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Department of Rangeland Ecosystem Science, Colorado State University, Fort Collins, CO 80523-1478
* Corresponding author (edr{at}cnr.colostate.edu)
Received for publication May 24, 2001.
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: B, biosolids • C, capillary barrier • M, mushroom compost • N, nonstockpiled topsoil • OM, organic matter • P, ProMac • S, stockpiled topsoil • SOM, soil organic matter • TRT, treatment
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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A variety of approaches have been considered for reclaiming acid mine waste including direct revegetation (no soil cover) of amended waste materials, topsoiling, and the use of capillary barriers. Many studies have illustrated numerous benefits of adding organic matter (OM), in addition to lime and fertilizer, to acidic mine waste (Berg et al., 1986; Pichtel et al., 1994; Bellitto et al., 1999; Prentice et al., 1999). Incorporation of OM can improve the fertility and physical and chemical properties of the mine waste, and may bind trace elements (Sposito, 1989), thereby facilitating reclamation efforts. Yet, topsoiling may be necessary given the adverse physical and chemical properties of mine waste, the economics of properly amending these materials, and/or state and federal regulations. Some researchers have advocated the placement of good quality topsoil over acidic mine waste (Richardson, 1980; Trlica et al., 1995) as the addition of topsoil may improve the water holding capacity and nutrient status of the mine waste, and provide a source of propagules and soil microorganisms (Howard and Samuel, 1979; Schuman and Power, 1980). Despite potential benefits of amending mine waste and/or topsoiling, problems may arise such as acidification (or reacidification) of surface layers (Barth, 1983; Boon, 1986), excessive plant uptake of trace elements (Levy et al., 1999; Paschke et al., 2000), and/or capillary rise of soluble salts (Stark and Redente, 1990; McFarland et al., 1994). Therefore, some researchers have investigated the use of capillary barriers between overlying topsoil and underlying wastes as a reclamation option (Stark and Redente, 1986, 1990; Barth, 1988) to reduce capillary rise of salts and trace elements and direct contact of plant roots with untreated waste materials.
Past research of alpine disturbances has indicated numerous techniques that may be useful for reclamation of these unique systems, but further research is needed to determine the best combination of these techniques. Objectives of this study were to (i) evaluate the effects of OM incorporation (mushroom compost vs. biosolids) into waste rock materials on plant community development, (ii) evaluate the effects of topsoil application (stockpiled vs. nonstockpiled) over waste rock materials on plant community development, and (iii) determine the effect of OM and topsoil treatments on plant trace element uptake (Mn, Cu, Zn, Cd, and Pb). This paper focuses primarily on results from data collected in the third and fourth growing seasons (1998 and 1999) of this field study.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Design and Construction
Experimental reclamation test plots were established on the North Waste Dump (one of two waste rock piles on site) in the fall of 1995. The field study was based on results from an initial greenhouse study (Redente et al., 1996). The study area was regraded to a slightly inward configuration to promote drainage, and individual plots were ripped to a depth of 30 cm with a D7 Caterpillar (Peoria, IL) to reduce compaction.
The study consists of eight treatments, replicated four times in a randomized complete block design (Fig. 1) . Individual treatments will be referred to as TRT-SMP, -SM, -NM, -M, -SB, -B, -NS, and -NSC, where M is mushroom compost, B is biosolids, S is stockpiled topsoil, N is nonstockpiled topsoil, P is ProMac (a combination of liquids and controlled release pellets formulated to inhibit iron-oxidizing bacteria; BFGoodrich, Akron, OH), and C is capillary barrier. Topsoil used in the study originated from Topsoil Stockpile 7 (stockpiled topsoil) and topsoil removed during construction of the Summitville Dam Impoundment (nonstockpiled topsoil). Organic amendments were acquired from Rocky Mountain Soil (mushroom compost) and the City of Alamosa (Grade A biosolids). Chemical analysis of the organic amendments prior to their use is presented in Table 1. Background data for the topsoil and waste material used in this study are not available. A description of each treatment is presented below:
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After individual treatments were in place, plots were disked to break up large soil aggregates and establish a good seedbed. Fertilizer was then applied with a tractor and fertilizer spreader. Treatments M and B did not receive topsoil, and were therefore treated with higher rates of fertilizer than the other treatments. Fertilizer rates were as follows: TRT-SMP, -SM, -NM, -M, -SB, and -B: ammonium nitrate (33–0–0) at 112 kg elemental N ha-1; triple super phosphate (0–44–0) at 168 kg elemental P ha-1; potassium chloride (0–0–60) at 112 kg elemental K ha-1. TRT-M and -B: ammonium nitrate (33–0–0) at 168 kg elemental N ha-1; triple super phosphate (0–44–0) at 336 kg elemental P ha-1; potassium chloride (0–0–60) at 168 kg elemental K ha-1.
Following fertilization, plots were disked again to 15 cm to incorporate the fertilizer, then broadcast seeded by hand (Table 2). After seeding, plots were lightly disked to provide good soil–seed contact. Plots were then covered with straw (6.7 Mg ha-1) and the straw was crimped into the soil with a D5 Caterpillar with wide tracks. Final plot dimensions were 10 by 20 m. Construction and seeding of test plots were completed on 18 Nov. 1995. Since 1997, all plots were fertilized annually at the beginning of each growing season with 56 kg ha-1 ammonium nitrate to accelerate organic matter buildup in this short-growing-season environment.
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= 0.05 level. Mean separation tests were performed with least significant difference (LSD) at the = 0.05 level. Plant canopy cover was measured in August 1996 though 1999. Cover was visually estimated in 1996 and 1997, with a 0.5- x 1.0-m quadrat placed every two meters along a 20-m randomly placed transect, for a total of 10 quadrats per plot. Cover was estimated in 1998 and 1999 with the point intercept method; four transects were randomly placed lengthwise in each plot and cover was observed and recorded for 150 points at 0.5-m intervals along the transects.
Soil pits (one per plot) were excavated in test plots, in 1999, to allow for observation of root growth and collection of soil samples. Soil samples were collected from all layers of soil and waste rock (except waste rock in TRT-NSC) in each test plot, for a total of 88 samples. Samples were air-dried and sieved through a 2-mm mesh sieve. The following analyses were performed on each soil sample: (i) pH with a 1:1 soil and water mixture (Self and Rodriguez, 1999); (ii) total organic carbon content with a LECO (St. Joseph, MI) CNH 1000 high-temperature induction furnace (Nelson and Sommers, 1996) and gravimetric inorganic carbon analysis (Self and Rodriguez, 1999); (iii) plant "available" trace element concentration (Mn, Cu, Zn, Cd, and Pb) with an ammonium-bicarbonate diethylenetriamine pentacetic acid (AB-DTPA) extraction (Soltanpour and Schwab, 1977) followed by inductively coupled plasma–atomic emission spectrometry (ICP) analysis; and (iv) total trace element content with a nitric–perchloric–hydrofluoric acid digest (Self and Rodriguez, 1999) followed by ICP analysis.
Tissue samples from two of the dominant grass species and the dominant forb species were collected from each test plot for trace element analysis in August 1999. Species analyzed were orchardgrass (Dactylis glomerata L.), slender wheatgrass [Elymus trachycaulum (Link) Malte], and western yarrow (Achillea millifolium L.). Plant materials were kept refrigerated (at approximately 4°C) and transported to Colorado State University where samples were washed with deionized water, dried at 60°C, and ground with a standard Model 4 Wiley mill. Samples were then analyzed for Mn, Cu, Zn, Cd, and Pb with a nitric–perchloric acid digest (Self and Rodriguez, 1999) followed by ICP analysis.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Aboveground biomass data from 1998 and 1999 showed distinct trends in plant growth relative to treatment, specifically the incorporation of organic amendments. During this period, the greatest overall plant growth occurred on treatments with mushroom compost incorporated into limed waste rock (in 1999: TRT-SMP = 198 g m-2, -SM = 210 g m-2, -NM = 177 g m-2, and -M = 172 g m-2) regardless of topsoil treatment (Fig. 2) . Treatments incorporating biosolids (in 1999: TRT-SB = 125 g m-2 and -B = 134 g m-2) and treatments that did not receive organic amendment (in 1999: TRT-NS = 121 g m-2 and -NSC = 108 g m-2) supported less plant growth, respectively. The mushroom compost used was found to be more effective than the biosolids as an organic amendment. It appears that mushroom compost was more effective in terms of its nutrient availability (Table 1), and therefore may have facilitated plant growth to a greater extent. For example, the mushroom compost contained a higher concentration of mineral nitrogen (NH4–N = 17.3 mg kg-1; NO3–N = 3.7 mg kg-1) than the biosolids (NH4–N = 3.25 mg kg-1; NO3–N = 0.2 mg kg-1) used in the study. This difference may partially account for the increased effectiveness of the mushroom compost, as available nitrogen is often low in alpine systems (Everett et al., 1981), and can therefore be a limiting factor to growth.
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The application of ProMac in TRT-SMP did not significantly alter aboveground biomass from that seen in TRT-SM (198 and 210 g m-2, respectively), which was the same treatment without ProMac. Therefore, any variation in biomass between TRT-SMP and other treatments was probably due to the presence of mushroom compost and/or stockpiled topsoil rather than the addition of ProMac. In both 1997 and 1999, it was noted that root growth in TRT-SM appeared to be more extensive than that in TRT-SMP (Redente and Richard, 1998; Winter, 2000). Some studies have reported that bactericides enhance reclamation success by reducing the activity of acid-producing bacteria (Maierhofer, 1988; Rastogi, 1996). Conversely, other studies have found the use of bactericides to result in decreased plant production and reduction in nontargeted microbial populations due to nonselective elimination of beneficial soil bacteria (Ingham et al., 1991; Zelles et al., 1985). Hence, ProMac may have negatively affected root growth due to nonselective elimination of beneficial soil microorganisms.
There was little change in plant composition relative to life form (i.e., grass vs. forb) during the first four years of this study. In 1996, grasses accounted for 76% of total herbaceous cover (Redente and Richard, 1998), while they accounted for 82% in 1999 (Table 3). However, shifts in species composition were observed during this period. Mountain brome (Bromus marginatus Nees ex Steud.) and common barley (Hordeum vulgare L.), which were two of the dominant grasses on site in 1996 (Redente and Richard, 1998), were absent or present in only trace amounts in 1999. This indicates that these species may be useful as cover crops at the site. Both barley and timothy (Phleum pratense L.) were probably introduced to the site through mulching, yet timothy, unlike barley, increased with time. This is a reminder that mulch selection is important as it can influence the long-term species composition of reclaimed areas. Overall, forb diversity at the site decreased with time. In 1996, western yarrow accounted for 21% of forb growth at the site, while it accounted for 89% in 1999. Finally, annual species that were observed during the first two growing seasons (Redente and Richard, 1998) were no longer present on the plots in 1999 (Winter, 2000).
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Soil pH
Variation in pH existed among soil–subsoil layers within each treatment (Table 4), and may have affected plant growth. Unamended waste rock and unlimed stockpiled topsoil were very acidic with an average pH of 2.8. Conversely, pH of the nonstockpiled topsoil, limed stockpiled topsoil, and amended waste rock was more favorable for plant growth and was more similar to those of native reference soils (pH 4.9 to 5.3; Richard and Redente, 1996). Hence, the addition of lime to acidic soil and mined-waste materials in this study was effective at raising the pH of the materials during the four years following application.
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Soil Organic Matter
The importance of incorporating OM into mine spoils to improve nutrient availability and soil physical properties has been well established (Berg et al., 1986; Chambers et al., 1988). The application of 89.3 Mg ha-1 of mushroom compost to agronomic crops has been shown to increase crop production by up to 900% due partially to increases in nitrogen availability (Rhoads and Olson, 1995). Municipal biosolids have also been widely recognized as an effective short-term fertilizer and source of long-term slow-release nitrogen (Hall, 1984). Hence, incorporation of OM may benefit the establishment of a perennial plant community necessary for achieving site-wide reclamation goals.
Soil organic matter appears to be a primary factor influencing aboveground biomass in this study. Treatments that incorporated mushroom compost (TRT-SMP, -SM, -NM, and -M) had relatively high SOM content in amended waste rock layers (Table 4) and supported the greatest aboveground biomass (in 1999: TRT-SMP = 198 g m-2, -SM = 210 g m-2, -NM = 177 g m-2, and -M = 172 g m-2). Treatments that incorporated biosolids (TRT-B and -SB) or no organic amendments (TRT-NS and -NSC) had amended waste rock layers with a lower SOM content and supported lower amounts of aboveground biomass (in 1999: TRT-SB = 125 g m-2, -B = 134 g m-2, -NS = 121 g m-2, and -NSC = 108 g m-2). The difference in SOM among treatments reflects differences in composition of the organic amendments used: on a dry weight basis, the mushroom compost applied was 29% OM compared with the biosolids that contained approximately 11% OM (calculated from C to N ratio).
In addition to its ability to supply plant nutrients, the ability of OM to bind trace elements has also been well established (Sposito, 1989; Kabata-Pendias and Pendias, 1984; Stevenson, 1982). Hence, the use of OM may be useful in mine land reclamation for reducing the bioavailability of potentially phytotoxic trace elements. In general, in this study the highest trace element tissue concentrations were found in plants from treatments that did not receive additional organic amendment (TRT-NS and -NSC). These treatments also supported the lowest amounts of aboveground biomass. Lower biomass could be due to either reduced nutrient availability (from lack of additional OM) or increased bioavailability of trace elements. This study was not designed to determine how OM might benefit plant growth due to its ability to reduce the bioavailability of trace elements, but this relationship should be examined more extensively in future studies.
Total and DTPA-Extractable Soil Trace Elements
Kabata-Pendias and Pendias (1984) have reported values for total concentrations of trace elements considered as phytotoxic to agronomic species in surface soils: 1500 to 3000 mg kg-1 Mn, 60 to 125 mg kg-1 Cu, 70 to 400 mg kg-1 Zn, 3 to 8 mg kg-1 Cd, and 100 to 400 mg kg-1 Pb. Analysis for total soil trace elements indicated that concentrations of Cu, Zn, Cd, and Pb were within levels considered potentially phytotoxic in all soil and subsoil layers (except Cu levels in the stockpiled topsoil layers), while concentrations of Mn were within normal levels in all samples (Table 5). Total soil concentrations of Cu, Zn, Cd, and Pb did not appear to be problematic in this study as no visual signs of phytotoxicity were observed and plant tissue analysis indicated "normal" uptake of these elements (see discussion under Plant Tissue Analysis, below). Furthermore, total trace element concentrations did not correspond with trends seen in aboveground biomass (Fig. 2). This indicates that species used in this study are more tolerant of trace elements than species used for determining maximum agronomic trace element limits, and that existing total soil trace element phytotoxicity standards are not applicable to this study.
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Extraction procedures with DTPA were developed to analyze the fertility of slightly acidic to alkaline agricultural soils (Soltanpour and Schwab, 1977). Recently, such procedures have been applied on mined-land soils for determination of trace element availability. It has been suggested that this method is of marginal use for comparing available soil trace element concentration to plant uptake and establishing nonagronomic toxicity thresholds useful for mined land reclamation (Levy et al., 1999). Due to a lack of more appropriate alternative analytical methods, an ammonium-bicarbonate diethylenetriamine pentacetic acid (AB-DTPA) extraction was used to examine trace element availability (Winter, 2000). Currently, there are no established DTPA-extractable trace element concentrations that can be cited for phytotoxicity in nonagronomic species. Levy et al. (1999) have suggested some possible DTPA-extractable trace element concentrations as potentially excessive: >70 mg kg-1 Zn, >6.5 mg kg-1 Cd, and >20 mg kg-1 Pb. Based on these suggested values, DTPA-extractable Pb was high in nonstockpiled topsoil samples (60.7–98.4 mg kg-1 Pb), but within normal limits for all other samples (0.2–4.6 mg kg-1 Pb), and DTPA-extractable Zn and Cd were within normal limits in all soil–subsoil samples (3.9–54.0 and 0.1–0.5 mg kg-1, respectively).
Plant Tissue Analysis
In 1999, orchardgrass, slender wheatgrass, and western yarrow were sampled from each test plot to evaluate uptake of trace elements (Mn, Cu, Zn, Cd, and Pb). Tissue concentrations of Mn in orchardgrass (from TRT-MN, -NS, and -NSC) and yarrow (from TRT-SM, -NM, -SB, -NS, and -NSC) exceeded toxicity thresholds established for agronomic species (Kabata-Pendias and Pendias, 1984; Table 6). Conversely, concentrations of Cu, Zn, Cd, and Pb were below levels considered phytotoxic; again based on agronomic standards. Additionally, monitoring of native reference areas near the study site showed similar plant uptake of these elements (Richard and Redente, 1996; Redente and Richard, 1998). Regardless of treatment or metal, yarrow constantly showed the highest levels of elemental uptake among the species sampled. It is typical for dicotyledonous plants to absorb trace elements at higher rates than monocots (Sauerbeck, 1991).
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Overall, uptake of Mn, Cu, Cd, Zn, and Pb did not appear to be problematic on the test plots as tissue concentrations of these elements did not show distinct patterns corresponding to those seen for aboveground biomass (Fig. 2). Although plant uptake of Mn was high compared with current standards, no visual signs of Mn toxicity, such as chlorotic spots and necrosis of young leaves and reduced turgor (Kabata-Pendias and Pendias, 1984), were noted. Purple leaf discoloration was noted, particularly early and late within the growing season, in some species growing on the test plots. This was attributed to accumulation of anthocyanins in plant tissues, rather than a deficiency of phosphorus or an excess of trace elements. Bright light and low temperatures, characteristic of high elevations, can result in low tissue chlorophyll content and a buildup of anthocyanins in leaf and young stem tissue (Billings, 1974).
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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1) Incorporation of OM into waste rock promotes plant growth. Treatments that incorporated OM supported more aboveground biomass than treatments that did not incorporate OM. Furthermore, the mushroom compost used was found to be more effective as an organic amendment than the biosolids (possibly due to its increased nutrient content), and therefore facilitated plant growth to a greater extent. These results are specific to the materials used, and will vary depending on a given material's composition, processing, application, etc. Therefore, analysis should be done on all organic materials prior to use to determine their potential effectiveness as amendments.
2) Properly amended, stockpiled topsoil is suitable for use in site-wide reclamation. Stockpiled topsoil that was limed supported aboveground biomass comparable with that supported by nonstockpiled topsoil overlying amended waste rock. The stockpiled topsoil used in this study had low biological activity due to the manner in which it was stored. Further research should be conducted to determine how differences in microbial activity of stockpiled and nonstockpiled materials affect reclamation at such sites.
3) Amended waste rock can directly support vegetation. Waste rock material properly limed and amended with OM supported aboveground biomass comparable with that produced when either nonstockpiled or limed stockpiled topsoils were applied over amended waste rock. This may be useful when sufficient quantities of topsoil are not available; however, state and federal regulations often dictate the use of topsoil for mined-land reclamation.
4) Use of a capillary barrier is equally as effective as a layer of limed waste rock. No difference was noted in soil pH, or soil and plant tissue trace element content relative to the placement of the capillary barrier or limed layer of waste rock between overlying topsoil and underlying unamended waste rock. Long-term monitoring of these study plots is necessary to better quantify treatment effects.
5) The use of ProMac has not significantly improved plant growth. Treatments that incorporated ProMac supported comparable aboveground biomass, yet showed decreased root growth, compared with similar treatments that did not incorporate ProMac. More intensive sampling of belowground biomass is necessary to better quantify treatment effects.
6) The low pH of unlimed soil and waste materials affects root growth. Root growth was diminished or nonexistent in the unamended waste rock and unlimed stockpiled topsoil layers observed in all test plots.
7) Plant trace element uptake is not problematic on the test plots. Plant tissue concentrations of Cu, Cd, Zn, and Pb were below levels considered phytotoxic. Tissue concentrations of Mn from some samples were elevated, but no visual signs of Mn toxicity were noted. Additionally, trace element tissue concentrations did not show distinct patterns correlating with those noted for aboveground biomass. Due to the deficiency of information regarding phytotoxicity of trace elements to nonagronomic species, toxicity values established for agronomic crops have been used. Yet, agronomic species typically have lower toxicity thresholds than the perennial species used in mine land reclamation (Chaney, 1983; Paschke et al., 2000), and therefore, comparisons with current standards are of marginal use. More research is needed to establish functional toxicity thresholds for reclamation species.
8) Total trace element content of soil and waste materials is not problematic on the test plots. Total concentrations of Mn were within levels considered normal. Concentrations of Cu, Zn, Cd, and Pb were elevated in some samples, but no visual signs of plant toxicity were noted. The potential effects of elevated soil trace elements on the growth of nonagronomic species have not been sufficiently characterized. Phytotoxicity levels for soil trace elements are difficult to establish due to the variability of toxicity thresholds in native plant species, chemical and physical soil properties, potential element interactions, and numerous environmental factors. More research is needed to develop methods for accurately analyzing trace element bioavailability in acidic mine soils and determining toxicity limits.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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