Journal of Environmental Quality 30:2188-2194 (2001)
© 2001 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
TECHNICAL REPORT
Waste Management
Seven Years of Biosolids versus Inorganic Nitrogen Applications to Tall Fescue
Craig G. Cogger*,a,
Andy I. Barya,
Steven C. Fransena and
Dan M. Sullivanb
a Washington State Univ. Puyallup Research and Extension Center, 7612 Pioneer Way E., Puyallup, WA 98371-4998
b Dep. of Crop and Soil Science, Oregon State Univ., Corvallis, OR 97331
* Corresponding author (cogger{at}wsu.edu)
Received for publication February 22, 2001.
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ABSTRACT
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Repeated applications of municipal wastewater biosolids is cost effective for biosolids managers, but may lead to undesirable accumulations of nutrients or contaminants. We evaluated the effects of seven years of biosolids applications on tall fescue (Festuca arundinacea Schreb.) production and nutrient availability. We compared two types of Class A biosolids applied to tall fescue on a sandy loam in western Washington. Mean annual biosolids rates of 290, 580, and 870 kg total N ha-1 yr-1 were compared with inorganic N and zero-N controls using a randomized complete block design. We measured yield and N uptake for each forage harvest, plant tissue metals at selected harvests, soil nitrate each fall, diethylenetriaminepentaacetic acid (DTPA)-extractable metals after five years of applications, and soil pH, available P, and organic C after seven years. Forage yields increased with biosolids rate. Apparent nitrogen recovery (ANR) for biosolids averaged 18% in 1993 (Year 1), 35% in 1994, and 46% in 1999. The ANR for inorganic N averaged 62% from 1994–1999. Residual soil nitrate was less than 25 kg ha-1 for all treatments through 1995, but increased beginning in 1996 for the high biosolids rate. Biosolids increased soil organic C levels by 2 to 5 g kg-1 and Bray-1 P levels by 300 to 600 mg kg-1 (0–15 cm depth). Plant tissue Zn increased from 24 to 66 mg kg-1 at the highest application rate. Nearly all of the DTPA-extractable metals remained in the 0- to 8-cm soil depth.
Abbreviations: ANR, apparent nitrogen recovery • DTPA, diethylenetriaminepentaacetic acid
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INTRODUCTION
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REPEATED applications of biosolids to the same site can have economic benefits for biosolids producers, because the costs of locating and permitting new sites are reduced. Repeated applications may be less desirable, however, when considering sustainable biosolids management. To ensure that repeated biosolids applications are managed sustainably, we must consider cumulative effects on the soil and plant environment, and the potential for contamination of ground water and surface water. Issues to address include long-term crop productivity, N management for repeated applications, levels of P and other nutrients, and the accumulation of trace elements.
Nitrogen management is a key concern because N accumulated from previous biosolids applications can have a significant effect on N requirements for subsequent crops (Sullivan et al., 1997; Kiemnec et al., 1987). Accumulation of excess P is a water quality concern on sites receiving repeated applications of animal manure (Sharpley et al., 1994), and biosolids may pose P concerns similar to those for manure (Maguire et al., 2000). Cumulative applications of trace elements from biosolids can increase bioavailable levels in soil (Alloway and Jackson, 1991), and may lead to phytotoxicity (Berti and Jacobs, 1996) or reduced microbial activity (McGrath et al., 1995; Chaudri et al., 1993). Most reports of trace element effects have come from studies using biosolids with much higher contents of trace elements than is common today, although McGrath et al. (1995) noted that microbial effects occurred at lower trace element accumulations than reported for phytotoxic effects. Barbarick et al. (1995) used biosolids with trace element levels more typical of current biosolids. They reported modest increases in the trace element content of grain, and an apparent plateau effect for Cu and Zn when the biosolids were applied repeatedly (five or six crop rotations) to dryland wheat.
Perennial grass forage can be a good choice for repeated applications of biosolids. It can utilize 300 kg ha-1 yr-1 or more of available N with little residual remaining in the soil (Steenvoorden et al., 1986; Whitehead, 1995). Under intensive management, grass forages make efficient use of high rates of biosolids N (Sullivan et al., 1997; Muchovej and Rechcigl, 1998). Other advantages of grass forages for biosolids application include accessible land close to urban areas, a long period of active growth, a potential long season for application on well-drained sites, and interest by farmers and ranchers.
Research with a variety of biosolids types applied to forage grasses across a range of locations has provided estimates of short-term (1 to 3 yr) effects on N availability and forage grass production (e.g., Zebarth et al., 2000; Cogger et al., 1999; O'Riordan et al., 1987; Soon et al., 1978). Lerch et al. (1990a)( b) and Barbarick et al. (1995) used long-term field experiments to assess the agronomic and environmental sustainability of repeated biosolids applications to dryland wheat, but little information is available on the effects of repeated biosolids applications in grass forage cropping systems.
The primary objective of this research was to evaluate the cumulative effects of repeated applications of biosolids on tall fescue production, N availability, and N leaching potential during a 7-yr period. The secondary objective was to evaluate the availability of other nutrients and trace elements during the same period.
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MATERIALS AND METHODS
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Site Description
Field plots were established in 1993 in Puyallup, Washington, located 55 km south of Seattle. The soil is a Puyallup fine sandy loam (coarse-loamy over sandy, mixed, mesic Vitrandic Haploxeroll), a deep, well-drained soil developed in recent alluvium. Annual crops (silage corn [Zea mays L.] alone or corn intercropped with sunflower [Helianthus annus L.]) were grown on the site from 1984 through 1992. Winter triticale (X Triticosecale Witt.) was planted in the fall of 1992, and harvested in April 1993. The site was moldboard-plowed and disced before planting the tall fescue. Inorganic fertilizers supplied nutrients during the annual cropping period, and the site received poultry manure before 1984.
Puyallup has a temperate climate typical of the maritime Pacific Northwest, with cool, wet winters and mild, dry summers. Mean annual temperature is 11°C with a January mean of 4°C and a July mean of 18°C. Mean annual precipitation is 1020 mm, falling mostly as rain between October and May. July and August are the driest months, receiving an average of 20 mm of rainfall each month.
Field Experiment
The experiment included eight treatments: two biosolids, each applied at three rates, ammonium nitrate (34–0–0 NPK) applied at a single rate, and a zero-N control. Treatments were arranged in a randomized complete block design with four replications. Plot size was 1.8 x 6 m.
A thermophilically digested, dewatered biosolids and a mesophilically digested, heat-dried biosolids were compared (Table 1). Both biosolids meet USEPA Exceptional Quality standards for trace element content, pathogen reduction, and vector attraction reduction (USEPA, 1993), and are described in more detail by Cogger et al. (1999). Dewatered biosolids were obtained fresh for each application, while the heat-dried biosolids were collected once and stored for use over the entire study period.
Tall fescue ‘A.U. Triumph’ was planted in May 1993 and the first treatments applied in July 1993. The plots were managed for grass silage production, with summer irrigation applied as needed to maintain near-maximum production. Grass was harvested at the early boot growth stage using a small plot forage harvester. The usual harvest interval during the growing season was 30 to 45 d. We harvested the tall fescue three times between August and early November 1993, and six times between April and October each year from 1994 through 1997 and in 1999. We harvested seven times in 1998 because of an early start to the growing season. At each harvest we cut a 1- x 6-m swath (7 cm above ground level) from the center of each plot. The harvested grass from each plot was weighed wet, and a 500-g subsample was collected and dried at 60°C for determination of dry matter. In the spring of 2000 we determined stand density by measuring plant occupation in 30- x 30-cm counting grids that were divided into 36 squares. We placed the grids at three random locations in each plot and counted the number of squares occupied by tall fescue plants.
Biosolids application rates were chosen to supply an estimated 100, 200, and 300 kg ha-1 plant-available N per year, split into three equal applications. These rates cover a wide range of N application, but remain within the expected linear portion of the N uptake curve for intensively managed forage grasses (Whitehead, 1995). We estimated plant-available N in the biosolids as 30% of the organic N plus 50% of the ammonium N, or approximately 15 kg N Mg-1 dry weight. Based on this estimate, we applied biosolids at rates of 2.2, 4.4, and 6.7 dry Mg biosolids ha-1 per application. The biosolids were broadcast by hand in July, August, and September of 1993, and after each of the first three grass harvests (April, May, and June) each year thereafter. Actual mean annual application rates were 283, 565, and 848 kg total N ha-1 yr-1 for the dewatered biosolids, and 302, 603, and 905 kg total N ha-1 yr-1 for the heat-dried biosolids.
Ammonium nitrate (34–0–0) was applied to the inorganic N treatment multiple times each year at a rate of 67 kg N ha-1 per application. In 1993, the first year of the study, the experiment did not start until midseason, and only three inorganic N applications were made (a total of 202 kg ha-1). From 1994 through 1996, 34–0–0 was applied in March and after each of the first four harvests, for a total of 336 kg N ha-1 yr-1. Beginning in 1997 we added a 34–0–0 application after the fifth harvest to supply N later in the season, increasing the total N rate to 403 kg ha-1 yr-1. The additional application had little effect on apparent N recovery as percentage of N applied.
Sampling and Sample Analysis
We collected six composite subsamples of the dewatered biosolids at each application date. Three subsamples were dried at 60°C for determination of total solids, and three subsamples were acidified to pH 4 to 5 by addition of 1 M H2SO4 and dried at 60°C before N analysis. Acidification to pH 5 has been shown to prevent ammonia loss from manure samples during sample drying (Derikx et al., 1994). In a preliminary experiment, sample preparation by acidification and drying gave N analysis results equivalent to established methods (American Public Health Association, 1992) using fresh biosolids.
Soil samples for basic nutrient analysis (0- to 30-cm depth) were collected across the plots before grass establishment in 1993, and in the high-rate dewatered biosolids and inorganic N treatments only each spring from 1996 to 1999. The preapplication sample had a pH of 5.8, Bray-1 extractable P of 315 mg kg-1, ammonium acetate extractable K of 220 mg kg-1, Mg of 0.83 cmolc kg-1, and Ca of 4.5 cmolc kg-1. These values were all adequate for perennial grass production (Hart et al., 1996). We applied supplemental S (40 kg ha-1) to all treatments annually to avoid S deficiencies in the inorganic N and zero-N treatments, and beginning in 1995 we applied supplemental K (220 kg ha-1) and Mg (20 kg ha-1) to all treatments each year. Lime (CaCO3) was applied to all plots at a rate of 4.5 Mg ha-1 in the fall of 1997.
We collected soil samples for nitrate analysis in October of each year, using a hydraulic hollow core probe with an inside diameter of 4 cm. Three cores were collected and composited for each plot. Cores were sampled in 30-cm depth increments to 120 cm. In 1997, we collected and composited additional cores (six per plot) at 0- to 8-, 8- to 15-, and 15- to 30-cm depths for analysis of extractable trace metals. In 2000, hand probes were used to sample the 0- to 15-cm depth in each plot (10 cores per composite) for analysis of available P, organic C, and pH.
Total N was determined for biosolids, grass, and soil samples using a combustion analyzer (Sweeney, 1989; LECO Instruments, St. Joseph, MI). Biosolids ammonium N was extracted with 2% acetic acid, and was determined by conductimetric analysis (Carlson, 1978). Soil and biosolids nitrate N was extracted with 2 M KCl (Gavlak et al., 1994), and determined by an automated Cd reduction method (American Public Health Association, 1992). Soil test P (Bray-1); soil test K, Mg, and Ca (1 M ammonium acetate at pH 7); soil pH (1:2 soil to water suspension); and organic C (LECO) were determined using standard procedures (Gavlak et al., 1994). Extractable Cd, Cu, Pb, and Zn were determined by atomic absorption spectrophotometry following DTPA extraction (Gavlak et al., 1994).
Tall fescue tissue samples from the dewatered biosolids treatments for the June harvest in 1994, 1996, and 1998 were analyzed for Ca, K, Mg, Cu, Ni, and Zn. Samples were digested using USEPA Method 3050 (USEPA, 1986) and the digestion extracts were analyzed using inductively coupled plasma atomic emission spectroscopy (ICP–AES).
Statistical Analyses and Calculations
Statistics for yield, N uptake, ANR, residual soil nitrate, stand density, nutrients, and metals were computed using analysis of variance (ANOVA) and least-significant difference (LSD) procedures (SAS Institute, 1996). Least-significant differences were compared following a protected (P < 0.05) F test. Data for residual soil nitrate from 1996 through 1999 were transformed to a log10 distribution, because the data had a large positive skew.
Apparent nitrogen recovery (ANR) was calculated as percentage of N applied for each year, using the difference method:
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where A = annual grass N uptake for the treatment of interest (kg ha-1), B = annual grass N uptake for the zero-N treatment (kg ha-1), and C = annual N applied for the treatment of interest (kg ha-1).
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RESULTS AND DISCUSSION
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Tall Fescue Production
Tall fescue yields tended to increase with biosolids rate (Table 2), and maximum yields were typical of peak yields measured for tall fescue under intensive, irrigated management (Yungen et al., 1977). Tall fescue grown with the medium biosolids rate had equivalent yields to the inorganic N (34–0–0) treatment. Yield differences between the medium and high biosolids rates were significant in all years for the heat-dried biosolids, but only in three years for the dewatered biosolids (Table 2).
Stand density declined with time at the high rate of dewatered biosolids, although the yields remained high. In the spring of 2000 tall fescue occupied 81% of the plot area for the high rate of dewatered biosolids, which was significantly lower than the other treatments. Stand density for the other biosolids treatments ranged from 92 to 98%, which was not significantly different from the inorganic N treatment measurement of 96%. The reason for this decline for the dewatered biosolids is not clear.
Nitrogen Recovery
Tall fescue N uptake increased with biosolids application rate for all rates and all years (Table 3). Nitrogen uptake was similar for the heat-dried and dewatered biosolids, and the medium biosolids rate had similar N uptake to the inorganic N treatment.
Apparent nitrogen recovery (ANR) is the fraction of fertilizer N that is recovered in the harvested part of the plant. For cool-season forage grasses fertilized with inorganic N, ANR typically ranges from 50 to 80% (Whitehead, 1995). Much of the unrecovered N is in the unharvested parts of the grass plant, while the remainder becomes part of the soil biomass or is lost to leaching or volatilization. Apparent N recovery from organic N sources, such as biosolids, is less than ANR from synthetic fertilizers, because only a portion of the N from the organic material becomes available for plant uptake during the year of application.
After a rapid increase between 1993 and 1994, ANR from biosolids increased slowly in subsequent years (Table 4). This suggests that with repeated applications, less biosolids N was needed to meet the N demand of the crop.
The large increase in ANR between 1993 and 1994 indicated increased efficiency of N uptake by the grass. In 1993 the grass stand was still becoming established and harvests did not begin until midseason, while in 1994 the grass was well established and was harvested throughout the season. Increases in ANR since 1994 were probably a result of increased N availability to the grass, caused by N mineralized from previous biosolids applications. Apparent N recovery for the medium rate of dewatered biosolids was greater than for the high rate. The difference was significant from 1997 through 1999, indicating that N recovery was less efficient at the high rate (Table 4). Apparent N recovery for the medium and low rates of dewatered biosolids also tended to be greater than for the same rates of heat-dried biosolids (Table 4). This suggests that the dewatered biosolids may supply slightly more plant-available N in the long term.
Residual soil nitrate measured in the fall is a test used to determine if available N exceeded plant uptake (Sullivan, 1994). Residual soil NO-3–N levels were less than 25 kg ha-1 for all treatments through 1995, and there was little effect of biosolids rate on soil NO-3–N (Fig. 1)
. This indicates that the grass was able to take up the N supplied by the biosolids, and available N was not excessive. Beginning in 1996 the residual soil NO-3–N at the high rate (848 kg N ha-1) of dewatered biosolids was significantly higher than for the inorganic N treatment and the lower biosolids rates (Fig. 1). Residual NO-3–N also was higher for the high rate of heat-dried biosolids in 1998 and 1999 (not shown). This is evidence that the cumulative effect of the high biosolids application rates had increased N availability to the point where it exceeded the uptake ability of the grass. The highest rate appears to be suitable in the short term (1 to 3 yr), but would not be sustainable for a longer term of repeated applications.
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Fig. 1. Fall residual soil nitrate in a 1.2-m soil profile for medium (565 kg N ha-1) and high (848 kg N ha-1) rates of dewatered biosolids, compared with inorganic N (34–0–0) and zero-N treatments. Inorganic N treatment was 202 kg ha-1 in 1993, 336 kg ha-1 in 1994–1996, and 403 kg ha-1 in 1997–1999. Means within a year followed by a different letter are significantly different at P < 0.05 by protected LSD.
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Other Nutrients and Trace Elements
Content of Ca, Mg, and K in plant tissue samples from dewatered biosolids and inorganic N treatments indicated adequate levels for tall fescue production (Mayland and Wilkinson, 1996). Calcium and Mg content increased with biosolids rate (Table 5), but there was no effect over years. Biosolids did not affect K levels. Tissue K, averaged over treatments, was 26 µg g-1 in 1994, 24 µg g-1 in 1996, and 28 µg g-1 in 1998. Potassium could have limited tall fescue production without the supplemental K applications made in this study.
After 7 yr, heat-dried biosolids had little effect on surface soil pH (0- to 15-cm depth) compared with the zero-N control, while pH declined slightly with increasing rate of dewatered biosolids (Table 6). Soil pH also declined with the inorganic N treatment. Surface soil organic C was higher by 2 to 5 g C kg-1 soil in the biosolids treatments compared with the inorganic N and zero-N treatments (Table 6). Surface soil Bray-1 P was very high (>400 mg P kg-1 soil) in all treatments because of high rates of P applied in the past, but biosolids treatments had substantially higher P levels in the surface soil (Table 6). The accumulation of P, especially on sensitive sites, could limit repeated applications.
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Table 6. Soil pH, Bray-1 P, and organic C in the 0- to 15-cm depth in biosolids, inorganic N, and zero-N treatments after 7 yr of biosolids applications.
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Five years of biosolids applications increased DTPA-extractable Cd, Cu, and Zn in the surface soil (Fig. 2)
. The effect of biosolids on DTPA-extractable Pb was not significant, with all treatments similar to the background level of 0.5 mg Pb kg-1 soil. Figure 2 shows levels of DTPA-extractable Cd, Cu, and Zn at 0- to 8-, 8- to 15-, and 15- to 30-cm depths for the heat-dried biosolids treatments, the inorganic N treatment, and the zero-N control. Nearly all of the increase in extractable metals occurred in the 0- to 8-cm depth. A small but statistically significant accumulation of Zn occurred in the 8- to 15- and 15- to 30-cm depths as well. Results for the dewatered biosolids treatments (not shown) were very similar to the heat-dried treatments.
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Fig. 2. DTPA-extractable Zn, Cu, and Cd at 0- to 8-, 8- to 15-, and 15- to 30-cm depths for heat-dried biosolids applied to tall fescue for 5 yr (1993–1997).
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Application of dewatered biosolids increased the Zn content of the tall fescue plant tissue (Table 7). Zinc content increased as annual application rate and years of application increased. We did not see evidence of Zn accumulation approaching a plateau under the conditions of this study. The highest plant Zn level observed (66 µg g-1 after approximately 100 kg ha-1 cumulative biosolids Zn application) was well below the typical critical Zn toxicity threshold of 200 to 500 µg g-1 leaf tissue (Römheld and Marschner, 1991).
Biosolids had a smaller effect on plant tissue Cu content. Copper content increased with annual application rate (Table 5), but remained in the normal range for forage grasses (Mayland and Wilkinson, 1996). We did not see a significant effect of biosolids applications on Cu content over years of application. The effect of biosolids on tissue Ni was even smaller, with a slight increase in tissue content observed only at the highest application rate (Table 5). Biosolids applications did not affect Ni content over time.
Accumulations of Cu and Zn in tall fescue in this study were in a similar range to those reported in grasses by Levine et al. (1989) following 10 yr of biosolids application at 9 Mg ha-1 yr-1 to an old field community on an acid soil. They measured Cu, Zn, and Pb content in tissue of bluegrass (Poa compressa L., Poa pratensis L.), Japanese brome (Bromus japonicum Thumb.), and giant foxtail (Setaria faberii Hermm.). They observed no change in lead content with biosolids, also similar to our results. Zinc levels in their biosolids were similar to the levels in this study, while Cu was lower (320 to 380 mg kg-1) and Pb was higher (250 to 480 mg kg-1).
Soil pH ranged from 5.3 to 5.9 (results from 0- to 30-cm samples, high rate and inorganic N treatments) during the course of our study, which was low enough to increase the availability of trace elements to plants (Alloway and Jackson, 1991). Despite the relatively low soil pH and relatively high cumulative biosolids application rates (up to 120 Mg ha-1 through 1998), accumulation of metals in plant tissue or soil probably will not limit repeated applications.
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CONCLUSIONS
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Under the intensive management conditions of this study, tall fescue maintained high yields and N uptake through 7 yr of repeated biosolids applications, although stand thinning was apparent at the high rate (848 kg ha-1) of dewatered biosolids. Increasing residual soil nitrate at the high application rate starting in 1996 (Year 4) indicated that this rate could not be maintained in the long run without increasing the risk of overfertilization and nitrate leaching. Soil P levels were high for all treatments because of previous management, but the increases seen with the biosolids treatments indicate that P accumulation may limit repeated applications of biosolids under the conditions of this study. Trace element levels increased in plant tissue and the surface soil, but levels were too low to be of concern. Biosolids slightly increased plant tissue Ca and Mg, but did not affect K.
An active soil testing program is a key to sustainable management of biosolids sites with repeated applications. Determining residual soil nitrate in the fall will help guide N application rates at long-term application sites, and basic soil tests will indicate the need for pH adjustment and supplemental K. Basic soil tests will also indicate soil P accumulation and guide decisions on site life. Our results suggest that routine analysis of soils and plant tissue for trace elements are not warranted when biosolids trace element levels are similar to those in this study.
Repeated application of biosolids to forage grasses is an acceptable practice, but application rates will decline with time based on N needs, and site life may be limited by P accumulation. Planning on a limited site life can reduce the potential for water quality problems and spread the benefits of biosolids nutrients to other sites.
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ACKNOWLEDGMENTS
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The Northwest Biosolids Management Association and the King County, WA Department of Natural Resources provided financial support for this project. We thank Liz Myhre of WSU-Puyallup for her technical assistance.
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NOTES
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WSU Crop and Soil Sciences Dep. Paper no. 0105-18.
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