Published online 4 April 2007
Published in Agron J 99:715-722 (2007)
DOI: 10.2134/agronj2006.0221
© 2007 American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA
Biosolids
Nutrient Assessment of a Dryland Wheat Agroecosystem after 12 Years of Biosolids Applications
K. A. Barbarick* and
J. A. Ippolito
Dep. of Soil and Crop Sci., Colorado State Univ., Fort Collins, CO, 80523-1170
* Corresponding author (ken.barbarick{at}colostate.edu)
Received for publication August 1, 2006.
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ABSTRACT
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Biosolids beneficial-use programs effectively recycle plant nutrients when these waste materials are applied at an agronomic rate. Plant-nutrient availability questions have arisen relating to Littleton and Englewood (L/E), Colorado Wastewater Treatment Plant biosolids applications to dryland wheat (Triticum aestivum L.)–fallow agroecosystems. What is the long-term estimated nitrogen equivalency (NE) of the biosolids? Can we estimate long-term micronutrient distribution with continuous biosolids application? How does plant-nutrient availability change with continuous application? Before each growing season we added biosolids at rates of 0 to 11.2 dry Mg ha–1 to plots arranged in randomized complete blocks with four replications per treatment. We found 12 years of application (6 applications to two sites in a 2-yr wheat–fallow rotation) produced N equivalencies, based on wheat-grain N uptake, of about 9 kg N Mg–1 biosolids. Estimated first-year mineralization rates were 
21 to 33%. Since P, Cu, Ni, and Zn grain removal were <1% of biosolids-applied concentrations, we estimated that tillage-layer (top 20 cm of soil) concentrations could be predicted within 5% of actual total soil contents based on biosolids additions of these nutrients. Biosolids additions produced linear increases in NH4HCO3–diethylenetriaminepentaacetic acid (AB-DTPA) soil extract concentrations of P, Cu, Ni, and Zn. Soils initially were Zn deficient; biosolids application provided plant-available Zn for dryland wheat. If biosolids agronomic rates were based on P instead of N availability, these soils could not receive biosolids. Also, if the Colorado Phosphorus Index was utilized, agronomic rates would continue to be based on N. Biosolids addition to dryland winter wheat according to N agronomic rates is a feasible method of recycling plant nutrients.
Abbreviations: AB-DTPA, NH4HCO3–diethylenetriaminepentaacetic acid • Bslope, slope of N uptake vs. biosolids application rate, kg N uptake Mg–1 biosolids • Epredicted, predicted soil nutrient content, kg • Esoil, total soil nutrient content, kg • ICP–AES, inductively coupled plasma–atomic emission spectrophotometer • L/E, Littleton and Englewood Wastewater Treatment Plant • Mr, estimated first-year mineralization rate, % • NE, nitrogen equivalency, kg N Mg–1 biosolids • Np, plant available nitrogen, kg N Mg–1 biosolids • Nslope, slope of N uptake vs. N fertilizer application rate, kg N uptake kg–1 N fertilizer • NU, grain nitrogen uptake, kg ha–1 • Y, grain yield, kg ha–1
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INTRODUCTION
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THE USEPA (1993) 40 CFR Part 503 regulations for beneficial use of biosolids (sewage sludge) promotes recycling of this material on some crop lands since it is an excellent source of several plant nutrients such as N, P, Cu, Ni, and Zn. For continuous application, an important environmental quality and protection question is: Will biosolids NE in a dryland wheat agroecosystem change during Years 7 through 12 as compared with results for the first 6 yr? Short- and long-term answers to this question are important because the Colorado Department of Public Health and Environment (2003) and most other states require biosolids be applied at the agronomic rate for N. And, because biosolids organic N concentrations dominate over inorganic N forms, one may ask the following questions: What is the apparent first-year N mineralization rate in a dryland wheat agroecosystem? What is the first-year N mineralization rate during years of increased precipitation as compared with that during years of drought? Also, how does soil distribution and plant-nutrient availability change with continuous biosolids application to a dryland wheat fallow agroecosystem?
Previous research provides some possible clues to help answer these questions. Barbarick and Ippolito (2000) found that continuous biosolids application for 6 yr in a Colorado dryland wheat-fallow agroecosystem (three applications to two sets of plots) produced an NE of 8.2 kg N fertilizer Mg–1 biosolids. They also estimated first-year N mineralization rates at 25 to 32% during years of above-normal precipitation. In Washington, Cogger et al. (1998) found that dryland winter wheat recovered 11 to 44% of biosolids-borne N. Using 12-wk laboratory incubations, Lerch et al. (1992) found a 55% mineralization for the L/E biosolids. He et al. (2000) reported 48% N mineralization from pelletized biosolids. Er et al. (2004) showed that temperature, biosolids application rate, and biosolids C/N ratio accounted for up to 87% of modeled mineralization variability.
When biosolids are land-applied based on mineralization and crop N requirements, trace metals are invariably added. If biosolids exceed the USEPA allowable trace-element concentrations for land application (USEPA, 1993), a trace-metal monitoring scheme must be implemented. The most common approach for tracking biosolids-borne plant nutrients or trace metals are mass-balance calculations. This method has met varying degrees of success. On large plots in Minnesota, Sloan et al. (1998) accounted for 119, 114, and 97% of the biosolids-applied Cu, Ni, and Zn, respectively, in the top 45 cm of soil. By physically isolating small plots to prevent lateral redistribution, Sukkariyah et al. (2005a) could account for 93 to 96% of biosolids-applied Cu, Ni, and Zn. Using extensive transects sampling, Yingming and Corey (1993) found 16 to 21% of applied Cd, Cu, and Zn moved laterally outside the targeted plot area. They were able to account for an average of 99% of the metal loading. By contrast, Berti and Jacobs (1998) recovered 45 to 155% of Cd, Cr, Cu, Pb, Ni, and Zn added to biosolids-amended soils. They also found significant lateral relocation of these elements and suggested that an exact accounting through mass-balance calculations may not be possible.
In addition to accounting for trace element application in biosolids-amended agroecosystems, tracking changes in plant-nutrient availability is an important agronomic consideration. Using a sequential-extraction procedure, Berti and Jacobs (1996) showed that biosolids additions significantly increased the plant-available (water-soluble and exchangeable) forms of Cu, Ni, and Zn. Chelating solutions such as diethylenetriaminepentaacetic acid (DTPA; Lindsay and Norvell, 1978) and AB-DTPA (Barbarick and Workman, 1987) are often used to indicate lability of micronutrients and trace metals. Sukkariyah et al. (2005b) reported significant increases in DTPA-extractable Cu and Zn 17 yr after single applications of biosolids rates of up to 210 Mg ha–1.
In some situations, agronomic-rate applications of biosolids must be based on P rather than N availability. Concerns about agricultural-P pollution of surface water prompted the state of Maryland to require P-based agronomic rates (Shober and Sims, 2003). Applying agronomic rates of biosolids based on NE leads to soil P accumulation since the biosolids-P amounts applied exceed crop removal (Shober and Sims, 2003). Consequently, tracking labile P levels is crucial in any biosolids beneficial-use program.
Our hypotheses for this research were as follows. 1. The NE of the L/E biosolids would be the same (±10%) for both (a) the second 6-yr period (1999–2005) and (b) 12-yr cumulative results (1993–2005) as it was for the first 6 yr (1993–1999) of biosolids application (8.2 kg N Mg–1 biosolids; Barbarick and Ippolito, 2000). 2. Average estimated first-year mineralization rates would be the same (±10%) for (a) the second 6-yr period (1999–2005; droughty period) and (b) 12-yr cumulative results (1993–2005) as it was for the first 6 yr (28% for 1993–1999; increased precipitation period) of biosolids application. 3. Average ratios of actual to predicted total soil concentrations of P, Cu, Ni, and Zn across all years of application (1993–2005) and across all biannual application rates (0–11.2 Mg ha–1) and total application rates (0 to 67.2 Mg ha–1) will be 1.00 ± 0.05. 4. After 12 yr, the AB-DTPA-extractable P, Cu, Ni, and Zn concentrations will have increased linearly as biosolids rate increases. Following 11 yr of biosolids application at another dryland-wheat research site, Barbarick et al. (1997) showed that AB-DTPA-extractable P, Cu, Ni, and Zn concentrations could be explained using a linear model.
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MATERIALS AND METHODS
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We initiated our biosolids nutrient assessment study in the summer of 1993 near Bennett, CO, USA. Mean annual precipitation for this area is 350 mm, mean maximum and minimum temperatures are 19 and 2°C, respectively, and the annual growing season is about 150 d (SCS, 1974). Two sets of plots were used (designated A for those established in 1993; B for those established in 1994), since the crop rotation was hard red winter wheat–summer fallow. The study sites were established on Weld loam soils (fine, montmorillonitic, mesic Aridic Paleustolls).
For both sites before biosolids application, organic matter content was 
1% to a depth of 200 cm; surface (0–20 cm) soil pH was 6.9, and subsoil pH ranged from 7.2 to 8.3 from 20 to 200 cm in depth. The electrical conductivity of saturated-soil extracts were <1 dS m–1 at all depths except the 150- to 200-cm depth at site B (2.8 dS m–1), and NO3–N plus NH4–N was <9 mg kg–1 for all depths at both sites.
The L/E biosolids were anaerobically digested and supplied by L/E after 60 d of sand-bed drying. Biosolids samples were collected before application and kept refrigerated at 3°C until analyses were completed. Table 1 shows the application sequence and the biosolids' characteristics. Biosolids were applied at rates of 0, 2.24, 4.48, 6.72, 8.96, and 11.2 dry Mg biosolids ha–1 to 1.8- by 17.1-m plots in 1993, 1995, 1997, 1999, 2001, and 2003 at site A and in 1994, 1996, 1998, 2000, 2002, and 2004 at site B. Urea (46–0–0) was hand applied to nonbiosolids 1.8- by 17.1-m plots at rates of 0, 22, 44, 66, 88, and 110 kg N ha–1 for each application year at both sites. Commercial N fertilizer was applied in the same sequence as shown in Table 1. These biosolids and N fertilizer rates were utilized since previous studies (Barbarick et al., 1992; Lerch et al., 1990) indicated this range would probably provide us with linear responses to the applied N. These rates also bracket those commonly used on dryland wheat in Colorado. Four replications of all treatments were used in a randomized complete block arrangement. In late July or early August (about 50 d before planting), the dried biosolids were weighed (solids content of 530–930 g kg–1), evenly spread over the plots with a front-end loader, hand raked to improve the uniformity of distribution, and immediately incorporated to a depth of 10 to 15 cm with a rototiller.
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Table 1. The solids, organic N, NH4–N, NO3–N, P, Cu, Ni, and Zn content of the Littleton/Englewood, CO, biosolids applied to dryland winter wheat at two sites near Bennett, CO, from 1993–2005.
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Grain samples from all cropping years were collected for yield determinations by harvesting a 1.8- by 15.2-m area. The 2000–2001 crop was lost due to hail damage. We collected grain subsamples from the harvested grain and determined N concentrations by dividing protein concentrations found with a Dickey John (Colombes, France) GAC III near infrared analyzer by 5.7. We calculated N uptake by the grain as follows:
 | [1] |
where NU = grain N uptake, kg ha–1; NC = grain N concentration, g kg–1; and Y = grain yield, kg ha–1. Linear regression analyses were completed for the effects of biosolids and N fertilizer rates on NU for each harvest and for the total grain yield and cumulative NU for the second 6-yr period and during the 12 yr of study. We took the average intercept for each material's linear regression model and completed a second set of regression analyses where the intercept for the biosolids and the N fertilizer models were set to the average intercept of both. This approach allowed us to then to equate the N fertilizer to the biosolids regression equation. The NE was then found by calculating the ratio of the slope of the biosolids curve to the slope of the N curve:
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where NE was measured in kg fertilizer N Mg–1 biosolids; Bslope = slope of N uptake vs. biosolids application rate, kg N uptake Mg–1 biosolids; Nslope = slope of N uptake vs. N fertilizer application rate, kg N uptake kg–1 N fertilizer.
Plant available N (Np) from the USEPA (1983) calculation was determined assuming an application rate of 1 Mg biosolids ha–1 and a first-year mineralization rate of 20%:
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where Np was measured in kg N Mg–1 biosolids; NNO3 = NO3–N content of biosolids, kg Mg–1; Kv = NH4–N volatilization factor (assumed to be a range of 0 for complete loss to 1.0 for complete recovery of NH4–N); NNH4 = NH4–N content of biosolids, kg Mg–1; NO = organic N content of biosolids, kg Mg–1; residual = 0.08 x NO from application 2 yr previous + 0.036 x NO from application 4 yr previous, kg Mg–1.
Using NE, Np, and assuming a 20% first-year mineralization rate (USEPA, 1983) and a proportional relationship, the effective N mineralization rates for continuous application were estimated as follows:
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where Mr = estimated first-year mineralization rate, %. The Mr does not account for residual N in the wheat straw; we did not include this fraction in our calculations since it was assumed to be returned to the organic N pool during the course of normal soil management at our sites.
Grain elemental concentrations of P, Cu, Ni, and Zn in concentrated HNO3 digests (Ippolito and Barbarick, 2000) were determined using an inductively coupled plasma–atomic emission spectrophotometer (ICP–AES; Soltanpour et al., 1996).
Immediately following each wheat harvest, composite soil samples were collected (two to three cores per plot) from the 0- to 20- (tillage layer) and 20- to 60-cm depth near the center of each plot. Samples were taken near the center of each plot to avoid biosolids redistribution problems that can occur following many tillage operations during many cropping years (Yingming and Corey, 1993). The soil samples were immediately air-dried and crushed to pass a 2-mm sieve. Concentrations of soil plant-available P, Cu, Ni, and Zn were determined in AB-DTPA extracts utilizing ICP–AES. We used 4 M HNO3 extractions as an approximation of total soil P, Cu, Ni, and Zn (Bradford et al., 1975; Chang et al., 1984; Barbarick et al., 1997).
For each rate and year for P, Cu, Ni, and Zn, the kilograms of element added with biosolids, the kilograms of element removed with grain, and the total kilograms of each nutrient found in the soil were determined. The predicted kilograms of total soil nutrient and the computed ratio between the actual kilograms to the predicted kilograms of each element was also determined. The kilograms of added nutrient was determined using the following equation:
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where Eadded = nutrient added by biosolids, kg; Cbio = concentration of nutrient in the biosolids, mg kg–1; Rbio = biosolids rate, Mg ha–1; Plength = plot length, 17.1 m; and Pwidth = plot width, 1.8 m. The grain removal of a nutrient was determined using the following equation:
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where Egrain = grain removal of nutrient, kg; Cgrain = concentration of nutrient in the grain, mg kg–1; For the total-soil quantity, we employed
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where Esoil = total soil nutrient content, kg; Bd = bulk density, g cm–3 (Bd was 1.3 g cm–3 for the 0–20cm depth and was 1.4 g cm–3 for the 20–60 cm depth); D = soil depth, m (0.2 m for the tillage layer, 0.4 m for the second layer).
Predicted amounts of nutrients (Epredicted) in the soil were found with
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where Esoil control = control (0 Mg ha–1 treatment) soil content of nutrient, kg; and Egrain control = control (0 Mg ha–1 treatment) grain removal of nutrient, kg. Then, the ratio between Esoil and Epredicted for each biosolids rate for each year and for the cumulative amounts for Esoil and Epredicted was determined.
The distribution of P, Cu, Ni, and Zn for each soil depth and grain removal was calculated as follows:
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RESULTS AND DISCUSSION
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NE and Mineralization of Biosolids
For NU, more consistent responses to biosolids and N fertilizer were found the first 6 yr (Barbarick and Ippolito, 2000) than the second 6 yr (Table 2). More droughty conditions and the 2001 crop loss to hail damage led to fewer significant regression analyses from 1999 through 2005. Evidence of the climatic conditions is illustrated in Fig. 1
, showing that the second 6-yr yields for biosolids and N fertilizer additions were between 50 and 60% of those for the first 6 yr.
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Table 2. Nitrogen uptake (NU) linear regression equations, nitrogen fertilizer equivalency (NE), and plant-available N (Np) for total NH4–N conservation (Kv = 1) and total NH4–N loss (Kv = 0) for Littleton and Englewood, CO, biosolids and N fertilizer application to Bennett dryland winter wheat sites, 2000 to 2005.
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Fig. 1. Total wheat-grain yields for the first 6 yr (1993–1999), second 6 yr (1999–2005), and for 12 yr (1993–2005) of Littleton/Englewood biosolids application.
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When scrutinizing the second 6-yr (Fig. 2
; Table 2) and the cumulative 12-yr results (Fig. 3
; Table 2), significant (P < 0.05) linear relationships between NU and biosolids rate were found. Using the linear models allowed us to determine NE with Eq. [2]. The biosolids was estimated to provide 7.7 and 9.2 kg N Mg–1 biosolids for the second 6 yr and across all 12 yr, respectively. For the first 6 yr of study, the NE was 8.2 kg N Mg–1 (Barbarick and Ippolito, 2000). We accepted Hypothesis 1(a) that the NE will be the same (±10%) as it was for the first 6 yr (7.7 compared with 8.2 kg N Mg–1). While the second 6-yr results only dropped 6.0% from the first estimated NE, the cumulative results were 12% higher than the first 6-yr estimate (9.2 compared with 8.2 kg N Mg–1). The cumulative results cannot be taken as a weighted average of the first and second 6-yr results since we used the average intercept for the total data set to determine Bslope and Nslope. From Table 2, the average intercepts were 330 kg N ha–1 for the first 6 yr, 235 kg N ha–1 for the second 6 yr, and 564 kg N ha–1 for the cumulative N uptake regression analyses. We rejected Hypothesis 1(b) that the NE at 12 yr will be the same (±10%) as it was for the first 6 yr.
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Fig. 2. Littleton/Englewood biosolids and N fertilizer effects on the second 6 yr (1999–2005) grain N uptake by dryland wheat near Bennett, CO.
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Fig. 3. Littleton/Englewood biosolids and N fertilizer effects on cumulative 12 yr (1993–2005) grain N uptake by dryland wheat near Bennett, CO.
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First-year mineralization rate estimates (Eq. [3–4]) for the second 6 yr of study ranged from 21 to 27% with an average of 24%, while the 12-yr estimates ranged from 27 to 33% with an average of 30%. By comparison, we found rates between 25 and 32% with an average of 28% for the first 6 yr (Barbarick and Ippolito, 2000). The plant-growth difficulties faced in Years 7 through 12 undoubtedly led to lower cumulative NE and then a lower estimated Mr. We rejected Hypothesis 2(a) that the average Mr for Years 7 through 12 (24%) would be the same as for Years 1 through 6 (28 ± 2.8%). However, we accepted Hypothesis 2(b) since the 12-yr average Mr (30%) was within 10% of the first 6-yr average Mr. These data indicate that the estimates for wetter years (1993–1999) will result in larger projected first-year mineralization rates than during drier years (1999–2005). Basing biosolids applications on N availability as the key to agronomic rates, our 12 yr of data suggest first-year mineralization of the organic N within the range of 21 to 33% and an average of 27%. Every N application to dryland wheat (Davis et al., 2005) should be based on residual soil NO3–N within the top 30 or 60 cm of soil.
Elemental Soil Accumulation
As Berti and Jacobs (1998) indicated, mass-balance calculations for tracking biosolids-borne trace elements are fraught with inaccuracies and accounting problems. They seem only to work if you take the incredible precautions used by Sukkariyah et al. (2005a) or if you complete exhaustive lateral and vertical soil sampling described by Yingming and Corey (1993). Another consideration is the magnitude of potential change in soil concentrations from agronomic applications. As shown in Table 3, our Cu, Ni, and Zn loading rates were at least approximately one third those of similar studies.
For large-scale field operations where the biosolids are applied at agronomic rates to dryland wheat, a simple prediction of total soil levels may provide valuable estimates without taking the extraordinary precautions presented by Sukkariyah et al. (2005a). Estimations, such as those offered in this research, could provide for a less exhaustive long-term sampling scheme and thus a cost-savings to municipalities. To this end, predicted soil contents in our plots using Eq. [5
–8] for all rates for each year and for the cumulative rates for both sites were determined. The ratio of the actual soil levels to the predicted content (Esoil/Epredicted) was then calculated. Average ratios for each biosolids rate and the overall ratios for P, Cu, Ni, and Zn are given in Table 4. The ratios ranged from 0.99 to 1.03 with a range of standard deviations of 0.02 to 0.06. Therefore, we accepted our third hypothesis that the Esoil/Epredicted would be in the range of 0.95 to 1.05. This simple approach would allow for estimation of elemental soil concentrations across time. These estimations may not work where application rates exceed the agronomic rate and significant lateral biosolids transfer occurs with tillage operations.
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Table 4. Average ratio between actual and predicted total-nutrient soil content for biosolids treatments at the two Bennett, CO, sites (fallow rotation), 1993–2005.
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Tables 5
through 8 provide the elemental distributions for the last soil sampling and 12-yr total grain removal. We observed significant (P < 0.10) linear increases in soil contents for P in both depths at both sites (Table 5) for surface Cu at both sites and grain removal at site A (Table 6), for surface Ni at site B and grain removal at site A (Table 7), and for subsurface Zn and grain removal at both sites (Table 8). All wheat-grain removals were 1% or less. Consequently, for P, Cu, Ni, and Zn, grain removal can be considered to be negligible and most of the applied nutrients will reside in the soil tillage layer.
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Table 5. Total soil P (0–60 cm) plus grain removal, % of total soil P plus grain removal, final soil AB-DTPA P concentrations, and correlation to quantity of applied biosolids or biosolids-borne P at the two Bennett, CO, sites (fallow rotation), 2003–2005.
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Table 6. Total soil Cu (0–60 cm) plus grain removal, % of total soil Cu plus grain removal, final soil AB-DTPA Cu concentrations, and correlation to quantity of applied biosolids or biosolids-borne Cu at the two Bennett, CO, sites (fallow rotation), 2003–2005.
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Table 7. Total soil Ni (0–60 cm) plus grain removal, % of total soil Ni plus grain removal, final soil AB-DTPA Ni concentrations, and correlation to quantity of applied biosolids or biosolids-borne Ni at the two Bennett, CO, sites (fallow rotation), 2003–2005.
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Table 8. Total soil Zn (0–60 cm) plus grain removal, % of total soil Zn plus grain removal, final soil AB-DTPA Zn concentrations, and correlation to quantity of applied biosolids or biosolids-borne Zn at the two Bennett, CO, sites (fallow rotation), 2003–2005.
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Plant-Nutrient Availability
For dryland winter wheat grown in Colorado, AB-DTPA soil extractions to determine P (Davis et al., 2005) and Zn fertilizer requirements (Mortvedt and Westfall, 2004) are used. No guidelines exist for Ni and Cu since deficiency symptoms for these micronutrients in Colorado have not been verified. The surface soil AB-DTPA P (Table 5), Cu (Table 6), and Zn (Table 8) concentrations increased linearly with biosolids addition. Biosolids application did not affect subsoil AB-DTPA P, Cu, Ni, or Zn concentrations (Tables 5–8).
All surface soil AB-DTPA P contents were high (>7 mg kg–1) for winter wheat (Davis et al., 2005). If biosolids application were regulated solely on the agronomic rate for P so as to prevent environmental contamination, biosolids could not be applied to any of our plots. The Natural Resources Conservation Service (NRCS) for animal-manure applications in Colorado (Sharkoff et al., 2005) developed a more comprehensive approach involving control of off-site P movement called the Colorado P Risk Index. The Risk Index utilizes runoff class (includes slope and soil-permeability class), P application rates and methods, best-management conservation practices (i.e., residue management), and soil-test P concentrations (Sharkoff et al., 2005). Using this approach for our biosolids study, the risk assessment of Ippolito et al. (2007) of our P management for our recommended rate of 6.7 Mg biosolids ha–1 was that we could continue to apply biosolids using the N agronomic rate.
Zinc is commonly deficient in eastern Colorado soils (Mortvedt and Westfall, 2004). As shown in Table 8, the AB-DTPA Zn in the control surface soil was low (<1 mg kg–1) at site A and marginal (1–1.5 mg kg–1) at site B, while subsurface concentrations were low at both sites. The six biosolids applications at both sites have elevated the AB-DTPA Zn surface concentrations to adequate levels (>1.5 mg kg–1), indicating that agronomic rates of biosolids could serve as Zn fertilizers on deficient soils.
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CONCLUSIONS
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Several key biosolids land-application management considerations for dryland wheat can be gleaned from this research. First, we found that the NE for L/E biosolids was about 8 to 9 kg N Mg–1 for either 6 or 12 yr of application. Estimated first-year biosolids N mineralization was 21 to 27% for the second 6 yr and 27 to 33% for our 12-yr determination as compared with about 25 to 32% for the first 6-yr evaluation. Droughty conditions and hail damage dominated the second 6 yr of our study and probably led to the lower Mr estimation. We suggest biosolids managers use an Mr of 25% to help avoid soil NO3–N accumulations and leaching.
As far as tracking P, Cu, Ni, and Zn soil accumulation where agronomic rates of biosolids are applied, we suggest assuming that wheat-grain removal is negligible and all of the added elements will reside in the tillage layer. This approach will not provide an accurate mass balance for the elements but would provide a reasonable estimate of projected surface-soil concentrations. This should also provide land applicators with a means of predicting long-term soil nutrient accumulation. The biosolids-borne P will pose a management challenge if agronomic rates are based solely on soil P levels and biosolids-P concentrations. If a more comprehensive biosolids land application approach is utilized, such as the Colorado P Risk Index (Sharkoff et al., 2005) for animal manures, then overall soil management could continue to allow agronomic rates based on crop N requirements. Biosolids agronomic-rate addition to dryland winter wheat based on N availability continues as a viable means for recycling plant nutrients.
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REFERENCES
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INTRODUCTION
