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a Department of Crop and Soil Environmental Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24060
b Conservation Specialist Peaks of Otter Soil and Water Conservation District, Bedford, VA 24523
c Department of Agronomy, The Ohio State University, Columbus, OH 43210
d Virginia Cooperative Extension, Powell River Project, Wise, VA 24293
* Corresponding author (gevanylo{at}vt.edu)
Received for publication August 25, 2004.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Monocultures or mixtures of perennial grasses and legumes can provide cover required for control of erosion, soil development, wildlife habitat, hay and pasture production, and biomass energy sources (Ditsch and Collins, 2000). Herbaceous species that are employed in pasture or hayland can be appropriate for restoration of Appalachian surface mined soils that are not reforested because livestock production is the major nonforestry agricultural enterprise in this region.
Cool season grasses usually establish quickly and develop extensive root systems that improve soil aggregation and reduce erosion and runoff potential. Heat and drought tolerance of cool season grasses range from excellent for endophyte-infected tall fescue to good for orchardgrass and reed canarygrass (Ditsch and Collins, 2000). The adaptation to wet soil conditions that often occur in reconstructed mine soils due to compaction-induced drainage limitations (Haering et al., 2004) is excellent for reed canarygrass, good for tall fescue, and poor for orchardgrass. All are adapted to a wide pH range (i.e., 5.0–8.0) and have excellent winter hardiness. Warm season grasses, such as switchgrass and caucasian bluestem, establish slower, and can exploit greater rooting depths and volumes than cool season grasses, which can be an advantage during periods of drought and under the low nutrient (especially P) availability common on surface mined soils (Jung, 1986). Both species also exhibit excellent winter hardiness.
The capability of legumes to fix nitrogen provides an establishment and productivity advantage of these species over grasses on nitrogen-limited soils. Furthermore, the potentially deep (>3 m in the absence of physical obstructions or unfavorable chemical conditions) taproot of many legumes (e.g., alfalfa, crown vetch, birdsfoot trefoil) enable these species to survive drought if the subsoil is sufficiently loose to allow root penetration. Legumes vary greatly in their tolerance of wet soil conditions (e.g., good for birdsfoot trefoil and sericea lespedeza, poor for alfalfa and crownvetch) and winter hardiness (excellent for alfalfa, good for birdsfoot trefoil, fair for crown vetch, and poor for sericea lespedeza) (Ditsch and Collins, 2000).
The addition of a legume to a grass will provide a mixture whose need for external N will be reduced, but all legumes are not equally proficient in supplying N to a companion grass. Powell et al. (1982) produced greater biomass yields with grass–legume mixtures than monocultures of the same species. Jefferies et al. (1981) determined that alfalfa was less effective as an N source for an associated grass than white clover. Unfortunately, legumes have high demand for P (Mengel and Kirkby, 1979), which is often limiting in surface mined soils constructed from overburden rock materials.
Soil organic matter is one of the most important attributes of soil quality, which has been defined as "...the capacity of a soil to promote the growth of plants, protect watersheds by regulating the infiltration and partitioning of precipitation, and prevent water and air pollution by buffering potential pollutants such as agricultural chemicals, organic wastes, and industrial chemicals" (National Research Council, 1993). The amount of organic carbon present in a soil affects soil attributes such as cation exchange capacity (CEC), pH buffering capacity, nutrient availability, leaching potential, water retention characteristics, compaction, porosity, and aggregate stability (Doran and Parkin, 1996). Revegetation of surface-mined coal land in Appalachia is most efficiently accomplished by the application of amendments that provide a steady supply of nutrients and the restoration of soil organic matter to levels that promote sound chemical, physical, and biological functions (Haering et al., 2000).
The chemical, physical, and biological properties of surface mined soils derived from overburden rock materials may be restored with high application rates of organic residuals, such as composted and uncomposted biosolids, manures, and other municipal and industrial by-products (Haering et al., 2000; Daniels et al., 2001, 2002). Biosolids meet the criteria for successful mined land amendments because they contain relatively high concentrations of N, P, and organic matter. The typical concentrations of potentially toxic heavy metals (e.g., Cd, Cu, Ni, Pb, and Zn) that occur in currently generated biosolids have been deemed low enough as to pose negligible risk to the environment and to the crops produced on soils amended with such biosolids (USEPA, 1993). The metal uptake rate by plants grown in biosolids-amended soils typically declines as the biosolids application rate increases. This occurs despite the increase in the metal content of soil, because the sorption capacity of biosolids-amended soil exceeds that of unamended soil (Corey et al., 1987; Chaney and Ryan, 1993). Chaney and Ryan (1992) concluded that the specific metal adsorption capacity added with biosolids will persist as long as trace metals of concern persist in the soil.
Despite the demonstrated benefits of biosolids in ameliorating metal-contaminated soils, critics of land application warn of potential degradation of soil quality and plant productivity where biosolids are applied according to current Federal and state regulations (McBride, 1995). McBride (1995) has proposed that the availability of potentially toxic trace elements may increase and result in phytotoxicity or excess accumulation of metals as the metal-binding attributes of biosolids organic matter decline years after application ceases. Reclaimed mine land that receives a single large application of biosolids offers an opportunity to study the change in metal availability with time.
The primary objective of this experiment was to assess the survivability and biomass productivity of individual plant species and mixes of species as long as 12 yr after establishment on a biosolids-amended Appalachian coal mine soil. A secondary objective was to assess the effects of the various revegetation treatments on long-term soil properties and phyto-accumulation of heavy metals from the biosolids-amended soil.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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A 50:50 mixture (by volume) of composted biosolids + woodchips and dewatered, anaerobically digested biosolids (termed "mine mix") produced by the Philadelphia Wastewater Treatment Authority's Biosolids Recycling Center was surface-applied to the entire experimental site and incorporated with a chisel plow. Before application, the blend was stacked in windrows and, while not actively aerated, partially composted as evidenced by the release of steam when the piles were disturbed. The mine mix was prepared during winter and early spring and allowed to "cure" for several months before delivery and application to the site in summer 1989. The mine mix was applied at a rate of 368 Mg ha–1 (dry weight) (Haering et al., 2000). The composition of the mix and the loading rate of the mix constituents are presented in Table 1.
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Plant Tissue Processing and Analysis
Plant tissue sampled and dried for biomass determination in 2002 was further processed for chemical analysis by grinding in a stainless steel Wiley mill to pass a 0.5-mm sieve and stored in paper bags, which were placed in an oven at 65°C until constant mass was achieved to remove moisture added during grinding and handling of the samples.
Plant tissue was digested in an Ethos Plus 800 Microwave Labstation (Milestone Microwave Lab Systems, Bergamo, Italy) using a nitric acid microwave method (USEPA, 1996b). Reagent blanks and NIST standard samples (1573a) were routinely included in the analysis. Samples were analyzed for Cd, Cu, Ni, and Zn using a Thermo (Fitchburg, MA) Jarrell Ash ICAP (inductively coupled argon plasma–atomic emission simultaneous spectrometer; ICP–AES).
Soil Sampling, Processing, and Analysis
The reconstructed, biosolids-amended soil was sampled as described below during the study duration to assess the effects of time and vegetation type on soil properties. Twenty cores (2.5 cm in diameter each) were collected to a depth of 15 cm from each of the four replications before planting in June 1990 for establishing baseline chemical properties. Ten soil cores (2.5 cm in diameter x 15 cm in depth) were collected from each treatment plot in fall 1996, 1997, and 2002 for analysis. Soil samples were air-dried and passed through a 2-mm sieve in preparation for chemical extraction or digestion.
Routine soil test laboratory analyses of pH and Mehlich I (0.05 M HCl and 0.0125 H2SO4)-extractable P, K, Ca, Mg, Mn, and Zn as determined by the Virginia Cooperative Extension Soil Test Laboratory procedures (Donohue, 1992) were performed on all samples at each of the four sampling times. Other analyses conducted in 1996 included total C by dry combustion and organic matter by Walkley–Black wet oxidation (Nelson and Sommers, 1982), total Kjeldahl N (TKN) by the method of Bremner and Mulvaney (1982), total P following digestion in perchloric acid (Olsen and Sommers, 1982), cation exchange capacity by saturation and replacement of unbuffered 1 M BaCl2 with 1 M KCl as described by Rhoades (1982), and available water at 33 kPa using a pressure plate (Klute, 1986).
The EPA-3050B (USEPA, 1996a) method, a strong acid digestion procedure recommended for the extraction of total metals (except those bound within crystalline structure), was employed to determine the total concentration of soil metals in the 2002 samples. Metal concentrations were determined using ICP–AES.
Statistical Analysis
Statistical analyses were performed on plant biomass, percent groundcover, and tissue metal concentration using analysis of variance (ANOVA) and least significant differences (LSD, 0.05 level of probability) to separate means with SAS (SAS Institute, 1990).
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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After 5 yr of revegetation, treatments had no effect on soil TKN (mean = 5.0 mg kg–1), total C (mean = 86 mg kg–1), organic matter (mean = 117 mg kg–1), cation exchange capacity (mean = 25.9 cmolc kg–1), and water holding capacity (mean = 195 g kg–1). The chemical properties that resulted from the application of the biosolids were greatly improved compared to adjacent reconstructed soils that had not received the biosolids' amendment.
The main soil limitation to plant establishment and growth appeared to be the physical constraints posed by a shallow (<25 cm) reconstructed topsoil underlain by a compacted zone of large impervious boulders and newly developing soil. We did not excavate soil pits to determine soil development, but penetration with a hand-operated soil corer to a depth greater than 25 cm was not possible. Previous investigations conducted on adjacent reconstructed mined lands, including both operational reclamation and experimental sites, revealed that horizons form within a few years after mining and reclamation. Weak A horizons can form within 2 yr of soil construction (Roberts et al., 1988), and well-developed A horizons can form within 4 yr (Roberts et al., 1988). The time required for formation of B horizons has been quite variable, ranging in time of development from as recent as 4 yr to greater than 20 yr (Haering et al., 2004). Examination of soil pits in adjacent mined lands reclaimed by similar methods during the same time period revealed that subsurface compaction similar to that present on our site limited any significant plant rooting to above the compacted zone (Haering et al., 2004).
Vegetation
Biomass
Within 1 yr of planting, ground coverage of the reclamation species ranged from less than 20% for reed canarygrass, switchgrass, and Caucasian bluestem to between 80 and 100% for ladino clover, birdsfoot trefoil, and alfalfa (Evanylo and Wolf, 1991). Five years after the establishment of vegetation (1996), alfalfa, ladino clover, and birdsfoot trefoil had nearly disappeared, and both the target biomass (i.e., the yield of the planted species; Table 5) and the bioefficiency (i.e., the yield of the target biomass divided by the total biomass, or the sum yield of all plant species; Fig. 1)
of these species were much lower than for all other species. The slowly establishing species (reed canarygrass, switchgrass, Caucasian bluestem, and crown vetch) performed better than the early adaptors (i.e., ladino clover, birdsfoot trefoil, alfalfa) by 1996. The order of biomass production among the monocultures after 5 yr was: switchgrass > sericea lespedeza > AULotan > reed canarygrass > tall fescue > Caucasian bluestem > crownvetch > orchardgrass. The order of biomass production among the mixed species at the same time was: switchgrass + AULotan > tall fescue + alfalfa > orchardgrass + birdsfoot trefoil > tall fescue + ladino clover > reforestation mix.
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Overall biomass production was lower in 1997 than in 1996 due likely to less favorable climatic conditions in 1997. Alfalfa, ladino clover, and birdsfoot trefoil disappeared from all plots (Table 5). Among the legume species, common sericea lespedeza was the dominant vegetation, while switchgrass was the dominant grass. Switchgrass continued to produce high biomass even under the poorer growing conditions in 1997 that reduced the yields of other reclamation species. Unlike 1996, no significant difference in 1997 target biomass production occurred between the pure stand of switchgrass and switchgrass in combination with AULotan (Table 5).
The biomass produced by species planted both in monoculture and in mixes was not affected by treatment combination in 1996 or 1997 (Table 6). Tall fescue, birdsfoot trefoil, alfalfa, orchardgrass, and switchgrass produced statistically identical yields whether grown alone or in combination with other species. AULotan biomass was greater in the AULotan treatment than in the switchgrass–AULotan treatment in 1996, but the differences disappeared by 1997. Combining several species in a mix can be a valuable reclamation practice if the species are compatible (i.e., if one species does not dominate).
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By 2002, nearly all of the surviving species had become well-established and were the dominant vegetation in the treatment (Table 5). Switchgrass, followed by reed canarygrass and tall fescue, produced the most biomass of the grasses. Orchardgrass was the only surviving grass species that continued to decline. Caucasian bluestem had largely disappeared. Among the legumes, the lespedezas (common sericea lespedeza and AULotan lespedeza), followed by crownvetch, continued to yield well.
Persistence
Long-term persistence and production of a plant species on mined land soils is a function of soil physical and chemical properties as well as the individual plant morphological and physiological characteristics that help the plant to withstand biotic and abiotic stresses. In addition to soil properties and environmental conditions, grazing and/or cutting management can be either detrimental or beneficial, depending on cutting frequencies and heights. Among the cool season grasses planted over 10 yr ago, tall fescue and reed canarygrass persisted better than orchardgrass (Table 5). The persistence of the two cool season grasses can be attributed to their low fertility requirements and their morphological characteristics. Both tall fescue and reed canarygrass spread by short rhizomes (underground stems), whose multiple growth initiation sites permit more rapid plant spreading. By contrast, orchardgrass is a bunch grass without rhizomes. Individual orchardgrass plants do not attain the density of those grasses that propagate by rhizomes and vigorous tillering; thus, orchardgrass stands typically thin and disappear 3 to 5 yr after establishment regardless of management.
Among the two perennial warm-season grasses, switchgrass was more persistent and out-yielded Caucasian bluestem years after establishment. Switchgrass, a native bunch grass that produces a short rhizome, can survive over 20 yr while tolerating annual cutting unlike many grasses that require frequent defoliation for tiller production. In addition, switchgrass is drought tolerant and grows well with minimal fertilization. Caucasian bluestem, also a bunch grass, is well adapted to most soil types and tolerant of acidic, infertile soils. The lack of persistence of caucasian bluestem on the reclaimed mined site may be attributed to management. Caucasian bluestem is often managed by burning old growth from the previous year or mowing the dead residue as short as possible in February or March. These practices increase tiller production and control disease and weed pressures (D. Wolf, personal communication). Our annual fall mowing provided less than optimal regrowth conditions for caucasian bluestem.
Tall fescue, a cool season perennial grass, was also persistent on the reclaimed soil. Tall fescue can survive on soil whose pH ranges from 4.7 to 9.5, but production on soils with native pH of 5.3 to 5.5 is typical (Burns and Chamblee, 1979). The grass' extensive root system allows it to sustain on droughty and poorly drained soils. The adaptability of tall fescue to a wide range of climatic and soil conditions has resulted in its cultivation on nearly 1 million ha in Virginia and 14 million ha throughout the United States (Ball et al., 2003).
Among the legumes, sericea lespedeza (both common and AULotan) and crown vetch were the most persistent, while alfalfa, birdsfoot trefoil, and ladino clover disappeared from the site. The persistence of sericea lespedeza can be attributed to its ability to grow well on marginal soils that are not suitable for many forage species. Sericea has a long taproot, which is capable of penetrating deep into the overburden to extract soil water that is inaccessible to more shallow-rooted species (Dove et al., 1991). The lack of close and frequent cutting allowed the sericea lespedezas to thrive throughout the study duration. The other persistent legume, crownvetch, is a low growing, long-lived perennial, that spreads via creeping underground roots. The species can withstand dry and infertile soils but is best adapted to well drained, fertile soils with a pH 6 or above (Barnes et al., 1995, p. 273–275).
Fertility and cutting management is very important for the maintenance and persistence of alfalfa. The disappearance of alfalfa from the experimental site soon after establishment can be attributed to the poor soil conditions (i.e., alternating drought and wetness) and less than optimum management (e.g., harvesting time and frequency). Maximum alfalfa persistence (for at least the first 6 yr of production) requires enough time after the final fall cutting to allow 30 to 45 cm of regrowth before the first killing frost (Chamblee et al., 1995). In most years, alfalfa was cut just before the first frost date. Birdsfoot trefoil can grow successfully on marginal lands; however, its persistence is highly dependent on its capability to reseeding. Trefoil must be allowed to reseed itself every 2 or 3 yr by accumulating growth during the spring until seed set to ensure a persistent stand. In general, the herbaceous species that survive environmental stress and less than optimal management are those with specialized organs such as stolons, rhizomes, or crowns that store large quantities of sugars for regrowth and possess sites for initiation of new growth.
Of the five most persistent species, sericea lespedeza and tall fescue dominate many abandoned hayland–pasture lands on coal surface mines. Sericea lespedeza is a dominant species, despite no longer being planted on such sites, because of the morphological attributes that contribute to its persistence and the manner in which its seeds are spread (i.e., by birds and other natural agents).
Weed Encroachment
Weed encroachment was insignificant for the duration of the study in the treatments where the originally planted species remained dominant. Tall fescue, common sericea lespedeza, switchgrass, crown vetch and, to a lesser extent, reed canarygrass were dominant to abundant, and weed species were rare. Less aggressive species (e.g., Caucasian bluestem) were invaded by weedy species such as devils beggarticks (Bidens frondosa L.), goldenrod (Solidago spp.), wild raspberries (Rubus idaeus L.), dewberries (Rubus caesius L.), blackberries (Rubus spp.), and other annual and perennial forbs.
Metal Concentration
The forage species exhibited differential uptake of the heavy metals that could affect the recommended use of forages planted on biosolids-remediated land (Table 7). Crown vetch accumulated the highest concentrations of Cd and Zn, AULotan sericea lespedeza accumulated the highest concentration of Ni, and switchgrass accumulated the highest concentration of Cu. While other differences existed, there were no obvious uptake patterns for the four metals among the various species.
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The metals Cu, Ni, and Zn are readily taken up by plants but pose phytotoxicity risks before food chain risks. This has been termed the "soil–plant barrier" by Chaney (1980). Although the plants accumulated trace elements differentially, the concentrations in the forages did not exceed thresholds for concern (i.e., >40 mg Cu kg–1, >50 mg Ni kg–1, and >400 mg Zn kg–1 as documented by Chaney, 1994).
Cadmium poses human or animal health risks at plant concentrations that are not generally phytotoxic; thus, Cd could be a concern due to bioaccumulation through the soil–plant–animal food chain. Even the highest concentration of Cd in this study (1.0 mg Cd kg–1 in crown vetch) was lower than established conservative levels of concern (Kabata-Pendias and Pendias, 1984).
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Among the grasses initially established, switchgrass grown in pure stand as well as in mixture with legumes outperformed the other grasses and grass mixtures for most of the experimental years. Switchgrass is well adapted to the moisture stress conditions that predominate shallow mineland soils. Reed canarygrass, likely due to its tolerance to both excessive moisture and drought, and tall fescue also produced well.
The establishment of a pool of nutrients and organic matter through the application of a higher than agronomic loading rate provided a root environment whose chemical properties were favorable for establishment and maintenance of most forages grown in the Appalachian Plateau region; however, long-term survival and productivity of the forages were dependent on the ability of the plants to root effectively in the physically limited medium and to compete with other species until the soil profile became better developed and on the type of management practice (i.e., mowing frequency) employed. The use of a high rate of biosolids containing considerably greater concentrations of heavy metals than the background soil did not reduce plant vigor nor increase metal uptake by the forages to concentrations of concern. Tall fescue and sericea lespedeza often emerge as dominant species in abandoned hayland–pasture areas on Appalachian coal surface mines today, even when initial seeding includes multiple grass and legume species. Our results indicate that these species retain their persistent attributes on Appalachian mine soils under the higher-nutrient conditions created by a heavy biosolids application.
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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  R. K. Shrestha and R. Lal Soil Carbon and Nitrogen in 28-Year-Old Land Uses in Reclaimed Coal Mine Soils of Ohio J. Environ. Qual., October 24, 2007; 36(6): 1775 - 1783. [Abstract] [Full Text] [PDF]
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