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Arsenic solubility and distribution in poultry waste and long term amended soil
The Science of the Total Environment 320 (2004) 51–61 0048-9697/04/$ - see front matter ᮊ 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0048-9697(03)00441-8 Arsenic solubility and distribution in poultry waste and long-term amended soil F.X. Han *, W.L. Kingery , H.M. Selim , P.D. Gerard , M.S. Cox , J.L. Oldham a,b, acda a Department of Plant and Soil Sciences, Mississippi State University, Mississippi State, MS 39762, USA a Diagnostic Instrumentation and Analysis Laboratory, Mississippi State University, 205 Research Blvd., Starkville, MS 39759, b USA Department of Agronomy, Louisiana State University, Baton Rouge, LA 70803, USA c Experimental Statistics Unit, Mississippi State University, Mississippi State, MS 39762, USA d Received 21 February 2003; accepted 15 July 2003 Abstract The purpose of this study was to quantify the solubility and distribution of As among solid-phase components in poultry wastes and soils receiving long-term poultry waste applications. Arsenic in the water-soluble, NaOCl- extractable (organically bound),NHOHØHCl-extractable (oxide bound) and residual fractions were quantified in an 2 Upper Coastal Plain soil (Neshoba County, MS) that received annual waste applications. After 25 years, As in the amended soil had a mean of 8.4 mg kg compared to 2.68 mg kg for a non-amended soil. Arsenic in the amended y1 y1 soil was mainly in the residual fraction (72% of total), which is generally considered the least bioavailable fraction. Arsenic in poultry waste samples was primarily water-soluble (5.3–25.1 mg kg ), representing 36–75% of the total y1 As. To assess the extent of spatial heterogeneity, total As in a 0.5-ha area within the long-term waste-amended field was quantified. Soil surface samples were taken on 10-m grid points and results for total As appeared negatively skewed and approximated a bimodal distribution. Total As in the amended soil was strongly correlated with Fe oxides, clay and hydroxy interlayered vermiculite concentrations, and negatively correlated with Mehlich III-P, mica and quartz contents. ᮊ 2003 Elsevier B.V. All rights reserved. Keywords: Arsenic; Poultry waste; Fractionation; Solubility; Spatial distribution 1. Introduction Arsenic (As) has become an increasingly impor- tant environmental concern due to its potential carcinogenic properties (Goyer et al., 1995). Recently, the USEPA announced a decrease in the allowable As level for drinking water from 50 to *Corresponding author. Tel.: q1-662-325-2897; fax: q1- 662-325-8465. E-mail address: han@dial.msstate.edu (F.X. Han). 10 mgl (USEPA, 2001). Arsenic is added to y1 poultry diets for the control of coccidial intestinal parasites and to improve feed efficiency (Moore et al., 1995; Wershaw et al., 1999). The organo- arsenical compounds, p-arsanilic acid (4-amino- phenylarsonic acid) and roxarsone 4 (3-nitro-hydroxyphenylarsonic acid), are typical feed additives (Wershaw et al., 1999; Jackson and Miller, 2000). Because these compounds are not readily absorbed in tissues, they can occur in 52 F.X. Han et al. / The Science of the Total Environment 320 (2004) 51–61 excreta. Therefore, the As in poultry wastes and waste-amended soils may be present primarily in organic forms (Morrison, 1969; Woolson, 1975). Moore et al. (1998) found that As concentrations in runoff from poultry-waste-amended soils increased as application rates increased. In addi- tion, because long-term applications to relatively small areas of land have been shown to lead to soil accumulation of nutrients and trace elements (Kingery et al., 1994; Han et al., 2000), the study of As behavior in waste-amended soil systems is crucial due to potential contamination of surface and groundwater via runoff and leaching. Several studies have documented As speciation and toxicity in soils (Onken and Hossner, 1995; Cox et al., 1996a). The bioavailability, toxicity and mobility of As in soil–water–plant systems are largely determined by its speciation and distri- bution, or partitioning between the solution and soil matrix. Moreover, As mobility and possible release into runoff from waste-amended fields may be governed by its distribution among various soil solid-phase components. Arsenic distribution among solid-phase components in poultry waste and in waste-amended soils is not well understood. Sequential dissolutionyextraction techniques, as opposed to a single extractant, have recently been adopted as indicators for As binding, mobility and bioavailability (Wenzel et al., 2001). Since As has similar physicochemical properties to P in soils, inorganic P fractionation techniques have been adapted for soil As (Johnston and Barnard, 1979; Onken and Adriano, 1997). Moore et al. (1988) divided As in sediments into oxyhydroxide (Fe and Mn)-bound, organically bound and sulfide- bound fractions. Recently, Wenzel et al. (2001) proposed a fractionation procedure that included non-specifically adsorbed, specifically sorbed, amorphous and poorly crystalline Fe and Al oxides-bound, crystalline Fe and Al hydrous oxides-bound and residual fractions. Since the main species of As in poultry waste is as an organic compound, about which little is know concerning its binding by solid phases, it is appro- priate to include an extractant for organic matter (OM). The primary objective of this study was to determine the solubility and distribution of As among solid-phase components in poultry waste and waste-amended soils receiving long-term applications. A second objective was to correlate soil properties to the spatial distribution of As in a long-term amended soil. 2. Materials and methods 2.1. Poultry-waste-amended soil and poultry wastes Six soil samples (0–20 cm) were taken from a waste-amended pasture on a poultry farm located in Neshoba County, Mississippi, where annual applications had occurred for 25 years. Although historical records of application rates are not com- plete, recent measurements of typical application management indicate that rates were approximately 10 Mg ha per application, one to three times y1 each year (Curtis, 1998). The pasture consisted predominantly of bermudagrass (Cynodon dacty- lon) harvested for hay each summer and ryegrass (Lolium multiflorum) sown each fall, after shallow plowing. Cattle grazed during the winter months. In order to assess the spatial variation of As resulting from the long-term poultry waste amend- ments, surface soil samples were collected on a grid from the waste-amended field. Specifically, 66 surface samples (0- to 5-cm depth) from the waste-amended field were sampled at 10-m inter- vals on a 50=100 m grid, located in the center 2 of the field. In addition, surface (0–5 cm) soil samples were collected from an adjacent, non- amended forest soil where loblolly pine (Pinus taeda) grew. Both soils were clayey, mixed, ther- mic Typic Hapludults (Upper Coastal Plain) from shale parent material. Properties of both soils were reported earlier in Han et al. (2000). Soil pH ranged from 4.7 to 6.3. The amended soil had higher pH, OM and CEC than the non-amended soil (Table 1)(Curtis, 1998). Soil samples were air-dried and ground to pass a 2-mm sieve. All soil samples were analyzed for total As. Furthermore, we carried out analysis for water-soluble As in 10 randomly selected surface samples from our grid scheme. Poultry waste samples were collected from two locations (Marshall and Newton counties) in Mis- 53F.X. Han et al. / The Science of the Total Environment 320 (2004) 51–61 Table 1 Selected properties of the non-amended and poultry-waste-amended surface soils Property Non-amended soil Amended soil pH 4.74–5.06 5.15–6.28 CEC (cmol kg ) y1 c 39.5"10.1 a 107.7"14.8 SSA (mg ) 2 y1 179"64 187"41 OM (%) b 3.4 4.45 Mineralogy of the clay fraction (%) b 2:1 type 51.6 46.5 Mica 10.6 15.9 Kaolinite 25 19.3 Quartz 3.6 12.2 Fe O 23 0.7 1.0 Mean of five samples and standard error. a From Ref. Curtis (1998). b Table 2 Total and water-soluble As concentrations in poultry wastes on oven-dry basis County Year Total As Water-soluble Water-soluble As (mg kg ) y1 (mg kg ) y1 (% of total) Newton 2000 32.6 (0.0) 22.3 (1.1) 68 31.1 (0.3) 20.7 (0.1) 67 33.3 (0.5) 25.1 (0.9) 75 31.8 (1.1) 21.2 (2.2) 67 31.3 (0.1) 22.5 (2.3) 72 30.9 (1.3) 23.2 (0.2) 75 Marshall 1997 32.0 (0.4) 19.8 (2.0) 62 1998 36.1 (0.1) 24.5 (1.4) 68 1999 26.7 (1.0) 9.7 (0.5) 36 11.1 (0.1) 6.0 (0.2) 54 34.2 (0.2) 15.6 (0.0) 46 12.4 (0.7) 5.3 (0.1) 43 28.0 (0.2) 12.5 (0.8) 45 2000 17.8 (0.4) 8.1 (0.4) 46 19.2 (1.1) 9.5 (1.2) 50 22.0 (0.5) 13.0 (0.1) 59 Summary Average 26.9 (7.8) 15.0 (7.6) 58 (13) Maximum 36.2 25.7 75 Minimum 11.1 5.3 36 Average followed by standard deviation in parentheses. sissippi between 1997 and 2000 (Table 2). In 1999 and 2000, multiple, composite waste samples were collected throughout the year. The samples were air-dried and ground to pass a 1-mm sieve. Solid- phase As fractionation was conducted on 10 sam- ples. This study focused on overall As solubility and its solid-phase distribution in both poultry wastes and long-term waste-amended soils, and no attempt was made to differentiate arsenate and arsenite forms. 2.2. Analytical methods Arsenic in soils and poultry wastes was meas- ured in four operationally defined solid-phase frac- 54 F.X. Han et al. / The Science of the Total Environment 320 (2004) 51–61 tions, which were obtained by selective sequential dissolution. This method is based on both solubil- ity of individual solid-phase components and the selectivity and specificity of chemical reagents. The procedure provides a gradient for the physi- cochemical association between trace elements and solid particles rather than actual chemical specia- tion (Martin et al., 1987). But, it can nonetheless provide an indication of their relative availability to plants or to further migration with percolating andyor runoff water. The terms of all fractions are more appropriately considered to be operationally rather than chemically defined (Han et al., 2001). Each extractant in the sequential selective proce- dures is assumed to effectively target one major solid-phase component. It is recognized that no extractant can remove all of a targeted solid-phase component without attacking other components. No selective dissolution scheme can be considered completely accurate in distinguishing between dif- ferent forms of an element, i.e. various organic inorganic solid-phase components. Despite possi- ble re-adsorption during sequential extraction, common to any chemical extraction procedure, sequential dissolution techniques still furnish use- ful information on metal binding, mobility and availability (Han et al., 2001) . The fractionation procedures employed in this study were modified from the sequential selective procedures developed by MacLeod et al. (1998), Shuman (1983), Moore et al. (1998) and Sposito et al. (1982). Arsenic in both waste-amended soil and wastes was fraction- ated into water-soluble, NaOCl-extractable, NH OHØHCl-extractable and residual fractions (4 2 M HNO ). 3 (1) Water-soluble arsenic: Twenty milliliter of distilled water was added to2gofsoil or waste (oven-dry weight basis) in 50-ml Teflon centrifuge tube and the mixture was shaken for 30 min at 25 8C. The sample was then centrifuged at 10 000=g and the supernatant decanted and filtered through a 0.45-mm filter. The soil residue was kept for the subsequent extraction. The same centrifugation– decantion steps were used after each of the follow- ing extractions. (2) NaOCl-extractable arsenic: Arsenic extract- ed in this way may be mainly bound to OM (Shuman, 1983). Twenty milliliter of 0.7 M NaOCl solution at pH 8.5 (pH adjusted with NaOH and HCl) was added to the soil residue. The mixture was boiled in a water-bath at 95–100 8C for 30 min. During digestion, the mixture was continu- ously stirred. (3) NH OHØHCl-extractable arsenic: Arsenic 2 extracted in this step may be mostly bound to oxides (Han and Banin, 1997): Twenty milliliter of 0.04 M NH OHØHClq25% HOAc solution was 2 added to the soil residue and boiled in the water- bath at 100 8C for 3 h. (4) Arsenic in the residual fraction (RES): Twenty milliliter of 4 M HNO solution was added 3 to the residue and the sample transferred to a glass digestion tube. Digestion was conducted in a water- bath at 80 8C for 16 h (Sposito et al., 1982; Han and Banin, 1997). This fraction includes As that was not extracted in the previous steps and repre- sents the very stable fraction in soil and wastes. Total As was extracted with heating with HNO –H SO (Ganje and Rains, 1982). Amended 324 and non-amended soils were analyzed both by Galbraith Laboratory (Knoxville, TN) and Missis- sippi State Chemical Laboratory (Mississippi State, MS) for total As concentrations. The results from the two laboratories were very consistent. A large number of samples were analyzed by Missis- sippi State Chemical Laboratory. Arsenic concen- trations in the extracts were determined using graphite furnace atomic absorption spectroscopy (GFAAS)(Perkin Elmer, Norwalk, CT) at 193.7 nm wavelength with background correction. A mixed matrix modifier containing 0.015-mg Pd and 0.01-mg Mg(NO ) was used for each 20-ml 32 standard or sample solution (Perkin Elmer, 1995). Arsenic concentration in each soilywaste sample was analyzed in duplicate. Due to perceived relationship between P and As mobility (Peryea, 1991), Mehlich III-extractable P was also determined (Mehlich, 1982). Organic carbon in soils was measured by wet combustion (Nelson and Sommers, 1982). Iron oxides were extracted by citrate-dithionate-bicarbonate (Dixon and White, 1997) and the Fe was determined by atomic absorption spectroscopy. Soil pH was meas- ured in 1:1 soilywater ratio using a combination glass pH electrode. Particle size distribution was determined with a hydrometer (Day, 1965). 55F.X. Han et al. / The Science of the Total Environment 320 (2004) 51–61 2.3. Mineralogical analyses Quantitative analyses of minerals in clay frac- tion of soils were determined following the meth- ods of Karathanasis and Hajek (1982) and Karathanasis and Harris (1994). Samples were pretreated to remove salts, carbonates, OM and Fe oxides, and the fine clay-sized fraction (-0.2 mm) was collected by sieving and centrifugation (Jack- son, 1956; Dixon and White, 1997). Clays were saturated with Mg and K by washing with 1 M MgCl or KCl, respectively. The Mg-clay was 2 solvated with glycerol, and the K-clay was sequen- tially heated to 300 and 500 8C for 4 h before analysis by X-ray diffraction (XRD). A Philips X’Pert-MPD PW 3050 diffractometer (Philips Electronics, Almelo, The Netherlands) equipped with a ceramic long, fine focus copper anode tube was used for XRD analysis. Samples were step- scanned from 28 to 308 2u at 1 s per step with a step size of 0.038 2u. Differential scanning calor- imetric (DSC) analysis (DSC 910S, TA Instru- ments Inc., New Castle, DE) was conducted on the Mg-clays that had been equilibrated at 54% relative humidity. Magnesium-clay was first cooled to 5 8C, and then heated in covered aluminum pans from 5 to 625 8Cat108C min in N y1 2 atmosphere. An empty, covered aluminum pan was used as the reference (Karathanasis and Harris, 1994). 2.4. Correlation analysis Pearson Product Moment correlation coefficients were calculated using available software (SAS Institute Inc., 1989) for all pairs of variables: pH, organic carbon, Fe oxide, particle size distribution, Mehlich III-P concentrations, total As concentra- tions and clay mineralogical composition. The effects of soil properties on As accumulation in the amended field were estimated. 3. Results and discussion 3.1. Arsenic in amended soil and poultry wastes Total as well as water-soluble As in poultry waste samples are presented in Table 2. Total As in poultry wastes ranged from 11.1 to 36.2 mg kg with an average of 26.9 mg kg . The y1 y1 As concentrations in more than 50% of the samples analyzed were between 30 and 35 mg kg , which y1 were similar to As ranges reported by Moore et al. (1995). Total As varied with sampling location and time (Table 2). We sampled poultry waste from Marshall county, Mississippi, from 1997 to 2000 and found that As concentrations in 2000 were smaller than that in 1997 and 1998. As a comparison, As concentrations in all samples of waste were less than the permitted monthly aver- age concentration of 41 mgAs kg for land appli- y1 cation of sewage sludge (USEPA, 1994). Solubility of As in poultry waste, indicated here by water-soluble As, is linked to its mobility and toxicity in soilywater systems. Water-soluble As concentrations in wastes varied from 5.3 to 25.1 mg kg with an average of 15 mg kg over the y1 y1 period samples were collected (Table 2). This As accounted for 36–75% of measured total As (Table 2). Some 35% of the samples were in the range of 20–25 mg kg of water-soluble As. There was y1 also large variation in As concentrations among sites and among years of sampling (Table 2).In addition, water-soluble As concentrations in the wastes were correlated (r s0.78, P-0.05) with 2 total As (Fig. 1). Jackson et al. (2000) reported that most water-soluble As was in organo-arsenical forms, such as p-arsanilic acid and roxarsone. In the 10 poultry waste samples used for frac- tionation analysis, As in the water-soluble fraction was the largest fraction with an average of 47%, followed by the NH OHØHCl-extractable fraction, 2 which represented 33% of the total (Fig. 2). The NaOCl-extractable As made up 13% and the resid- ual fraction accounted for 7% of the total As (Fig. 2). The high solubility of As in poultry waste may be due to a large portion of it existing as organic species, and lack of solid-phase components, such as Fe oxides, with high binding affinity for As. It has been shown that incorporation of alum into poultry-house bedding materials significantly decreases soluble As concentrations in poultry wastes and in runoff from amended soils (Sims et al., 2001; Moore et al., 1998). Arsenic accumulation in surface soils (0–5 cm) over approximately 25 years of poultry waste 56 F.X. Han et al. / The Science of the Total Environment 320 (2004) 51–61 Fig. 1. Water-soluble vs. total As concentrations in poultry waste samples. applications is indicated by the results given in Table 3. Total As concentration in the amended soil ranged from 1.7 to 15.2 mg kg with an y1 average of 8.4 mg kg . By comparison, total As y1 in the non-amended soils was from 0.59 to 4.5 mg kg and averaged 2.68 mg kg with a stan- y1 y1 dard deviation of 1.35 mg kg . Thus, total As in y1 the amended soil was four times greater than that in the adjacent non-amended soils. If we assume recent application rates of 10 Mg ha per appli- y1 cation and two applications per year over the history of the field (Curtis, 1998), As input is estimated to be 5.3 mgAs kg in the surface soil y1 (0–20 cm). In other words, the average As input rate was approximately 0.54 kgAs ha yr , y1 y1 which is equivalent to 0.21 mgAs kg yr in the y1 y1 top 20 cm of soil. This suggests that annual As loading at current application rates is below the annual ceiling rates (2.0 kgAs ha yr ) for safe y1 y1 land application of sewage sludge (USEPA, 1994). It should be noted, however, that this field was plowed annually and therefore subject to a rela- tively high degree of erosion. This practice is typical for the region. Removal of As by eroded soil particles is unknown. Arsenic in the long-term waste-amended soils was mostly present in the residual fraction (72%), followed by NH OHØHCl-extractable fraction 2 (21%) and NaOCl-extractable fraction (6%)(Fig. 2). Compared to As distribution in poultry wastes, As in the long-term amended soil appears to be in more stable forms, resulting in decreased As bio- availability and mobility. This indicates that pos- sible quick leaching of water-soluble As into surface water probably occurs shortly after wastes are applied to fields. Jackson and Miller (2000) have shown that aryl-organoarsenical compounds are well adsorbed on amorphous Fe oxides and on goethite. Arsenic is known to become rapidly recalcitrant in soil with time after application (Onken and Adriano, 1997). At present there are no detailed studies on As distribution in poultry wastes and poultry-waste-amended soils available in the literature. However, studies on As solid- phase fractionation in soils that received long-term pesticide applications showed oxide-bound As to be the dominant fraction. It was reported that oxyhydroxides of Fe, Al, Mn are the primary solid phases influencing soil As solubility (Woolson et al., 1971; Johnston and Barnard, 1979; Livesey and Huang, 1981). However, the present study indicates that in addition to Fe-oxide-bound As, the residual As is a major solid-phase fraction in amended soils. 3.2. Spatial heterogeneity Results of spatial measurements of total As in the waste-amended field indicate extensive hetero- 57F.X. Han et al. / The Science of the Total Environment 320 (2004) 51–61 Fig. 2. Comparisons of distributions of As among solid-phase components in poultry waste and long-term poultry-waste-amended soil. Table 3 Total As concentrations and As fractionation in poultry wastes and the surface layer of a long-term poultry-waste-amended and a non-amended soil Soilypoultry wastes Depth n a Mean Standard deviation Maximum Minimum (cm) (mg kg ) y1 Amended soil Total As 0–5 66 8.4 3.5 15.2 1.7 As fraction 0–20 6 Water-soluble As 0.12 0.04 0.18 0.08 Organically bound 0.99 0.644 2.5 0.28 Oxide bound 2.77 1.404 5.77 0.84 Residual fraction 9.52 2.42 12.77 5.45 Non-amended soil Total As 0–5 6 2.68 1.35 4.5 0.59 Poultry wastes Total As 16 26.9 7.8 36.1 11.1 As fraction 10 Water-soluble As 13.9 6.7 25.5 5.2 Organically bound 3.7 2.9 8.1 0.06 Oxide bound 10.7 6.7 20.1 1.6 Residual fraction 2.0 1.2 12.8 1.4 n, sample number. a geneity. This is illustrated in Fig. 3 where the lowest values were in the northeast section and the highest values of As tended to be in the southwest section of the field. On the basis of the coefficient of variation (CV), a high degree of variability in As concentrations was observed. Reasons for this variability are not obvious and reflect non-uniformity of waste applications, soil adsorption–desorption properties for As, plant uptake, slopes and others. The CV is consistent with that of soil Cd measured by Murray and Baker (1992) of 91.5%. This indicates that trace element concentrations in field soils are highly variable. Test for normality using frequency distri- butions and the Kologorov–Smirnov D-statistic suggested that As concentrations were not normal- ly distributed. Such finding has been reported by others. For example, Murray and Baker (1992) 58 F.X. Han et al. / The Science of the Total Environment 320 (2004) 51–61 Fig. 3. Spatial distribution of total As (mg kg ) in surface soils of a long-term poultry-waste-amended field. The water flows of y1 two small streams were indicated as arrows. showed that total Cd concentrations taken on a 15.2=15.2 m grid from a 2.1-ha site were nega- 2 tively skewed and approximated a lognormal dis- tribution. A histogram of As concentrations from our waste-amended field is shown in Fig. 4. It suggests that As distribution is somewhat nega- tively skewed with an apparent bimodal distribu- tion. We are not aware of such distributions for heavy metal observations at the field scale. If spatial analysis of As data shown in Fig. 3 is desired, such as in ordinary kriging, variogram analysis of the data is necessary. Semi-variogram analysis (not shown) exhibited a gradual increase and then leveling off and reaching a sill after four separation distances or lags. 3.3. Correlation with soil properties Total As concentrations in poultry-waste-amend- ed surface soils were positively correlated with clay content, Fe oxide and hydroxy-interlayer ver- miculite content, and negatively correlated with mica, quartz, silt and Mehlich III-P concentrations (Table 4). In the clay fraction, hydroxy interlay- 59F.X. Han et al. / The Science of the Total Environment 320 (2004) 51–61 Fig. 4. Frequency distribution of total As in a 0.5-ha site within a long-term poultry-waste-amended field. Table 4 Correlation analyses of total As concentrations with selected soil properties and clay mineral composition in the clay fraction (ns 60) of poultry-waste-amended field Properties As pH Organic Fe O 23 Clay Silt Sand P Mehlich HIV Kaol Mica (mg kg ) y1 carbon (%)(%)(%)(%)(%)(mg kg ) y1 (%)(%)(%) pH 0.16 Organic carbon 0.18 0.28 * Fe O 23 0.64 * 0.22 0.39 * Clay 0.60 * 0.20 0.61 * 0.70 * Silt y0.35 * y0.09 y0.49 * y0.40 * y0.69 * Sand y0.20 y0.10 y0.04 y0.25 * y0.17 y0.59 * P a Mehlich y0.31 * 0.07 y0.04 y0.23 y0.15 0.21 y0.11 HIV 0.34 * 0.21 0.30 * 0.39 * 0.51 * y0.37 * y0.08 y0.02 Kaol 0.14 0.14 0.14 0.20 0.17 y0.16 0.03 y0.05 y0.03 Mica y0.52 * y0.06 y0.35 * y0.47 * y0.61 * 0.72 * y0.29 * 0.42 * y0.52 * y0.27 * Quartz y0.52 * y0.20 y0.42 * y0.53 * y0.58 * 0.62 * y0.18 0.43 * y0.45 * 0.09 0.70 * P , HIV and Kaol represent Mehlich P in the soils, and HIV and kaolinite in the clay fraction, respectively. a Mehlich III Correlation is significant at P-0.05 level. * ered vermiculite (HIV) was the major mineral, followed by kaolinite and mica (Curtis, 1998, data not shown). These correlations suggest that more As may accumulate in soils with higher clay contents. Total As was negatively correlated with Mehlich III-extractable P in the poultry-waste-amended soil (Table 4). Enhanced As mobility, phytoavailability and phytotoxicity were reported in lead arsenate- contaminated soils amended with monoammonium phosphate (Peryea, 1991). Arsenic is adsorbed on soil mineral surfaces through ligand exchange with surface hydroxide or hydrated metal-oxide miner- als (Goldberg, 1986). Phosphate and AsO 3y 4 exhibit similar physicochemical behavior in soils and compete directly for specific adsorption sites in soil particles (Hingston et al., 1972; Woolson, 1983). Total P in the poultry-waste-amended sur- face soil was 2000 mg kg and Mehlich III-P y1 was 500 mg kg (Curtis, 1998). Thus, As solu- y1 bility and mobility in both waste and waste- amended soils may be enhanced by these high 60 F.X. Han et al. / The Science of the Total Environment 320 (2004) 51–61 concentrations of P. Similarly, Cox et al. (1996b) showed that addition of As increased solution P concentrations in the soil. 4. Conclusions At the current application rates, arsenic accu- mulated over 25 years of poultry waste applica- tions is estimated at 5.9 mgAs kg in the surface y1 20 cm or with an input rate of approximately 0.54 kgAs ha yr . This annual As loading is much y1 y1 lower than the rates established by USEPA for land application of sewage sludge. Moreover, arsenic in the amended soil was mainly in the residual fraction (72% of total), which is the least susceptible fraction to runoff losses as soluble As or downward movement. However, since As in the applied poultry waste was primarily water-soluble (5.3–26 mg kg ), representing 36–75% of total y1 As, an excessive application of poultry wastes per time could release soluble As from amended fields. 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