm soil depth in the backfill region. Each value is pooled across six bi-monthly measurement dates (n = 24) and error bars depict standard error of mean. Within each species, letters denote significant effects due to soil treatment (α = 0.05).
Soil amendment had no effect on root mass, total root length, or fine root length during the 12-month sampling period (Table 2). However, there was a marginally significant tree species × soil amendment interaction for both root length metrics. Further analysis of the interaction revealed that soil amendment was significant for red maple (P < 0.05), but not for chestnut oak or pin oak (Figure 1). Averaged across sampling dates, both total and fine root length of red maple were greatest in LBC and lowest in PM and CON (Figure 1; only fine roots depicted). Red maple root length in BBC was intermediate, but not statistically different from the other treatments.
Soil Carbon–Nitrogen Dynamics
Although no species-related differences were observed in soil carbon content, nitrogen content, or C/N ratio during the second year after treatment, soil amendment significantly affected (P < 0.01) all three metrics (Table 2). Because there was a significant tree species × measurement date interaction, amendment effects on soil carbon, nitrogen, and C/N ratio were analyzed within species. Soil carbon was stable through time (no significant date effect) and averaged about 40% lower in CON than in the three amended soils, which did not differ from one another (Figure 2). In contrast, nitrogen content was more dynamic through time for all tree species (all date main effects, P < 0.05), generally showing modest increases during the sampling year (Figure 3). Within species, there was no interaction between soil amendment and measurement date. Although the response varied across species, nitrogen content was generally higher in BBC and LBC than in CON and PM, which never significantly differed in all cases (Figure 3).
Soil carbon content at 0–10 cm soil depth in root zones of bare root tree seedlings 33 months after transplant into field soil amended with three organic amendments. Each value is pooled across six bi-monthly measurement dates (n = 24) and error bars depict standard error of mean. Within each species, letters denote significant effects due to soil treatment (α = 0.05).
Soil nitrogen content at 0–10 cm soil depth in root zones of bare root tree seedlings 33 months after transplant into field soil amended with three organic amendments. Within each date, lowercase letters denote significant differences in soil treatments (n = 4). Within each species, uppercase letters denote significant differences in measurement dates (n = 16). For all analyses, α = 0.05.
Both two-way and three-way interactions were found between the independent variables for soil C/N ratio (Table 2). Further analyses by species revealed a significant soil treatment × measurement date interaction for pin oak, but not for chestnut oak or red maple (hence the three-way interaction in the three-factor model, see Table 2). Within species, soil C/N ratio was dynamic through time (all date main effects, P < 0.05). In red maple and chestnut oak, C/N ratio was greatest on the first date irrespective of soil treatment (Figure 4). In contrast, a date effect was only present with the PM treatment for pin oak; here, C/N ratio showed a more progressive decline through the year (Figure 4). Averaged across species and dates, C/N ratio of PM-amended soil was about 52% higher than the other treatments.
Soil C/N ratio at 0–10 cm soil depth in root zones of bare root tree seedlings 33 months after transplant into field soil amended with three organic amendments. Within each date, lowercase letters denote significant differences in soil treatments (n = 4). Within each species, uppercase letters denote significant differences in measurement dates (n = 16). For pin oak, the date effect was only significant for peat moss and therefore uppercase letters apply to date differences for peat moss treatments only. For all analyses, α = 0.05.
Soil Microbial Biomass Carbon
Soil MBC was measured at the end of the third growing season after transplant differed across tree species as well as soil treatments; there was no interaction between the two independent variables (Table 3). On average, MBC of PM-amended soil was 47% less than CON (P < 0.0001; Table 3). In contrast, LBC increased MBC of backfill soil by 12% compared to CON (P = 0.0989) and 65% compared to PM (P = 0.001); although MBC was higher in BBC backfill soil than in CON, there was little or no evidence this was due to treatment (P = 0.2444). Averaged across backfill treatments, MBC was significantly lower in chestnut oak than in red maple and pin oak, which did not differ from one another. Regression analyses revealed significant (P < 0.001) but weak relationships between MBC and cumulative fine root length, C/N ratio, and soil nitrogen (Figure 5). A multivariate model for MBC with these parameters explained 61% of the variance in MBC across all species and treatments:
Microbial biomass measured in the root zone of bare root tree seedlings 33 months after transplant into field soil amended 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.
Relationship of fine root length, C/N ratio, and Soil N to microbial biomass carbon at 0–10 cm soil depth in root zones of bare root tree seedlings 33 months after transplant into field soil or field soil amended with one of three organic amendments.
MBC = 77.4 + 0.00589lf + 162.7n − 3.04c
where lf = cumulative fine root length, n = % soil N, and c = C/N ratio. MBC was unresponsive to C/N ratio when C/N ratio was below 14. However, since only peat moss addition resulted in higher C/N ratios in this study, it cannot be determined if the resulting lower MBC was due to higher C/N ratios or some other characteristic of peat moss. Likewise, percent soil N was relatively consistent within a given treatment, and thus its influence on MBC cannot be easily separated from treatment effects.
Soil Respiration
Although soil CO2 efflux rate (respiration) did not differ across tree species, differences were observed among soil backfill treatments as well as sampling dates. Because there were significant tree species × measurement date, and soil amendment × measurement date interactions, amendment effects on soil respiration were analyzed within species. For all species, soil respiration was lowest during the winter months (minimum in February) and highest during the summer months (maximum in July) (Figure 6). However, soil treatment did not significantly affect soil respiration of chestnut oak. In red maple and pin oak, soil respiration differed among treatments on three dates and two dates, respectively. Although the compost-amended soils had a tendency to have higher soil respiration than CON or PM-amended soil, the effect was not consistent. The effect was most pronounced for red maple in July 2006. Neither soil temperature nor soil moisture was a significant covariate of respiration in the presence of other effects in the MIXED model (data not shown).
Soil respiration measured in the root zone of bare root tree seedlings 33 months after transplant into field soil amended with three organic amendments. Error bars depict standard error of mean (n = 4). Letters indicate significant effects due to soil amendment (α = 0.05).
DISCUSSION
Organic Amendment Effects
Did the single application of organic matter alter carbon dynamics and result in increased soil microbial activity within the root zone?
Organic amendments sustained enhanced carbon content in amended root zones for up to 33 months after treatment (Figure 2). Researchers also found good evidence that compost amendment enhanced microbial activity, with leaf-based compost increasing MBC compared to the control (P = 0.0989; Table 3) across all species and a strong spike in summer soil respiration occurred in both compost-based treatments for red maple (Figure 6). The relative contributions of increased root length (in red maple) and increased microbial biomass to enhanced soil respiration were not quantified. However, past research has shown that roots contribute about 45% of total soil respiration annually in a forested environment (Hanson et al. 2000).
Other studies have found increased microbial activity after compost amendment. However, the amended soils studied were in arid or semi-arid environments with an inherently low microbial activity and were of relatively short duration (<90 days) (Roldan et al. 1994; Pascual et al. 1997). In addition, many studies have used uncomposted materials or immature compost, where decomposition of the material may result in temporary increases in microbial activity. Annabi et al. (2007) found immature compost produced a short-term spike in microbial activity in the first two months after application, whereas mature compost did not increase microbial activity until approximately two months had passed. Cheng and Grewal (2009) found that both subsoil and topsoil amended with compost and seeded to turfgrass experienced a rapid increase in microbial biomass, but this enhancement declined markedly in the subsequent year. In contrast, it was found that mature compost marginally enhanced microbial biomass carbon as long as 33 months after application in a temperate zone soil with somewhat low initial organic matter content (Table 3). Although microbial biomass was only assessed late in the study, consistent soil carbon content and soil respiration patterns during the measurement period suggest that enhancement of microbial activity had been achieved. Because a source of ongoing carbon input (i.e., tree roots) was present, soil carbon may not have been as limited in the soil at the study site as it might be in soil on newly developed urban land with sparse vegetation (Scharenbroch et al. 2005).
Did the type of organic matter and tree species influence these outcomes?
Soil C/N ratio was unaffected except for root zones amended with peat moss, where ratios were consistently higher than for unamended and compost-amended soils (Figure 4). Original C/N ratio of the peat moss was very high (125) compared to leaf-based compost (18) and biosolids-based compost (17). In addition, pH of the peat moss was very low (4.8) and perhaps inhibitory to decomposing microbes. In this study, peat moss significantly suppressed microbial biomass carbon (Table 3) and soil respiration (Figure 6). Past work has investigated peat moss amendment to improve soil physical properties to enhance growth of trees (e.g., Day et al. 1995) and row crops (e.g., Pietola and Tanni 2003). The stability of peat moss perhaps contributes to its popularity as a soil amendment since its effect on soil physical properties can be long lasting. However, increased microbial activity has a positive effect on the building of soil structure (Lynch and Bragg 1985) as well as on aggregate resilience (Annabi et al. 2007), offering the potential for a longer term positive effect on soil quality.
In this study, incorporating peat moss decreased microbial biomass carbon (Table 3). These data are supported by the decreased soil respiration recorded during warm summer months for peatamended versus compost-amended backfill soil (Figure 6). Peat-based growing media are known for their limited microbial communities and thus their ability to suppress certain media-borne plant diseases although there is variation among peats from different sources (Chen et al. 1988). In contrast, there are very few studies examining the influence of peat on microbial biomass in mineral soils. Niklasch and Joergensen (2001) found that peat moss, in contrast to composts, did not increase microbial biomass C when incorporated into a silt loam soil. More recently in a study of strawberry cropping systems, Vestberg et al. (2009) found that microbial biomass C and N were lower in peat-amended field soil than in unamended soil. These results combined with the work presented here indicate that sphagnum peat moss may suppress soil biological activity; whether this is a direct effect or the result of limiting microbial resources is unknown. Notably, although red maple fine root length was lower in PM-amended soils than in soils with compost-based amendments (Figure 1), this was not the case for other species while MBC in peat-amended root zones was lower regardless of species (Table 3). This suggests that reductions in microbial activity did not likely inhibit fine-root growth per se. In addition, although sphagnum peat moss has historically been a common horticultural soil amendment and container media, peat harvest is largely considered a non-sustainable practice (Chapman et al. 2003), and is considered unacceptable for sustainable site development by some certification organizations (e.g., Sustainable Sites Initiative 2009), whereas composting utilizes what might otherwise be waste products. Amending soil with compost rather than peat may therefore be considered desirable for both sustainability and soil improvement practices.
Organic amendments clearly differed in their influence on microbial activity. Overall, the two composts had similar effects on MBC and soil respiration, while PM suppressed both MBC and soil respiration (Table 3; Figure 6). There were no significant effects of amendment on root growth for the two oak species. However, the effect on root length of red maple (Figure 1) followed the same pattern as the effects on MBC, suggesting MBC and root activity were both responding to amendment type. The complex relations among roots, root exudates, microbial activity, microbial exudates, and soil structure could be expected to be reflected in soil respiration. Indeed, PM-treated trees generally had lower soil respiration levels during the growing season, although only occasionally could these differences be reliably 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 ratios 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 competition to be uncoupled from C/N ratios of 20, 31, and 34 – similar 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 proliferation 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 nucleation of mineral particles around these plant residues to form aggregates (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 community (Mitchell et al. 2007; Mitchell et al. 2010). In this study, chestnut oak had lower root zone microbial biomass carbon accumulation than pin oak overall (see Table 3). Chestnut oak also had distinctively less root length than either red maple or pin oak (Figure 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 performance and the extent of root growth or total root growth. Instead, the study aimed to assess the relations between root length and soil biological response. Thus, pin oak also had greater MBC 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 organic 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 chestnut 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 explained 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 backfill 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 approximately 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 responses to amendments are not surprising when comparing experiments that assumedly differ in a multitude of influential factors, the current experiment has shown that some tree species may respond 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 improve plant growth. Conversely, amendment-induced root growth may seem counterintuitive given that increased soil resource levels generally result in decreased root growth (as demonstrated for red maple by Canham et al. 1996) as plants efficiently reallocate carbon to other sinks. However, amendment effects on soil nutrient content may be confounded by effects on soil physical properties, which may exert great control on root responsiveness, particularly 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 enhancing the intimate relations between roots, microbial communities, and soil mineral particles that have the potential to result in enhanced 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 amendments can increase tree root growth (Smalley and Wood 1995; Prince et al. 2000), as was found in red maple, or have no effect (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 urban soils where the microbial community may be more impoverished (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 species, pin oak and chestnut oak. In addition, peat moss amendment suppressed microbial biomass and respiration compared to leaf-based and biosolids-based compost. The very high initial 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 increases 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 backfilled area. In the face of global climate change and rapid urbanization, society must find sustainable uses for urban organic resources. Because soils in disturbed urban environments often lack suitable properties for sustainable tree development, amending impoverished soils with composts may hold promise for restoring soil function and enhancing urban canopy development.
Acknowledgments
This work was supported in part by a Hyland R. Johns grant from the Tree Research and Education Endowment Fund. The technical support 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 Evanylo in the Virginia Tech Department of Crop and Soil Environmental Sciences.
Appendix 1. Response variable means and standard errors for each treatment factor level combination (n = 4).
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