Influence of water treatment residuals on phosphorus solubility and leaching

Laboratory and greenhouse studies compared the ability of water treatment residuals (WTRs) to alter P solubility and leaching in Immokalee sandy soil (sandy, siliceous, hyperthermic Arenic Alaquod) amended with biosolids and triple superphosphate (TSP). Aluminum sulfate (Al-WTR) and ferric sulfate (Fe-WTR) coagulation residuals, a lime softening residual (Ca-WTR) produced during hardness removal, and pure hematite were examined. In equilibration studies, the ability to reduce soluble P followed the order Al-WTR > Ca-WTR = Fe-WTR > hematite. Differences in the P-fixing capacity of the sesquioxide-dominated materials (Al-WTR, Fe-WTR, hematite) were attributed to their varying reactive Fe- and Al-hydrous oxide contents as measured by oxalate extraction. Leachate P was monitored from greenhouse columns where bahiagrass (Paspalum notatum Flugge) was grown on Immokalee soil amended with biosolids or TSP at an equivalent rate of 224 kg P ha(-1) and WTRs at 2.5% (56 Mg ha(-1)). In the absence of WTRs, 21% of TSP and 11% of Largo cake biosolids total phosphorus (PT) leached over 4 mo. With co-applied WTRs, losses from TSP columns were reduced to 3.5% (Fe-WTR), 2.5% (Ca-WTR), and <1% (Al-WTR) of applied P. For the Largo biosolids treatments all WTRs retarded downward P flux such that leachate P was not statistically different than for control (soil only) columns. The phosphorus saturation index (PSI = [Pox]/ [Al(ox) + Fe(ox)], where Pox, Al, and Fe(ox) are oxalate-extractable P, Al, and Fe, respectively) based on a simple oxalate extraction of the WTR and biosolids is potentially useful for determining WTR application rates for controlled reduction of P in drainage when biosolids are applied to low P-sorbing soils.     

Phosphorus leaching from biosolids-amended sandy soils

Increasing emphasis on phosphorus (P)-based nutrient management underscores the need to understand P behavior in soils amended with biosolids and manures. Laboratory and greenhouse column studies characterized P forms and leachability of eight biosolids products, chicken manure (CM), and commercial fertilizer (triple superphosphate, TSP). Bahiagrass (Paspalum notatum Flugge) was grown for 4 mo on two acid, P-deficient Florida sands, representing both moderate (Candler series: hyperthermic, uncoated Typic Quartzipsamments) and very low (Immokalee series: sandy, siliceous, hyperthermic Arenic Alaquods) P-sorbing capacities. Amendments were applied at 56 and 224 kg P(T) ha(-1), simulating P-based and N-based nutrient loadings, respectively. Column leachate P was dominantly inorganic and lower for biosolids P sources than TSP. For Candler soil, only TSP at the high P rate exhibited P leaching statistically greater (alpha = 0.05) than control (soil-only) columns. For the high P rate and low P-sorbing Immokalee soil, TSP and CM leached 21 and 3.0% of applied P, respectively. Leachate P for six biosolids was <1.0% of applied P and not statistically different from controls. Largo biosolids, generated from a biological P removal process, exhibited significantly greater leachate P in both cake and pelletized forms (11 and 2.5% of applied P, respectively) than other biosolids. Biosolids P leaching was correlated to the phosphorus saturation index (PSI = [Pox]/[Al(ox) + Fe(ox)]) based on oxalate extraction of the pre-applied biosolids. For hiosolids with PSI < or = approximately 1.1, no appreciable leaching occurred. Only Largo cake (PSI = 1.4) and pellets (PSI = 1.3) exhibited P leaching losses statistically greater than controls. The biosolids PSI appears useful for identifying biosolids with potential to enrich drainage P when applied to low P-sorbing soils.     

Relationships between biosolids treatment process and soil phosphorus availability

Laws mandating phosphorus (P)-based nutrient management plans have been passed in several U.S. Mid-Atlantic states. Biosolids (sewage sludge) are frequently applied to agricultural land and in this study we evaluated how biosolids treatment processes and biosolids P tests were related to P behavior in biosolids-amended soils. Eight biosolids generated by different treatment processes, with respect to digestion and iron (Fe), aluminum (Al), and lime addition, and a poultry litter (PL), were incubated with an Elkton silt loam (fine-silty, mixed, active, mesic Typic Endoaquult) and a Suffolk sandy loam (fine-loamy, siliceous, semiactive, thermic Typic Hapludult) for 51 d. The amended soils were analyzed at 1 and 51 d for water-soluble phosphorus (WSP), iron-oxide strip--extractable phosphorus (FeO-P), Mehlich-1 P and pH. The biosolids and PL were analyzed for P, Fe, and Al by USEPA 3050 acid-peroxide digestion and acid ammonium oxalate, Mehlich-1, and Mehlich-3 extractions. Biosolids and PL amendments increased extractable P in the Suffolk sandy loam to a greater extent than in the Elkton silt loam throughout the 51 d of the incubation. The trend of extractable WSP, FeO-P, and Mehlich-1 P generally followed the pattern: [soils amended with biosolids produced without the use of Fe or Al] > [PL and biosolids produced using Fe or Al and lime] > [biosolids produced using only Fe and Al salts]. Mehlich-3 P and the molar ratio of P to [Al + Fe] by either the USEPA 3050 digestion or oxalate extraction of the biosolids were good predictors of changes in soil-extractable P following biosolids but not PL amendment. Therefore, the testing of biosolids for P availability, rather than total P, is a more appropriate tool for predicting extractable P from the biosolids-amended soils used in this study.     

Phytoavailability of biosolids phosphorus

Efficient utilization of biosolids P for agronomic purposes requires accounting for differences in the phytoavailability of P in various biosolids. Greenhouse studies were conducted with a common pasture grass grown in two P-deficient soils amended with 12 biosolids and a commercial fertilizer (triple superphosphate, TSP) to quantify P uptake and to assess the relative phytoavailabilities of the P sources. Biosolids were grouped into three general categories of phytoavailability relative to TSP: high (> 75% of TSP), moderate (25-75% of TSP), and low (< 25% of TSP). Two biosolids, produced via biological phosphorus removal (BPR) processes, were in the high category, and mimicked fertilizer P with regard to P phytoavailability. Most biosolids produced by conventional wastewater and solids digestion and additional treatments like composting were in the moderate category. Also included in this category was a BPR that had been pelletized and another BPR supplemented with Al. The low category included biosolids containing greater than normal (> 50 g kg(-1)) total Fe and Al concentrations and processed to high (> 60%) solids content.     

Phosphorus saturation in spodosols impacted by manure

Significant amounts of phosphorus (P) accumulate in soils receiving animal manures that could eventually result in unacceptable concentrations of dissolved P loss through surface runoff or subsurface leaching. The degree of phosphorus saturation (DPS) relates a soil's extractable P to its P sorbing capacity, and is reportedly a predictor of the P likely to be mobilized from a system. A DPS value (DPS-1) was derived that expressed the percentage of Mehlich 1-extractable P to the sorbing capacity of a Spodosol (expressed as the sum of oxalate-extractable Fe and Al). Values of DPS-1 were determined in various horizons of soil in current and abandoned dairy systems in South Florida's Lake Okeechobee watershed to assess P release potential. Land use within the dairies was classified as highly impacted by cattle (intensive and holding), and minimally impacted by cattle (pasture, forage, or native) areas. The A and E horizon of soils in heavily manure-impacted intensive and holding areas for both active and abandoned dairies generally had higher DPS-1 values than the pasture, forage, and native area soils, which were minimally impacted by manure. Degree of P saturation was also calculated as a percentage of Mehlich 1-extractable P to the sum of Mehlich 1-extractable Fe and Al (DPS-2). Both DPS-1 and DPS-2 were shown to be significantly (P = 0.0001) related to water-extractable P for all soil horizons, suggesting that either index can be used as an indicator for P loss potential from a soil.     

Evaluating phosphorus loss from a Florida spodosol as affected by phosphorus-source application methods

Incorporating applied phosphorus (P) sources can reduce P runoff losses and is a recommended best management practice. However, in soils with low P retention capacities, leaching can be a major mechanism for off-site P loss, and the P-source application method (surface or incorporation) may not significantly affect the total amount of off-site P loss. We utilized simulated rainfall protocols to investigate effects of P-source characteristics and application methods on the forms and amounts of P losses from six P sources, including five biosolids materials produced and/or marketed in Florida, and one inorganic fertilizer (triple superphosphate). A typical Florida Spodosol (Immokalee fine sand; sandy, siliceous, hyperthermic Arenic Alaquods) was used for the study, to which the P sources were each applied at a rate of 224 kg P ha(-1) (approximately the P rate associated with N-based biosolids applications). The P sources were either surface-applied to the soil or incorporated into the soil to a depth of 5 cm. Amended soils were subjected to three simulated rainfall events, at 1-d intervals. Runoff and leachate were collected after each rainfall event and analyzed for P losses in the form of soluble reactive P (SRP), total dissolved P (TDP), total P (TP), and bioavailable P (BAP) (in runoff only). Cumulative masses (runoff + leachate for the three rainfall events) of P losses from all the P sources were similar, whether the amendments were surface-applied or incorporated into the soil. The solubility of the amendment, rather than application method, largely determines the P loss potential in poorly P-sorbing Florida Spodosols.     

Site-specific phosphorus application based on the kriging fertilizer-phosphorus availability index of soils

Site-specific phosphorus management is done to optimize crop production and minimize P loss from soils. The spatial variability of the available P prior to fertilizer application and the P-fixation tendency of soil both need to be taken into account for variable-rate P application. The objectives of this research were to document the spatial variability of the fertilizer-P availability index, which shows the P-fixation tendency, and to develop a strategy that takes the spatial distribution of this index into account for site-specific phosphorus application. In this study, the spatial patterns of the fertilizer-P availability index were characterized by using geostatistics. The ordinary kriging was used for spatial interpolation of the fertilizer-P availability index. Because the fertilizer-P availability index of soil is related to oxalate-extractable Fe and Al and because measuring oxalate-extractable Fe and Al is much easier than directly determining the fertilizer-P availability index, the spatial distribution of the fertilizer-P availability index can be obtained using the oxalate-extractable Fe and Al data. The spatial distribution of Olsen-extractable P, which was used to measure the available-P status prior to fertilizer-P application, was also estimated by using ordinary kriging. The required fertilizer-P amounts were then determined using the kriging estimates of the fertilizer-P availability index and Olsen-extractable P. A fertilizer-P recommendation map for the 430-ha study site in Changhua county, Taiwan was generated by using this approach for illustration. The proposed method for generating fertilizer-P recommendation maps can be used for variable-rate application to maintain an adequate P status for crop production and to potentially reduce the P loss from soils.     

Runoff phosphorus losses from surface-applied biosolids

Runoff losses of dissolved and particulate phosphorus (P) may occur when rainfall interacts with manures and biosolids spread on the soil surface. This study compared P levels in runoff losses from soils amended with several P sources, including 10 different biosolids and dairy manure (untreated and treated with Fe or Al salts). Simulated rainfall (71 mm h(-1)) was applied until 30 min of runoff was collected from soil boxes (100 x 20 x 5 cm) to which the P sources were surfaced applied. Materials were applied to achieve a common plant available nitrogen (PAN) rate of 134 kg PAN ha(-1), resulting in total P loading rates from 122 (dairy manure) to 555 (Syracuse N-Viro biosolids) kg P ha(-1). Two biosolids produced via biological phosphorus removal (BPR) wastewater treatment resulted in the highest total dissolved phosphorus (13-21.5 mg TDP L(-1)) and total phosphorus (18-27.5 mg TP L(-1)) concentrations in runoff, followed by untreated dairy manure that had statistically (p = 0.05) higher TDP (8.5 mg L(-1)) and TP (10.9 mg L(-1)) than seven of the eight other biosolids. The TDP and TP in runoff from six biosolids did not differ significantly from unamended control (0.03 mg TDP L(-1); 0.95 mg TP L(-1)). Highest runoff TDP was associated with P sources low in Al and Fe. Amending dairy manure with Al and Fe salts at 1:1 metal-to-P molar ratio reduced runoff TP to control levels. Runoff TDP and TP were not positively correlated to TP application rate unless modified by a weighting factor reflecting the relative solubility of the P source. This suggests site assessment indices should account for the differential solubility of the applied P source to accurately predict the risk of P loss from the wide variety of biosolids materials routinely land applied.     

Solid phosphorus phase in aluminum- and iron-treated biosolids

Stabilization of phosphorus (P) in sewage sludge (biosolids) to reduce water-soluble P concentrations is essential for minimizing P loss from amended soils and maximizing the capacity of the soil to safely serve as an outlet for this waste material. The chemical form at which P is retained in biosolids stabilized by Al(2)(SO(4))(3) x 18H(2)O (alum) or FeSO(4) x 7H(2)O (FeSul) was investigated by scanning electron microscopy (SEM) equipped with energy-dispersive X-ray elemental spectrometry (EDXS) and by X-ray diffraction (XRD). Both treatments resulted in the formation of a Ca-P phase, probably brushite. Phosphorus was further retained in the alum-treated biosolids by precipitation of an Al-P phase with an Al/P molar ratio of about 1:1, while in the FeSul-treated biosolids, P was retained by both precipitation with Fe/P molar ratios of 1:1 or 1.5:1, and by adsorption onto newly formed Fe hydroxides exhibiting an Fe/P molar ratio of up to 11:1. All of these mechanisms efficiently reduced P solubility and are crucial in biosolids environmentally safe agronomic beneficial use for this waste product; however, each P phase formed may react differently in the amended soil, depending on soil properties. Thus, the proper P stabilization method would depend on the target soil.     

Plant and soil responses to biosolids application following forest fire

Soil stability and revegetation is a great concern following forest wildfires. Biosolids application might enhance revegetation efforts and enhance soil stability. In May 1997, we applied Metro Wastewater Reclamation District (Denver, CO, USA) composted biosolids at rates of 0, 5, 10, 20, 40, and 80 Mg ha(-1) to a severely burned, previously forested site near Buffalo Creek, CO to improve soil C and N levels and help establish eight native, seeded grasses. The soils on the site belong to the Sphinx series (sandy-skeletal, mixed, frigid, shallow Typic Ustorthents). Vegetation and soils data were collected for four years following treatment. During the four years following treatment, total plant biomass ranged from approximately 50 to 230 g m(-2) and generally increased with increasing biosolids application. The percentage of bare ground ranged from 4 to 58% and generally decreased with increasing biosolids rate. Higher rates of biosolids application were associated with increased concentrations of N, P, and Zn in tissue of the dominant plant species, streambank wheatgrass [Elymus lanceolatus (Scribn. & J.G. Sm) Gould subsp. lanceolatus], relative to the unamended, unfertilized control. At two months following biosolids application (1997), total soil C and N at soil depths of 0 to 7.5, 7.5 to 15, and 15 to 30 cm showed significant (P < 0.05) linear increases (r2 > 0.88) as biosolids rate increased. The surface soil layer also showed this effect one year after application (1998). For Years 2 through 4 (1999-2001) following treatment, soil C and N levels declined but did not show consistent trends. The increase in productivity and cover resulting from the use of biosolids can aid in the rehabilitation of wildfire sites and reduce soil erosion in ecosystems similar to the Buffalo Creek area.     

On-farm assessment of biosolids effects on soil and crop tissue quality

Agronomic use of biosolids as a fertilizer material remains controversial in part due to public concerns regarding the potential pollution of soils, crop tissue, and ground water by excess nutrients and trace elements in biosolids. This study was designed to assess the effects of long-term commercial-scale application of biosolids on soils and crop tissue sampled from 18 production farms throughout Pennsylvania. Biosolids application rates ranged from 5 to 159 Mg ha(-1) on a dry weight basis. Soil cores and crop tissue samples from corn (Zea mays L.), soybean (Glycine spp.), alfalfa (Medicago sativa L.), orchardgrass (Dactylis spp.) hay, and/or sorghum [Sorghum bicolor (L.) Moench] were collected for three years from georeferenced locations at each farm. Samples were tested for nutrients, trace elements, and other variables. Biosolids-treated fields had more post-growing season soil NO3 and Ca and less soil K than control fields and there was some evidence that soil P concentrations were higher in treated fields. The soil concentrations of Cu, Cr, Hg, Mo, Mn, Pb, and Zn were higher in biosolids-treated fields than in control fields; however, differences were < or = 0.06 of the USEPA Part 503 cumulative pollutant loading rates (CPLRs). There were no differences in the concentrations of measured nutrients or trace elements in the crop tissue grown on treated or control fields at any time during the study. Commercial-scale biosolids application resulted in soil trace element increases that were in line with expected increases based on estimated trace element loading. Excess NO3 and apparent P buildup indicates a need to reassess biosolids nutrient management practices.     

Fate of biosolids trace metals in a dryland wheat agroecosystem

Biosolids land application for beneficial reuse applies varying amounts of trace metals to soils. Measuring plant-available or total soil metals is typically performed to ensure environmental protection, but these techniques do not quantify which soil phases play important roles in terms of metal release or attenuation. This study assessed the distribution of Cd, Cr, Cu, Mo, Ni, Pb, and Zn associated with soluble/exchangeable, specifically adsorbed/carbonate-bound, amorphous Mn hydroxyoxide-bound, amorphous Fe hydroxyoxide-bound, organically complexed, and residual inorganic phases. Biosolids were applied every 2 yr from 1982 to 2002 (except in 1998) at rates of 0, 6.7, 13.4, 26.8, and 40.3 dry Mg biosolids ha(-)(1) to 3.6- by 17.1-m plots. In 2003, 0- to 20-cm and 20- to 60-cm soil depths were collected and subjected to 4 mol L(-1) HNO(3) digestion and sequential extraction. Trace metals were concentrated in the 0- to 20-cm depth, with no significant observable downward movement using 4 mol L(-1) HNO(3) or sequential extraction. The sequential extraction showed nearly all measurable Cd present in relatively mobile forms and Cr, Cu, Mo, Ni, Pb, and Zn present in more resistant phases. Biosolids application did not affect Cd or Cr fractionation but did increase relatively immobile Cu, Mo, and Zn phases and relatively mobile Cu, Ni, and Pb pools. The mobile phases have not contributed to significant downward metal movement. Long-term, repeated biosolids applications at rates considered several times greater than agronomic levels should not significantly contribute to downward metal transport and ground water contamination for soils under similar climatic conditions, agronomic practices, and histories.     

Using data mining to predict soil quality after application of biosolids in agriculture

The amount of biosolids recycled in agriculture has steadily increased during the last decades. However, few models are available to predict the accompanying risks, mainly due to the presence of trace element and organic contaminants, and benefits for soil fertility of their application. This paper deals with using data mining to assess the benefits and risks of biosolids application in agriculture. The analyzed data come from a 10-yr field experiment in northeast France focusing on the effects of biosolid application and mineral fertilization on soil fertility and contamination. Biosolids were applied at agriculturally recommended rates. Biosolids had a significant effect on soil fertility, causing in particular a persistent increase in plant-available phosphorus (P) relative to plots receiving mineral fertilizer. However, soil fertility at seeding and crop management method had greater effects than biosolid application on soil fertility at harvest, especially soil nitrogen (N) content. Levels of trace elements and organic contaminants in soils remained below legal threshold values. Levels of extractable metals correlated more strongly than total metal levels with other factors. Levels of organic contaminants, particularly polycyclic aromatic hydrocarbons, were linked to total metal levels in biosolids and treated soil. This study confirmed that biosolid application at rates recommended for agriculture is a safe option for increasing soil fertility. However, the quality of the biosolids selected has to be taken into account. The results also indicate the power of data mining in examining links between parameters in complex data sets.     

Hybrid poplar and forest soil response to municipal and industrial by-products: a greenhouse study

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.     

Vertical distribution of phosphorus in agricultural drainage ditch soils

Pedological processes such as gleization and organic matter accumulation may affect the vertical distribution of P within agricultural drainage ditch soils. The objective of this study was to assess the vertical distribution of P as a function of horizonation in ditch soils at the University of Maryland Eastern Shore Research Farm in Princess Anne, Maryland. Twenty-one profiles were sampled from 10 agricultural ditches ranging in length from 225 to 550 m. Horizon samples were analyzed for total P; water-extractable P; Mehlich-3 P; acid ammonium oxalate-extractable P, Fe, and Al (P ox, Fe ox, Al ox); pH; and organic C (n = 126). Total P ranged from 27 to 4882 mg kg(-1), P ox from 4 to 4631 mg kg(-1), Mehlich-3 P from 2 to 401 mg kg(-1), and water-extractable P from 0 to 17 mg kg(-1). Soil-forming processes that result in differences between horizons had a strong relationship with various P fractions and P sorption capacity. Fibric organic horizons at the ditch soil surface had the greatest mean P ox, Fe ox, and Al ox concentrations of any horizon class. Gleyed A horizons had a mean Fe ox concentrations 2.6 times lower than dark A horizons and were significantly lower in total P and P ox. Variation in P due to organic matter accumulation and gleization provide critical insight into short- and long-term dynamics of P in ditch soils and should be accounted for when applying ditch management practices.     

Runoff and leachate losses of phosphorus in a sandy Spodosol amended with biosolids

Florida Spodosols are sandy, inherently low in Fe- and Al-based minerals, and sorb phosphorus (P) poorly. We evaluated runoff and leachate P losses from a typical Florida Spodosol amended with biosolids and triple superphosphate (TSP). Phosphorus losses were evaluated with traditional indoor rainfall simulations but used a double-deck box arrangement that allowed leaching and runoff to be determined simultaneously. Biosolids (Lakeland, OCUD, Milorganite, and Disney) represented contrasting values of total P, percent water-extractable P (PWEP), and percentage of solids. All P sources were surface applied at 224 kg P ha(-1), representing a soil P rate typical of N-based biosolids application. All biosolids-P sources lost less P than TSP, and leachate-P losses generally dominated. For Lakeland-amended soil, bioavailable P (BAP) was mainly lost by runoff (81% of total BAP losses). This behavior was due to surface sealing and drying after application of the slurry (31 g kg(-1) solids) material. For all other P sources, BAP losses in leachate were much greater than in runoff, representing 94% of total BAP losses for TSP, 80% for Milorganite, 72% for Disney, and 69% for OCUD treatments. Phosphorus leaching can be extreme and represents a great concern in many coarse-textured Florida Spodosols and other coastal plain soils with low P-sorption capacities. The PWEP values of P sources were significantly correlated with total P and BAP losses in runoff and leachate. The PWEP of a source can serve as a good indicator of potential P loss when amended to sandy soils with low P-retention capacities.     

Estimating soil phosphorus requirements and limits from oxalate extract data

Excessive fertilizer and manure phosphorus (P) inputs to soils elevates P in soil solution and surface runoff, which can lead to freshwater eutrophication. Runoff P can be related to soil test P and P sorption saturation, but these approaches are restricted to a limited range of soil types or are difficult to determine on a routine basis. The purpose of this study was to determine whether easily measurable soil characteristics were related to the soil phosphorus requirements (P(req), the amount of P sorbed at a particular solution P level). The P(req) was determined for 18 chemically diverse soils from sorption isotherm data (corrected for native sorbed P) and was found to be highly correlated to the sum of oxalate-extractable Al and Fe (R2 > 0.90). Native sorbed P, also determined from oxalate extraction, was subtracted from the P(req) to determine soil phosphorus limits (PL, the amount of P that can be added to soil to reach P(req)). Using this approach, the PL to reach 0.2 mg P L(-1) in solution ranged between -92 and 253 mg P kg(-1). Negative values identified soils with surplus P, while positive values showed soils with P deficiency. The results showed that P, Al, and Fe in oxalate extracts of soils held promise for determining PL to reach up to 10 mg P L(-1) in solution (leading to potential runoff from many soils). The soil oxalate extraction test could be integrated into existing best management practices for improving soil fertility and protecting water quality.     

Sorption and desorption of cadmium by different fractions of biosolids-amended soils

To evaluate the importance of both the inorganic and organic fractions in biosolids on Cd chemistry, a series of Cd sorption and desorption batch experiments (at pH 5.5) were conducted on different fractions of soils from a long-term field experimental site. The slope of the Cd sorption isotherm increased with rate of biosolids and was different for the different biosolids. Removal of organic carbon (OC) reduced the slope of the Cd sorption isotherm but did not account for the observed differences between biosolids-amended soils and a control soil, indicating that the increased adsorption associated with biosolids application was not limited to the increased OC from the addition of biosolids. Removal of both OC and Fe/Mn further reduced the slopes of Cd sorption isotherms and the sorption isotherm of the biosolids-amended soil was the same as that of the control, indicating both OC and Fe/Mn fractions added by the biosolids were important to the increased sorption observed for the biosolids-amended soil samples. Desorption experiments failed to remove from 60 to 90% of the sorbed Cd. This "apparent hysteresis" was higher for biosolids-amended soil than the control soil. Removal of both OC and Fe/Mn fractions was more effective in removing the observed differences between the biosolids-amended soil and the control than either alone. Results show that Cd added to biosolids-amended soil behaves differently than Cd added to soils without biosolids and support the hypothesis that the addition of Fe and Mn in the biosolids increased the retention of Cd in biosolids-amended soils.     

Cadmium, copper, nickel, and zinc availability in a biosolids-amended piedmont soil years after application

Concerns over the possible increase in phytoavailability of biosolids-applied trace metals to plants have been raised based on the assumption that decomposition of applied organic matter would increase phytoavailability. The objectives of this study were to assess the effect of time on chemical extractability and concentration of Cd, Cu, Ni, and Zn in plants on plots established by a single application of biosolids with high trace metals content in 1984. Biosolids were applied to 1.5 by 2.3 m confined plots of a Davidson clay loam (clayey, kaolinitic, thermic Rhodic Kandiudults) at 0, 42, 84, 126, 168, and 210 Mg ha(-1). The highest biosolids application supplied 4.5, 760, 43, and 620 kg ha(-1) of Cd, Cu, Ni, and Zn, respectively. Radish (Raphanus sativus L.), romaine lettuce (Lactuca sativa L. var. longifolia), and barley (Hordeum vulgare L.) were planted at the site for 3 consecutive years, 17 to 19 yr after biosolids application. Extractable Cd, Cu, Ni, and Zn (as measured by DTPA, CaCl(2,) and Mehlich-1) were determined on 15-cm depth samples from each plot. The DTPA-extractable Cu and Zn decreased by 58 and 42%, respectively, 17 yr after application despite a significant reduction in organic matter content. Biosolids treatments had no significant effect on crop yield. Plant tissue metal concentrations increased with biosolids rate but were within the normal range of these crops. Trace metal concentrations in plants generally correlated well with the concentrations extracted from soil with DTPA, CaCl(2), and Mehlich-1. Metal concentrations in plant tissue exhibited a plateau response in most cases. The uptake coefficient values generated for the different crops were in agreement with the values set by the Part 503 Rule.     

Recovery and distribution of biosolids-derived trace metals in a clay loam soil

The long-term mobility of trace metals has been cited as a potential hazard by critics of EPA 503 rule governing the land application of biosolids. The objectives of this study were to assess the accumulation of Cu, Ni, Cd, and Zn within the soil profile; the distribution of exchangeable, specifically adsorbed, organic, and oxide fractions of each metal; and mass balance of Cu, Ni, and Zn 17 yr after a single biosolids application. Biosolids were applied to 1.5- x 2.3-m confined plots of a Davidson clay loam (fine, kaolinitic, thermic Rhodic Kandiudult) in 1984 at 0, 42, 84, 126, 168, and 210 Mg ha(-1). The highest biosolids application supplied 4.5, 750, 43, and 600 kg ha(-1) of Cd, Cu, Ni, and Zn, respectively. Soils were sampled to a depth of 0.9 m and sectioned into 5-cm increments after separating the Ap horizon. Total (EPA-3050B), bioavailable (Mehlich-I), sequential extraction, and dispersible clay analyses were performed on samples from the control, 126 Mg ha(-1), and 210 Mg ha(-1) treatments. Trace metals are still concentrated in the top 0.2 m with slight enrichment down to 0.3 m. More than 85% of applied Cu, Ni, and Zn are still found in the topsoil where biosolids was incorporated and 95% or more of the applied metals were accounted for with mass balance calculations. Mehlich-I results showed a slight increase in metal concentration down to 0.35 m. Biosolids application increased the concentrations of trace metals in all the extracted fractions. The major portions of Cu, Zn, and Ni are associated with the metal-oxides fraction. Dispersible clay content and water-soluble metal contents were low and except for water-soluble Zn they were not affected by biosolids application. Results from this study showed that 17 yr after biosolids application there was negligible movement of trace metals through the soil profile and consequently there is little risk of contamination of ground water at this site.