Published in J. Environ. Qual. 33:1055-1061 (2004).
© ASA, CSSA, SSSA
677 S. Segoe Rd., Madison, WI 53711 USA
TECHNICAL REPORTS
Waste Management
Hybrid Poplar and Forest Soil Response to Municipal and Industrial By-Products
A Greenhouse Study
Molly A. Cavaleria,
Daniel W. Gilmore*,b,
Morteza Mozaffaric,
Carl J. Rosend and
Thomas R. Halbachd
a Department of Forest, Rangeland, and Watershed Stewardship, Colorado State University, Fort Collins, CO 80523
b Department of Forest Resources, University of Minnesota, North Central Research and Outreach Center, 1861 Highway 169 East, Grand Rapids, MN 55744-3396
c Department of Crop, Soil, and Environmental Sciences, University of Arkansas, Marianna, AR 72360
d Department of Soil, Water, and Climate, University of Minnesota, 439 Borlaug Hall, 1991 Upper Buford Circle, St. Paul, MN 55108
* Corresponding author (dgilmore{at}umn.edu).
Received for publication December 16, 2002.
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ABSTRACT
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Little research has been conducted in the Lake States (Minnesota, Wisconsin, and Michigan) to evaluate the effects of municipal and industrial by-product applications on the early growth of short rotation woody crops such as hybrid poplar. Anticipated shortages of harvestable-age aspen in the next decade can be alleviated and rural development can be enhanced through the application of by-products to forest soils. This study was conducted to evaluate the effects of inorganic fertilizer, boiler ash, biosolids, and the co-application of ash and biosolids application on tree growth and soil properties by measuring hybrid poplar clone NM-6 (Populus nigra L. x P. maximowiczii A. Henry) yield, nutrient uptake, and select post-harvest soil properties after 15 wk of greenhouse growth. Treatments included a control of no amendment; agricultural lime; inorganic N, P, and K; three types of boiler ash; biosolids application rates equivalent to 70, 140, 210, and 280 kg available N ha–1; and boiler ash co-applied with biosolids. All of the by-products treatments showed biomass production that was equal to or greater than inorganic fertilizer and lime treatments. A trend of increased biomass with increasing rates of biosolids was observed. Soil P concentration increased with increasing rates of biosolids application. None of the by-products treatments resulted in plant tissue metal concentrations greater than metal concentrations of plant tissue amended with inorganic amendments. Biosolids, boiler ash, and the co-application of biosolids and boiler ash together on forest soils were as beneficial to plant growth as inorganic fertilizers.
Abbreviations: ENP, effective neutralizing power
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INTRODUCTION
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RESEARCH ON SITE ENHANCEMENT of forest soil for fast-growing hybrid poplar has intensified due to a combination of factors including projected fiber shortages of the aspen resource in the Lake States (David et al., 2001), research illustrating that tree growth can be enhanced through intensive silviculture (e.g., Gilmore and David, 2002), and the availability of municipal and industrial by-products as site ameliorating material (Matysik et al., 2001; Cavaleri et al., 2002). Sustainable hybrid poplar tree farms can supplement native aspen fiber stocks and contribute to maintaining the forest industry as a part of the rural economy in the Lake States. By-products have been used for many decades to improve yields of agricultural crops and recycle nutrients that would otherwise end up in landfills or be incinerated (Sommers, 1977; Etiegni et al., 1991; Linden et al., 1995; Cabral et al., 1998). It has only been in the past couple of decades, however, that these by-products have been used as forest soil amendments (Matysik et al., 2001).
Biosolids, the by-products of residential and industrial wastewater treatment facilities, are land-applied based on the amount of potentially available N (Linden et al., 1995). Boiler ash is produced from the incineration of wood, coal, or paper mill waste. The material incinerated determines, to a large extent, the chemical composition of this residual product. Ash is land-applied based on the effective neutralizing power (ENP) of the material. Ash increases soil pH and has been found to be beneficial to tree growth on acidic soils (Weber et al., 1985; Pritchett and Fisher, 1987; Silfverberg and Hotanen, 1989; Etiegni et al., 1991).
A 15-wk greenhouse study was conducted to evaluate the effects of biosolids, boiler ash, inorganic fertilizers, lime, and various co-applications of these soil amendments on hybrid poplar growth and soil properties. The objectives of this study were to (i) determine the most limiting supplement based on a nutrient exclusion study with N, P, K, and lime; (ii) evaluate the use of ash alone as a liming agent; (iii) characterize hybrid poplar response to biosolids based on N content; and (iv) evaluate the effects of co-application of biosolids and ash on hybrid poplar growth.
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MATERIALS AND METHODS
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Plant Materials and Maintenance
We selected the hybrid poplar clone NM-6, which was cultivated at the University of Minnesota Natural Resources Research Institute (NRRI) in Duluth and propagated at the University of Minnesota North Central Research and Outreach Center in Grand Rapids for inclusion in the study. NM-6 is an industry standard used by the NRRI for comparisons in their hybrid poplar research program. Based on visual assessment, uniform 15-cm cuttings were selected from 1-yr-old sprouts and planted one per pot. All new shoots except two were removed 1 wk after planting, and thereafter any additional new shoots were removed as they appeared. Greenhouse temperatures were maintained at 29°C during the day and 24°C at night. Sodium and metal halide lights provided supplemental light for 12 h each day. Greenhouse lights dried the soil quickly and pots were watered daily to maintain moisture at approximate field capacity. A 20-cm-diameter saucer was placed under each pot. Liquid collected in the saucers was poured back onto the soil surface to minimize nutrient losses from leaching.
Soil
Mineral soil was collected from the B and upper BC soil horizons to a depth of 18 cm from a mixed-species forest clear-cut in 1999 (Cavaleri et al., 2002). Site preparation treatments had eliminated the A and O soil horizons. The soil, representative of the Cloquet fine sandy loam (coarse-loamy over sandy or sandy-skeletal, mixed, superactive, frigid Typic Dystrudepts) soil series (Lewis, 1978) was collected from the Cloquet Forestry Center located in Carlton County, Minnesota (46°42' N, 92°31' W). Soil was mixed in an industrial soil mixer to ensure homogeneity before amendment application and characterized by the University of Minnesota Department of Soil, Water, and Climate Research Analytical Laboratory (Table 1). Twenty 22.7-L buckets, one bucket per treatment, were then each filled with 7.62 kg dry soil. Amendments were pre-weighed and incorporated into the soil by hand. The amended soil from each bucket was portioned into three 15-cm-diameter pots (one pot for each replication). Each pot had a volume of 2240 cm3, not including the 2.5-cm watering space left at the top.
Experimental treatments (Table 2) included a control of no amendment; six inorganic fertilizer treatments including various combinations of N, P, K, and lime; three boiler ash treatments from different sources plus P and N; seven biosolids treatments, five of which contained an addition of P; and three co-application treatments in which the above three types of ash had additions of P and biosolids at a single rate. Treatments included four different rates of biosolids application based on available N with P and lime, and three treatments of biosolids that lacked P or lime or both. A randomized block design with three replications of all 20 treatments was used.
Inorganic Fertilizers and Lime Amendments
Ammonium nitrate (NH4NO3) was applied at a base rate (1.0X) of 140 kg available N ha–1 (0.0707 g available N kg–1 soil) to supply inorganic nitrogen in all treatments without biosolids application (Treatments 2–6 and 8–10), with the exception of the N controls (Treatments 1 and 7) (Table 2). To minimize N leaching, one-third of the total ammonium nitrate application rate was incorporated dry into the soil before planting, one-third was applied dissolved in solution at Week 5, and the last one-third was applied in solution at Week 10.
Our experience with potted studies in a greenhouse environment has shown phosphorus to be limiting in nearly all instances. Thus, a yield response could be observed from P application. To eliminate P as a limiting factor and to more directly compare inorganic to organic sources of N, calcium phosphate monobasic [Ca(H2PO4)2] was added to all treatments except 1, 6, 11, and 17 at a rate equivalent to 39 kg P ha–1 (0.020 g P kg–1 soil), very near the 40 kg P ha–1 rate recommended for poplar (Pritchett and Fisher, 1987), to assure that a lack of phosphorus did not limit plant growth.
Liming materials were analyzed and distinguished from one another on the basis of ENP. The ENP is calculated from the calcium carbonate equivalent and particle size (Rosen and Eliason, 1996). Agricultural lime with an ENP of 87.1% was applied at a liming rate required to raise the soil pH to 6.5 in Treatments 4 through 7 and 13 through 17 (Table 2).
Potassium is the most mobile soil nutrient and is seldom a limitation to tree growth (Fisher and Binkley, 2000). Therefore, one-half the rate of the ash treatment (Treatment 8) containing the lowest K concentration was used to determine the application rate for inorganic K. Minnesota Power ash was applied at a rate of approximately 168 kg K ha–1 (0.095 g K kg–1 soil), so inorganic KCl was applied at half that rate of K, or 84 kg K ha–1 (0.047 g K kg–1 soil) in Treatments 3, 4, 6, and 7.
By-Products Amendments
Ash was obtained from the Minnesota Power Company in Duluth (Treatments 8 and 18), Potlatch Wood Products in Cloquet (Treatments 9 and 19), and Potlatch Paper Company in Cook (Treatments 10 and 20). Ash from Minnesota Power (ENP = 21.8) is derived from coal, wood, and railroad tie chips. Potlatch Cloquet ash (ENP = 36.8) is derived from bark, sawdust, nonrecyclable paper, cardboard, rejected knots from the pulping process, coal, and primary paper mill sludge (approximately 50% organic cellulose fiber and 50% inorganic coating chemicals such as clay, starch, and calcium carbonate). Potlatch Cook ash (ENP = 82.5) is derived from sawdust, wood, and bark. Ash application rates were based on ENP and a liming rate equivalent calculated to raise soil pH to 6.5. Liming rates were kept constant but due to differing ENPs the actual mass of ash amendment was different for each ash type.
Anaerobically digested biosolids were acquired from the Grand Chute Menasha Water Treatment Plant in Winnebago County, Wisconsin. Biosolids had a pH of 7.2 and were not lime-stabilized. Typical Ca concentrations in biosolids range from 1.0 to 250 g kg–1 with a median value of 2.7% (Edwards and Someshwar, 2000). On average, biosolids in this study contained 5.4% Ca (Table 3), which is higher than the typical median for anaerobically digested biosolids, but within the low range of typical Ca concentrations in lime-stabilized biosolids. Biosolids and N addition treatments were based on an "X" application rate of 140 kg available N ha–1 (0.0707 g available N kg–1 soil). Fertilizer recommendations for Populus spp. cuttings are few. We used a minimum N fertilization level of 70 kg ha–1, slightly less than the 80 kg ha–1 rate recommended by Pritchett and Fisher (1987). Our base rate of 140 kg ha–1 was selected to keep the maximum rate of 280 kg available N ha–1, or 2.0X, from greatly exceeding the maximum application rate of biosolids N gleaned from the literature (Wells et al., 1986; Riddell-Black, 1998; Matysik et al., 2001).
By-products were characterized by the University of Minnesota Department of Soil, Water, and Climate Research Analytical Laboratory (Table 3). Total elemental content was determined by USEPA Method SW 846-3051 (USEPA, 1992) and available K and P were determined by methods described by AOAC International (Johnson, 1990a, 1990b).
Data Collection
Shoot height and shoot caliper at a distance of 5 cm from the planted cutting were measured for both shoots on each plant at the end of 15 wk of growth. Roots and shoots were removed from the original cuttings and washed. Material from the original cuttings was not included in any analysis. Roots were washed thoroughly in four water baths before drying and analyses. Plant material was dried in a forced air oven at 65°C for 1 wk or until tissue had a constant mass. Oven-dry mass was recorded and plant material was ground to pass through a 1-mm screen. Ground plant tissue was then digested in a microwave by a wet ashing procedure (Miller, 1998) and concentrations of P, K, Ca, Mg, Al, Pb, Cr, and Cd were determined by the University of Minnesota Research Analytical Laboratory using an ARL 3560 inductively coupled plasma–atomic emission spectrometer (ICP–AES; Fisons, Sunland, CA) (Dahlquist and Knoll, 1978). Total N was measured using the semi-micro Kjeldahl digestion followed by continuous flow analysis (Bremner, 1996). Following the removal of the plants, the soil within each pot was thoroughly mixed and subsampled. Concentrations of NH+4 and NO–3 were determined following a 2 M KCl extraction (Carlson et al., 1990). The soil was then dried at 35°C in a forced air dryer for 3 d and ground to pass through a 2-mm screen. Soil pH and electrical conductivity were measured in water (1:1 w/w soil to water) (Thomas, 1996). Exchangeable cations were then extracted with 1 M ammonium acetate and determined by inductively coupled plasma (ICP) (Sumner and Miller, 1996). Extractable P was measured by the Bray method (Kuo, 1996).
Data Analysis
For plant tissue biomass and chemical composition, both shoots from each plant were weighed and analyzed together. All data are presented with treatment means (Tables 4–6). For ease of readership, the treatments are placed in the following experimental groupings: control (Treatment 1), nutrient exclusion (Treatments 2–7), ash as a liming agent (Treatments 8–10), biosolids application rate (Treatments 11–17), and ash and biosolids co-applied (Treatments 18–20) (Table 2).
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Table 4. Effect of fertilizers, lime, and by-products on shoot dry mass, total dry mass, root to shoot dry mass ratio, and volume index after 15 wk of growth. Treatment means are displayed.
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Table 6. Effect of fertilizers, lime, and by-products on soil pH, P, K, and ammonium concentrations after 15 wk of growth. Treatment means are displayed.
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Table 5. Effect of fertilizers, lime, and by-products on N, P, K, Ca, and Mg concentrations in shoot tissue after 15 wk of growth. Treatment means are displayed.
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Results of one-factor analyses of variance (ANOVA) and Fisher's LSD values (P = 0.05) were used to evaluate treatment effects on shoot mass; total mass (shoot + root); stem volume index (Li et al., 1998); the root to shoot dry mass ratio; shoot and root N, P, K, Ca, Mg, Cd, Pb, Al, and Cr concentrations; soil pH; and N, P, and K concentrations. Volume index is calculated as volume index = [(caliper of Shoot 1)2 x height of Shoot 1] + [(caliper of Shoot 2)2 x height of Shoot 2]. Categorical variables were the treatments (Table 2). Normality of data was evaluated with a Lillefors test performed on residual values from the separate ANOVAs.
The biosolids treatments were analyzed with planned orthogonal contrasts to determine if increasing rates of biosolids had linear or quadratic effects on plant growth and yield. These analyses included four different biosolids rates (Treatments 13–16), and a control of no biosolids (Treatment 7). The mean squared error terms from the full ANOVAs performed with all twenty treatments were used in the calculation of P values for the orthogonal contrasts.
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RESULTS AND DISCUSSION
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Nutrient Exclusion Experiment
Applications of inorganic N (NH4NO3) (Treatments 2, 3, 4, 5, and 6) produced a growth response greater than the control as determined by shoot dry mass, shoot and root mass, or volume index. In Treatments 1 and 7, N appeared to be the limiting nutrient. These two treatments that lacked N in any form (Treatments 1 and 7) produced plants with the lowest shoot mass, total mass, and volume indexes the highest root to shoot ratios (Table 4). Proportionally more biomass was allocated to roots of the plants that lacked N. This supports the generally accepted hypothesis that nutrition has an affect on root to shoot ratios. The plants amended with the full complement of inorganic fertilizers and lime (Treatment 4) had some of the lowest root to shoot ratios (Table 4).
There were trends of increasing growth from P and K additions and a decreasing volume index when lime was absent (Table 4). The percentage of N in the shoot, however, was greater in treatments where inorganic N was applied because of the mobility of this nutrient and more frequent applications (Table 5). The concentration of P, K, Ca, and Mg did not differ appreciably from the control or other treatments indicating that the concentrations of these nutrients in the growing medium was probably adequate. As expected, soil pH increased for all treatments that included lime. Concentrations of soil P, K, and ammonium did not differ from the control (Table 6). Concentrations of all measured metals (Cd, Pb, Al, and Cr) for all treatments were well below foliage toxicity levels (Linden et al., 1995) and therefore are not discussed further.
Evaluation of Ash as a Liming Agent
No differences were detected among ash types (Treatments 8, 9, and 10) for shoot dry mass, shoot and root mass, or volume index but these measures did differ from the control (Treatment 1) for all ash types (Table 4). Shoot N concentrations for the Minnesota Power ash treatment (Treatment 8) were greater than those for the other two ash treatments (Treatments 9 and 10) (Table 5). The concentration of N in the plant tissue for all three ash treatments was generally higher then those for plants amended with biosolids or co-application treatments because the ash treatments included three applications of inorganic N. With the exception of soil pH, there were no differences among ash treatments for variables measured in soil solution (Table 6). Although the target soil pH for all ash treatments was 6.5, this was not achieved possibly because of the differing concentrations of other elements in each ash type (Table 3) that are not included in the ENP calculations.
Biosolids Application Rate Study
There was a trend of an increase in shoot dry mass, total dry mass, and volume index with an increase in the amount biosolids applied (Table 4). Using planned orthogonal contrasts, linear (P < 0.001) and quadratic (P = 0.062) effects were detected on shoot dry mass and linear (P < 0.001) and quadratic (P = 0.048) effects were detected on total dry mass with increasing rates of biosolids application. The significant quadratic effect suggests the existence of a biosolids application efficiency threshold for available N that would require additional research to determine.
There was less N detected in the tissue of plant material amended with biosolids than in those amended with inorganic N (Table 5). This is probably due to a slower rate of N mineralization in biosolids. There was a nonsignificant trend of decreasing shoot K concentrations with increasing rates of biosolids (Table 5). The risk of K deficiency on low K sites should be evaluated when biosolids application programs are considered. Withholding P or lime from biosolids treatments did not reduce shoot dry mass, total dry mass, or volume index (Table 4). Soil P concentrations increased over the control in treatments that included biosolids because of the addition of P and a buildup of soil organic matter containing P with increasing rates of biosolids application (Table 6). A higher concentration of ammonium was detected in all biosolids treatments as compared with inorganic fertilizer treatments (Table 6). Soil pH was increased in biosolids treatments that did not include lime (Treatments 11 and 12) relative to the control (Table 6). The effects of biosolids application on forest soil pH are variable and partially dependant on how the biosolids are processed. Biosolids have been shown to increase the pH of low pH forest soils (Brockway, 1983; Hallett et al., 1999). The range of results observed in various studies (Matysik et al., 2001) leads us to suggest that results are highly dependent on the pretreatment condition of both the soil amended and the biosolids added.
Evaluation of the Co-Application of Ash and Biosolids
Plants amended with ash and biosolids co-application treatments (Treatments 18, 19, 20) had shoot dry mass, total dry mass, and volume index increases relative to the control (Treatment 1) but did not differ among themselves (Table 4). Measured growth also did not differ greatly from the nutrient exclusion and biosolids rate components of this study (Table 4). The concentration of N in the shoot tissue for plants amended with co-applied ash and biosolids (Treatments 18, 19, 20) was the same as the biosolids rate application treatments (11 through 17) and lower than the inorganic N treatments (Table 5). Again, this is probably due to a slower rate of N mineralization in biosolids. Phosphorus shoot concentrations did not differ among treatments or the control (Table 5). Concentrations of K, Ca, and Mg in shoot tissue did not differ among co-application treatments and all treatments (Table 5). We are unable to provide an explanation other than to suggest that these nutrients were not limiting in this study and their respective uptakes were probably related to unmeasured variables.
Soil pH following co-application of ash and biosolids increased to levels similar to those in the treatments that included ash or lime. Soil P levels were similar to those for biosolids treatments (Table 6). Interestingly, there was a trend of increasing soil K in the co-application treatments suggesting that co-application may be beneficial in K deficient soils. Ammonium concentrations in the soil from the co-application treatments were similar to those in the soil from the biosolids treatments (Table 6).
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CONCLUSIONS
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All by-products treatments showed increases in biomass production that were equal to or greater than increases achieved through inorganic fertilizers and lime. All three by-products co-application treatments of ash and biosolids showed increases of plant biomass that were equal to or greater than the corresponding treatments of ash with inorganic N or biosolids with lime. Measured metal concentrations in plant tissue in the by-products treatments did not differ from metal concentrations detected in plants amended with inorganic fertilizers. All measured metal concentrations were below plant toxicity levels.
When lime and ash were applied based on their respective ENPs to raise soil pH to 6.5, plant and soil responses were similar. Ash from the sources evaluated could serve as a replacement for agriculture lime when applied to soils from northern Minnesota but field trials should be evaluated before widespread application of this practice. Increased application rates of biosolids based on available N content produced a trend of increased plant growth. Co-application of ash and biosolids did not increase plant growth relative to plant growth amended from inorganic fertilizer or biosolids. However, co-application created a soil environment more favorable to plant growth in the form of higher pH, greater K, and less Al.
Forest soil and by-products composition is highly variable. Before the commencement of a by-products land application program, both soils and the by-products to be added should be thoroughly characterized. Based on our results, further research is warranted to determine threshold application rates of available N to optimize poplar growth. Further research on the ameliorating effects of the co-application of ash and biosolids for additional soil types and plant species is also suggested.
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ACKNOWLEDGMENTS
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Funding was provided by the Departments of Forest Resources and Soil, Water, and Climate, University of Minnesota; the Minnesota Legislature ML 1999, Chap. 231, Sec. 16, Subd. 10(g) as recommended by the Legislative Commission on Minnesota Resources; and the Northeast Sustainable Development Partnership. We thank Matthew McNearney and Emily Jaklitsch for their assistance with laboratory analyses.
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REFERENCES
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ABSTRACT
INTRODUCTION
