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Journal of Environmental Quality 31:910-916 (2002)
© 2002 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORTS
Plant and Environment Interactions

Copper Release from Chemical Root-Control Baskets in Hardwood Tree Production

Sonja Kosutaa, Chantal Hamel*,a, Yolande Dalpéb and Marc St-Arnaudc

a Dep. of Natural Resource Sciences, McGill Univ., Macdonald Campus, 21 111 Lakeshore Road, Ste-Anne-de-Bellevue, QC, Canada H9X 3V9
b Eastern Cereal and Oilseed Research Centre, 960 Carling Ave., Ottawa, ON, Canada K1A 0C6
c Institut de Recherche en Biologie Végétale, 4101 rue Sherbrooke E., Montréal, QC, Canada H1X 2B2

* Corresponding author (hamel{at}nrs.mcgill.ca)

Received for publication July 14, 2000.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSION
 REFERENCES
 
The City of Montreal, Canada, evaluated the environmental impact and usefulness of in-ground copper (Cu)-treated baskets in controlling root growth of hardwood trees in nursery culture. Using baskets planted with 5-yr-old Norway maple (Acer platanoides L.) trees, the amount and temporal pattern of Cu release from the basket surface into soil was determined for two copper formulations: Cu metal powder and Cu(OH)2. Release of both Cu formulations from the basket surface decreased exponentially over time, with Cu concentration at the basket surface dropping to 2% of the initial Cu applied by the end of the second field season. Total Cu content increased significantly in the soil around the baskets (from 7 to 28 mg Cu kg-1 soil) and in the baskets (from 7 to 50–70 mg Cu kg-1 soil) over the two years of the study. Three levels of phosphorus application (33, 66, and 100% of the regular nursery rate of 465 kg ha-1 yr-1) did not affect release of Cu from the basket surface. The release of Cu metal at 28 and 105 d in the field was significantly increased by inoculation with the symbiotic arbuscular mycorrhizal fungus (AMF) Glomus intraradices Schenck & Smith; however, AMF inoculation had no affect on Cu(OH)2 release. Trees grown in Cu-treated baskets and inoculated with G. intraradices had similar colonization to non-inoculated trees, suggesting that inoculation was not very effective and that AMF inoculum was already present in the root ball of the trees at planting. After two years, copper basket–grown trees had significantly less root colonization than isolated control trees growing in the open field. This strongly suggests that conditions inside the baskets were not favorable to AMF.

Abbreviations: AMF, arbuscular mycorrhizal fungus


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSION
 REFERENCES
 
IN CANADA, NURSERY PRODUCTION of high-quality trees for transplant into urban environments requires 8 to 10 yr and costs approximately Can$400 per tree after transplant. Labrecque (1993) found that only 4% of trees planted in downtown Montreal reach 10 yr of age. Furthermore, trees planted along busy downtown streets survive only 4 to 5 yr, and only 1 to 2 yr in the harshest locations. In an effort to improve production and post-transplant survival of hardwood trees, the City of Montreal, Canada, examined two new nursery production techniques: inoculation of trees with arbuscular mycorrhizal fungi (AMF) and the use of in-ground Cu-treated baskets to control root growth. This study presents data relevant to the potential environmental impact of Cu-treated basket use under field production conditions, and in combination with AMF inoculation.

Symbiosis with AMF has been shown to enhance the growth of several woody species under low to moderate soil fertility (Huang et al., 1985; Plenchette et al., 1981; Purcino et al., 1986; Miyasaka et al., 1993; Siqueira et al., 1998). Recently, AMF inoculation of hardwoods growing at high-fertility levels produced positive growth responses in 1-yr-old seedlings (Chapdelaine et al., 1998) and 3-yr-old trees (Morrison et al., 1993) transplanted into nonsterile field sites. This illustrates that, although fertilization, especially with phosphorus, has long been thought to interfere with AMF colonization of plant hosts (Amijee et al., 1989), AMF inoculation may be viable under nursery production conditions. As peat-based potting mixes used in container production contain no natural AMF propagules, inoculation of container-grown stock seems necessary.

Container production of landscape trees is preferred over bare-root production because it can reduce the root damage caused by tree transplanting. However, the long-term use of containers leads to "root bound" trees with subsequent poor growth (Harris et al., 1971), mechanical instability (Burdett, 1978), and water stress (Davidson and Mecklenburg, 1981) after transplanting. Containers treated with Cu(OH)2 or CuCO3 decrease circling, matted, and kinked roots at the container–soil interface (Arnold and Struve, 1989a,b, 1993; Beeson and Newton, 1992; Struve et al., 1994), making container production possible. These containers release Cu2+ into the soil solution where it is available for plant uptake. Localized Cu toxicity effectively "pinches off" roots at the container–soil interface, resulting in chemical root control. This arrestment of apical root growth has also been related to increased root branching (Arnold and Young, 1991), increased root density, more uniform distribution of roots within the root ball (Arnold and Struve, 1989a, 1993), increased number of actively growing root tips (Arnold and Young, 1991), and improved root regeneration potential (Struve et al., 1994), all desirable horticultural characteristics that improve the quality of the trees. Together these effects can improve post-transplant survival and establishment of landscape trees (Burdett, 1978; Arnold and Struve, 1989b; Brass et al., 1996).

Copper-treated containers in various shapes and sizes are widely used in horticultural production. A water-permeable Cu-treated synthetic fabric placed directly into the ground, such as those studied here, may perform differently in terms of root control and release of Cu to the soil, plant, and ground water. The soil environment can be acidic and humid, qualities that enhance the solubility of Cu compounds. Exudates from plant roots and fungal hyphae can acidify the rhizosphere (Jones, 1998) and fertilization with some nitrogen sources can lower soil pH (Tisdale et al., 1985), making Cu(OH)2 more soluble. Some soil solution Cu2+ is necessary in the vicinity of the container wall to control root growth and prevent root circling within the container; however, dissolved Cu2+ can move both horizontally and vertically with soil water and be lost by leaching. Copper cations in the soil solution can be immobilized by adsorption to clay colloids and other soil particles and organic complexes (McLaren and Crawford, 1973). Copper will also become complexed by soluble organic acids and could trickle down the soil profile to ground water (McBride et al., 1997).

The in-ground Cu-treated growing basket is currently seeking product registration for use in Canada. Recent work on Cu(OH)2–treated in-ground fabrics found that 75% of the Cu had been lost from the fabric surface after 90 d in the field and up to 70% of the Cu could be lost within the first month (Iraqi, 1995; Kosuta and Hamel, 2000). Loss of a large amount of the Cu within the first few months is costly because more Cu must be applied than actually needed for root control. The large loss of Cu also raises an environmental concern, for although Cu is an essential plant nutrient, it can be toxic when present in excess. The effects of Cu-treated containers on tree production and root morphology are well documented, but the release of Cu from the container wall to the soil system and the fate of the Cu released from the in-ground baskets under field conditions must be known in order to determine the environmental impact of this practice.

The objective of this study was to determine the extent of Cu-release from in-ground Cu-treated baskets under field production conditions. In addition, the influence of two production parameters, AMF inoculation and phosphorus fertilization, were considered. We hypothesized that fungal hyphae would increase the solubility of the Cu on the fabric surface by lowering the pH of the rhizosphere. Phosphorus fertilization was hypothesized to influence the colonization success of AMF inoculation.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSION
 REFERENCES
 
In the spring of 1996, 288 five-year-old trees were planted into in-ground growing baskets in the field at the City of Montreal Nursery in Terrebonne (45° N, 73° W). According to data collected by Environment Canada (2000), Montreal's average monthly low for the coldest month is -10°C, average monthly high for the warmest month is 20°C, and average annual precipitation is 756 mm yr-1. The trees were grown under normal nursery conditions until the fall of 1995, when they were removed from the field and stored at 0°C until spring 1996.

The treatments included three P fertilization levels (33, 66, and 100% of the regular nursery application of 465 kg P ha-1 yr-1) and two levels of AMF inoculation (inoculated with Glomus intraradices Schenck & Smith or not inoculated). The experiment was a 3 x 2 factorial in 8 blocks. Each experimental unit was composed of three trees (replications) arranged randomly in one-row plots. In-ground growing baskets (80-cm diameter, 40-cm depth) were comprised of a metal wire frame covered by a geotextile fabric made from post-industrially recycled synthetic fibers (Tex-R; Texel, St-Elzéar-de Bauce, QC, Canada). The fabric was factory treated with a latex resin containing either Cu metal powder or Cu(OH)2 (Spinout; Griffin Corp., Valdosa, GA), applied at a rate of 6 ± 2 g m-2 Cu. Two very similar cultivars of Norway maple grafted to the same rootstock were used: Deborah and Emerald Queen. As trees of the cultivar Emerald Queen were planted into baskets treated with Cu metal powder while trees of the cultivar Deborah were planted into baskets treated with Cu(OH)2, neither cultivar nor Cu formulation were considered as experimental factors in the statistical analysis of the data.

Mycorrhizal inoculum contained one effective propagule of G. intraradices per gram fresh weight (Mycorhize; Premier Tech, Rivière-du-Loup, QC, Canada). Trees were grown in a horticultural peat soil with a bulk density of 0.34 ± 0.01 g mL-1, determined by the method of Hendershot et al. (1993). The texture of the nursery field soil around the baskets varied between sand and sandy loam, determined by the hydrometer method (Sheldrick and Wang, 1993), with a bulk density of 1.06 ± 0.08 g mL-1. The pH values in 0.01 M CaCl2 (Hendershot et al., 1993) of the nursery and basket soils were 5.29 ± 0.04 and 5.69 ± 0.01, respectively.

In May 1996, growing baskets were placed into pre-dug holes and half-filled with soil. One liter of inoculum (268 g) was spread evenly over the soil surface of inoculated baskets. The tree root ball was placed on top of this and the basket topped-up with soil. Granular fertilizer was applied by hand to the top of each basket. The P3 treatment received a one-time application of 928 kg ha-1 (the 100% nursery application rate of 465 kg ha-1 yr-1 for the two years of the study) of a granular slow-release fertilizer (Nutricote 14–14–14; Agrivert, Wayne, NJ) on 11 June 1996, while the P1 and P2 treatments received 33 and 66% of this quantity, respectively, on the same date. The nitrogen and potassium required to reach the 100% level in the P1 and P2 treatments were supplied as feather meal (10–0–0) and K-Plus (0–0–22) (Greensmiths, Frisco, TX), in equal applications on 11 June and 8 July of both 1996 and 1997. Trees were drip-irrigated throughout the summer season with well water treated by reverse osmosis (pH 7.0–7.5).

Soils were sampled with a 5-cm-diameter soil core. Ten soil cores of the bulk horticultural soil were taken before planting. The sandy field soil was sampled before planting and after two seasons in the following manner: 10 vertical (to a depth of 40 cm from the soil surface) and 10 horizontal (20-cm soil core taken at a depth of 20 cm from the soil surface) soil cores were taken randomly from each block, mixed thoroughly in a bucket, and subsampled.

The soil in the Cu-treated baskets was sampled horizontally and vertically at 0, 14, 28, 60, 105, and 470 d after planting in Blocks 1 through 5. In the field, holes were dug at the outside of each basket allowing cores to be taken horizontally 20 cm into the basket at a depth of 20 cm from the soil surface. Vertical soil samples consisted of a vertical core taken down into the basket to a depth of 40 cm, roughly the bottom of the basket. Cores of the three treatment replications of each plot were bulked and a subsample taken, providing representative vertical and horizontal samples for each treatment of each block.

All soil samples were digested in 15 mL HNO3 plus 5 mL HClO4 for 20 h at 135°C (Cook, 1998). Copper analysis of the digest solutions was done using a 2380 Perkin Elmer (Wellesley, MA) 7 atomic absorption spectrophotometer. An audit soil sample obtained from the National Institute of Standards and Technology (1992) and reported to contain 2950 mg Cu kg-1 dry weight was analyzed and yielded 2912 ± 123 mg Cu per kg of soil.

Six strips (5-cm width, 50-cm length) of the Cu-treated fabric were placed vertically into baskets of the P1 and P3 treatments of Blocks 1 through 5 at planting. The strips were collected at 0, 14, 28, 60, 105, and 470 d after planting. As digestion was incomplete using only HNO3 and HClO4, each strip was digested in 10 mL H2SO4, 10 mL HClO4, and 15 mL HNO3 for 20 h at 135°C. Digest solutions were analyzed for total Cu in the same process described above.

The roots protruding from a 20- x 20-cm section of the side of each basket at a depth of 20 to 40 cm were counted at the end of the second season. This number was extrapolated to determine the number of perforating roots per basket.

Roots from each tree were sampled with a vertical soil core 30 cm from the trunk at the end of September 1996 and 1997. All root fragments that came up with the sample core were cleared with 10% KOH and stained for AMF determination (Phillips and Hayman, 1970). Although sample roots were not intentionally selected, they were necessarily younger roots because older, lignified roots were too tough to be sampled in this manner. The percentage colonization by arbuscular mycorrhizae was determined by the grid-line intersect method (Giovanetti and Mosse, 1980).

All data were treated as repeated measures in time using the modified analysis of variance (ANOVA) procedure (Von Ende, 1993). Copper metal and Cu(OH)2 data were analyzed separately to avoid confounding results from cultivar effects. Where necessary, treatment means were compared using Tukey's method. All statistical analyses were performed using PROC GLM procedures of SAS statistical software (SAS Institute, 1984). The exponential curves were determined by regression analysis using SigmaPlot software (SPSS, 1997).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSION
 REFERENCES
 
Copper release from both Cu(OH)2 and Cu metal–treated baskets followed the same temporal trend, showing a second-order exponential decay [r2 = 0.87 for Cu metal and r2 = 0.75 for Cu(OH)2] (Fig. 1) . Mycorrhizal inoculation had no effect on the movement of Cu from the Cu(OH)2–treated basket. However, Table 1 shows that Cu metal–treated baskets inoculated with G. intraradices released more Cu at 28 and 105 d after planting (P < 0.05). By the end of the second field season, only 2% of the Cu initially applied as either Cu(OH)2 or Cu metal remained on the basket fabric surface. The release of Cu from Cu(OH)2– and Cu metal–treated baskets was not affected by the level of P application (Table 1).



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  		Fig. 1. Exponential release of Cu from in-ground chemical root-control baskets in the field. Best-fit curves were calculated using regression analysis.

 

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  		Table 1. Effect of mycorrhizal inoculation and phosphorus fertilization level on copper release by in-ground chemical root-control baskets.

 
The average Cu content of the soil inside baskets treated with Cu(OH)2 increased from 8.7 to 14.7 mg kg-1 over the first season and to 50.0 mg kg-1 by the end of the second field season (Table 2). Copper metal–treated baskets showed a similar trend; an increase of Cu content from 7.3 to 10.6 mg kg-1 in the first year and to 71 mg kg-1 in the second year, showing an exponential growth [r2 = 0.90 for Cu metal and r2 = 0.88 for Cu(OH)2] (Fig. 2) . Copper accumulation in the basket soil was not affected by inoculation with AMF (data not shown) nor level of P fertilization (Table 2).


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  		Table 2. Effect of mycorrhizal inoculation and phosphorus fertilization level on copper accumulation in soil inside the in-ground chemical root-control baskets.

 


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  		Fig. 2. Exponential increase of soil Cu content inside the root-control baskets in the field. Best-fit curves were calculated using regression analysis.

 
The field soil of the nursery contained significantly more Cu at the 20-cm depth than at 40 cm (Table 3) before the trees were planted. After the two seasons of the study, average Cu content of the nursery field soil around and below the baskets had increased significantly to 28.4 mg kg-1 with no significant difference between sampling depths (Table 3). Both Cu metal– and Cu(OH)2–treated in-ground growing baskets effectively controlled the roots of Norway maple trees for the duration of the study. After two years in the field, however, small roots crossed the Cu barrier at a rate of 16.6 roots per 20- x 20-cm sampling area, or 415 roots per square meter, for both Cu-treated baskets. The number of roots protruding from the sack did not differ among P levels or AMF inoculation treatments (data not shown).


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  		Table 3. Copper accumulation in nursery soil resulting from the use of in-ground root-control baskets.{dagger}

 
Mycorrhizal colonization of trees growing in baskets was not significantly different than that of trees growing "bare root" in the open field at the end of the first growing season (Fig. 3) . However, by the end of the second season, roots of the field trees were significantly more colonized than those of both inoculated and non-inoculated trees growing in baskets. Phosphorus fertilization had no effect on colonization in either year (data not shown).



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  		Fig. 3. Arbuscular mycorrhizal colonization of tree roots in root-control baskets. Control trees were grown bare-root in the open field. Error bars indicate standard error.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSION
 REFERENCES
 
Copper Release from the Basket Fabric
The 75% release of the Cu from the fabric of Cu(OH)2–treated baskets over the first field season is similar to the results of previous studies where in-ground Cu-treated baskets were filled with soil where no tree was included (Iraqi, 1995) and where the Cu-treated basket system was contained within a waterproof barrier (Kosuta and Hamel, 2000). We hypothesized that tree roots and fungal hyphae might increase the solubility of the Cu compounds on the fabric surface by lowering the pH of the rhizosphere (Jones, 1998), but the rate of Cu(OH)2 release did not seem to be affected by G. intraradices. Considering the already-low pH of the organic and nursery field soils in this study, the effect of roots and hyphae on soil acidification should be limited, as seems to be the case.

In the field, baskets treated with Cu metal responded differently than those treated with Cu(OH)2. The Cu metal decay curve was more gradual, particularly over the first month. Copper metal may represent a solution to the problem of rapid Cu loss because of its much lower solubility in both water and weak acids. Elemental Cu metal must first undergo an oxidation reaction to Cu2+ before it becomes soluble. The greater retention of Cu metal at the basket fabric at 60 and 105 d indicates that this reasoning may be correct. Perhaps Cu metal of a larger particle size would oxidize even more slowly and be retained longer at the fabric surface. A cultivar effect is also possible. Inoculation with G. intraradices seemed to hasten the dissolution of Cu metal from the basket fabric surface despite the low level of colonization observed in tree roots. Much evidence suggests that fungi can solubilize metals by exuding complexing organic acids into the rhizosphere (Jones, 1998), which may explain the greater loss of Cu metal from the basket fabric of inoculated baskets observed in this study. However, AMFs are not well characterized in this respect. In a complementary study, leaves of AMF-inoculated maple shrubs growing in Cu-treated baskets contained significantly more Cu than leaves of non-inoculated shrubs (Kosuta and Hamel, 2000), which supports the hypothesis that AMFs increase the solubility of Cu in the rhizosphere.

Accumulation of Copper in Soil
After two seasons, Cu content in the organic soil was roughly double that of the surrounding nursery soil. As peat can form very stable complexes with Cu (Ennis, 1962), the abundance of organic matter in the bulk horticultural soil was expected to retain Cu more strongly than the sandy nursery soil surrounding the baskets. Though average soil Cu concentrations did not exceed the acceptable parkland soil Cu level of 100 mg Cu kg-1 soil (Canadian Council of Ministers of the Environment, 1991), Cu content of several individual samples approached and even exceeded this guideline limit after two seasons in the field.

Outside the baskets, Cu content at the 20-cm depth from the side of the basket was similar to that at the 40-cm depth directly below the basket, indicating that dissolved Cu2+ or Cu–organic complexes traveled laterally and vertically. The migration of Cu2+ to soil outside the basket may have been made easier by the flow of irrigation water from its drip source at the top center of the basket, though the high pH (7.0–7.5) of the irrigation water used is not likely to favor Cu solubility. As only total Cu was determined for the two soils, the size of the mobile Cu fractions and therefore the stability of this Cu at the various sampling dates is unknown (Sauvé et al., 1996). Even strongly bonded Cu can be mobile in association with colloidal humified organic matter and in solution as ionic complexes (Bloomfield et al., 1976).

Leachate from these baskets tested in a recent study contained soluble Cu concentrations between 0.5 and 8.5 mg L-1, and exceeded 1 mg L-1, the Canadian Council of Ministers of the Environment limit for total Cu in water, in 60% of samples (Kosuta and Hamel, 2000), though the total Cu recovered in leachate over one season represented only 0.004% of the total Cu present on the basket fabric at planting. In the same study, the amount of Cu taken up by maple shrubs growing in Cu-treated baskets over one season accounted for less than 1% of the basket Cu (Kosuta and Hamel, 2000). A Cu balance was determined for the data collected in this study (Table 4). It shows that 98.2% of the initial Cu present on the basket fabric can be accounted for after two years in the field, with the majority (67.7%) of initial Cu present in the nursery soil around the baskets, and 27.7% remaining in the soil inside the basket. Assuming that mobilization of Cu accumulated in the sandy nursery soil is negligible, with repeated use of new Cu-treated baskets every two years at the same planting density in the same field, soil Cu levels will increase at a rate of 10 mg Cu kg-1 soil yr-1, reaching 500 mg kg-1 and requiring remediation (Canadian Council of Ministers of the Environment, 1991) in approximately 50 yr.


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  		Table 4. Copper balance determined for Cu-treated root-control baskets after two years in the field.

 
Inoculation Success
Level of P fertilization did not affect arbuscular mycorrhizal colonization of roots in the baskets, suggesting either that the lowest P level was already inhibitory or that the highest P level was not inhibitory. The substantial colonization observed in trees growing two years in the open field at the 100% P level (Fig. 3) seems to suggest that fertilization at normal nursery P levels is not inhibitory to AMF. However, the AMF species in the field were likely more efficient colonizers than the G. intraradices used in the baskets, either because of selection through repeated cultivation of these same tree species fertilized at high rates in this field, or because the field soil contained many more viable AMF propagules than were added to the baskets in the form of G. intraradices inoculum. Root colonization of both inoculated and non-inoculated trees in the baskets was very similar over both years. This indicates that inoculation with G. intraradices was not very effective, because root colonization may have been the result of AMF propagules already present in the root ball of the tree when it was planted, contamination from boots, tools, or air-borne deposition rather than inoculation. In addition, two years should be long enough for roots in the baskets to become colonized by AMF in the surrounding field soil if conditions in the baskets were favorable to AMF. High soil Cu2+ inside and around the baskets may have inhibited AMF development (Koomen et al., 1990), though other authors report no correlation between heavy metal bioavailability and reduction in AMF colonization in contaminated soils (Weissenhorn et al., 1995a, b). Colonization with endomycorrhizal fungi has been observed in metal-polluted sites, indicating that metal-tolerant species exist and tolerance can develop over time (Leyval et al., 1997). Perhaps a Cu-tolerant strain of fungi would be more appropriate for nursery application.


   CONCLUSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSION
REFERENCES
 
This research showed that Cu is released from the fabric surface of in-ground growing baskets following an exponential decay curve. Inoculation of Norway maple with G. intraradices increased the solubilization of Cu metal powder, while the rate of solubilization of Cu(OH)2 was unaffected. Inoculation may have influenced the biodecomposition of the resin, although how or why is unclear. Copper reached the soil both inside and outside the basket in quantities that do not exceed soil quality guidelines; however, the stability of this new soil Cu and the extent of Cu mobility to the nursery soil must be investigated to ensure the sustainability of this practice. Mycorrhizal colonization in the baskets was low, possibly due to P fertility, insufficient or ineffective inoculum, high Cu2+ bioavailability, or adverse soil conditions.


   ACKNOWLEDGMENTS
 
The financial support of the National Science and Engineering Research Council, Texel Inc., Premier Tech Inc., Agriculture and Agri-Food Canada, and the City of Montreal is greatly appreciated in making this study possible. Thanks to Anne Chapdelaine, Jamie Sauder, and the staff of the City of Montreal Nursery in Terrebonne for technical aid.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSION
 REFERENCES
 





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