RESEARCH ARTICLE

Degree of phosphorus saturation of an Oxisol amendedwith biosolids in a long-term field experiment

Luís Reynaldo Ferracciú Alleoni &Antonio Rodrigues Fernandes & Murilo de Campos

Received: 22 May 2013 /Accepted: 16 December 2013# Springer-Verlag Berlin Heidelberg 2014

Abstract When applied to agricultural soils, phosphate fer-tilizers and the mineral or organic compounds present in solidand/or liquid waste may raise phosphorus (P) content andincrease soil P saturation. The degree of phosphorus saturation(DPS) is a good indicator of potential P loss from agriculturalsoils. The purpose of this study was to calculate the DPS ofsamples from an Oxisol amended for 5 years with biosolidsand mineral fertilizer. DPS was calculated based on P, iron,and aluminum extracted by ammonium oxalate and oxalicacid (DPSox) or by Mehlich-1 solution (DPSM1). Treatmentsincluded NPK mineral fertilization (175 kg ha−1 of P), B1=19.02 t ha−1 of biosolids (350 kg ha−1 of P), B2=38.17 t ha−1

of biosolids (703 kg ha−1 of P), B3=76.26 t ha–1 of biosolids(1,405 kg ha−1 of P), and a control (no P added). Water-extractable P (WEP) was also measured. Critical levels ofDPSox and DPSM1 (21 and 24 %, respectively) were onlyachieved in the topsoil (0–0.1 m) at the highest biosolid dose.Concentration ofWEP was positively correlated to DPSox andDPSM1. The DPSM1 method may be an alternative to DPSoxfor assessing the environmental risk of P loss from soil intosurface runoff.

Keywords Tropical soil . Fertility . Organic waste .

Water-extractable P . Oxalate . Mehlich-1

Introduction

When applied to agricultural soils, phosphate fertilizers andthe mineral or organic compounds present in solid and/orliquid wastes may raise phosphorus (P) content and increasesoil P saturation. The P adsorption capacity of soil is con-trolled by several attributes, such as the amount and the type ofclay minerals, organic matter (OM), iron (Fe) and aluminum(Al) oxides, and soil pH (Souza et al. 2010). Sandy soils withlow-OM content have a low P adsorption capacity. Therefore,they have a high potential for P saturation and for runoff lossesas well as leaching, thus contributing to the degradation ofwater quality (eutrophication) (Alleoni et al. 2008). By con-trast, clayey and acidic soils with high contents of OM and/orFe and Al oxides adsorb larger amounts of P and are conse-quently less susceptible to P losses (Elliott et al. 2002;O’Connor et al. 2005).

The use of biosolids in agriculture is a viable alternative forimproving soil quality due to its low cost and high concentra-tions of some plant nutrients such as P. However, their use isallowed only when it does not cause environmental pollution(Silveira et al. 2003). The concentration of P in biosolids isrelated with biosolid chemical composition (Elliott et al.2002), whereas the solubility of P varies as a function ofconcentration of noncrystalline Al and/or Fe compounds insewage sludge treatments (Ippolito et al. 2011). In highlyweathered soils as Oxisols and Ultisols, P deficiency problemsare common because of the strong acidic reactions and abun-dance of Al and Fe ions (Hoseini and Taleshmikaiel 2013).Under the Brazilian legislation, the total N biosolids content isused to calculate the amount of biosolids that can be legallyapplied to soils, taking into account the N requirement ofcrops as well as the average N mineralization rate in thebiosolids. The total P concentration in biosolids ranges from20 to 40 g kg−1 in the USA (O’Connor et al. 2004) and from13 to 31 g kg−1 in Brazil (Bettiol and Ghini 2011). Thus, the

Responsible editor: Zhihong Xu

L. R. F. Alleoni (*) :M. de CamposDepartment of Soil Science, ESALQ, University of São Paulo, C.P.09, Piracicaba, SP 13418-900, Brazile-mail: alleoni@usp.br

A. R. FernandesInstitute of Agricultural Sciences, Rural Federal University ofAmazon, Belém, PA 66077-530, Brazil

Environ Sci Pollut ResDOI 10.1007/s11356-013-2469-0

legally recommended rates usually exceed the P requirementsof crops (Agudelo et al. 2011). As a consequence, P mayaccumulate in the soil, increasing the risks of environmentalpollution (Butler and Coale 2005, Mostaghimi et al. 1988).

The degree of phosphorus saturation (DPS) is a goodindicator of potential P loss from agricultural soils(Hooda et al. 2000; Casson et al. 2006; Guérin andParent 2007) and is calculated based on the Fe, Al,and P content of soils, as extracted with ammoniumoxalate. The extraction of Fe, Al, and P contents insoils is not a common procedure in Brazilian laborato-ries. These elements are commonly extracted by acidicsolutions such as Mehlich-1 (0.05 M HCl+0.0125 MH2SO4) and Mehlich-3 (0.2 M CH3COOH+0.25 MNH4NO3+0.015 M NH4F+0.013 M HNO3+0.001 MEDTA). Because it is easily carried out and widelyaccessible, the Mehlich-1 procedure is an attractive optionfor obtaining the nutrient concentrations needed to calculatethe DPS in soils that have been amended with biosolids andmineral and organic fertilizers. In soils in the USA that hadbeen amended with manure, Nair et al. (2004) observed thatthese three extractants were effective at calculating DPS.

Soils with low capacity to retain additional P can quicklypresent high-DPS environmental problems. They are thusmore susceptible to P loss via leaching and runoff than soilswith a low DPS (Hooda et al. 2000; Pautler and Sims 2000).According to Pautler and Sims (2000), soils with DPS greaterthan 20% face a higher risk of P loss. In Ultisols, Entisols, andAlfisols amended with biosolids of different compositions,Maguire et al. (2000) observed a positive correlation betweensoluble P and the P extracted with ammonium oxalate (r=0.93), the P extracted with Mehlich-1 solution (r=0.89), andDPS (r=0.80).

The prevalent soils in Brazil are highly weathered withdifferent textures and high contents of Fe and Al oxides.Although the P retention capacity in these soils is usuallyhigh, there are few studies assessing the DPS of soils in thehumid tropics that have been amended with biosolids in long-term field experiments. The results of such soils should becompared carefully with soils from temperate regions sincethey have different properties and characteristics.

The purpose of this study was to determine the DPS,calculated from P, Fe, and Al contents obtained by usingsolutions with ammonium oxalate and Mehlich-1, from sam-ples of an Oxisol amended with biosolids and mineral fertil-izers for 5 consecutive years. Our hypothesis is that evenwhenapplied at rates higher than the recommended (based on Ncontents), the addition of biosolids will not increase the DPSof soils from the humid tropics. It is not expected to observeenvironmental problems such as P losses via leaching andrunoff because of the high capacity of such Oxisols to retainP, as a result of the type of clay minerals and iron andaluminum oxides contents.

Materials and methods

Samples of a clayey Rhodic Hapludox (Soil Survey Staff2010) were collected in a field experiment carried out inJaguariuna, State of São Paulo, Brazil (22°41′S, 47°W,570 m above sea level). The results of soil chemical analysescarried out prior to the experiment are shown in Table 1. Thefollowing analysis were performed: pH in water (soil/solutionratio of 1:2.5), exchangeable Ca and Mg extracted with 1 MKCl and determined by atomic absorption spectrometry (vanRaij et al. 2001), exchangeable Al extracted with 1MKCl anddetermined by titration with 0.025 M NaOH (van Raij et al.2001), extractable K extracted with the Mehlich-1 solution(Mehlich 1953) and determined by flame photometry, andexchangeable P extracted with an anionic exchange resin(van Raij et al. 1986) and determined colorimetrically(Murphy and Riley 1962). Organic carbon (OC) content wasdetermined by the potassium dichromate/sulfuric acid diges-tion method (Nelson and Sommer 1982). Aluminum and Feoxides were extracted with 9 M H2SO4 using a 1:1soil/solution ratio (Embrapa 1997). Particle size was deter-mined by chemical dispersion using the densimeter method(Gee and Bauder 1986).

Biosolids classified as class B according to the US Envi-ronmental Protection Agency (EPA) 40 CFR part 503 stan-dard were obtained from the sewage treatment station of thecity of Franca, São Paulo, Brazil. The biosolids were analyzedfor OC with the potassium dichromate/sulfuric acid digestionmethod (Nelson and Sommer 1982); total Kjeldahl N; ammo-nium N and nitric N via extraction with 2 M KCl (Bremnerand Mulvaney 1982); and total P, K, Ca, Mg, Al, and Fe byinductively coupled plasma-optical emission spectrometer(ICP-OES) after extraction with aqua regia (HNO3:HCl, 1:3,v/v) according to EPA 3051 method (United StatesEnvironmental Protection Agency USEPA 1995). Soil mois-ture was determined by mass loss at 60 °C (Table 2).

A completely randomized block design field study wasestablished with three replicates of five treatments. Individualplots measured 20×10 m had 12 rows of plants each, withborders of at least 5 m on each side. Borders were planted withBrachiaria decumbens grass. The treatments were control (nobiosolids added), Min=mineral fertilization with NPK, B1(rate of biosolids based on the total N of the biosolids), B2(twice the amount of biosolids applied in B1), and B3 (fourtimes the amount of biosolids applied in B1) (Table 3). Themineral fertilization consisted of urea (N source), triple super-phosphate (for P), and KCl (for K). The biosolids rate wasbased on the N requirement of the crop, assuming that 30% ofthe nitrogen is mineralized each year (Cetesb 1999). Amountsof N added in each year were (in kg ha−1), in 1999, B1=405,B2=810, B3=1,260; in 2000, B1=260, B2=520, B3=1,040;in 2001, B1=220, B2=440, B3=880; and in 2002, B1=240,B2=480, B3=960.

Environ Sci Pollut Res

In 1999, soils were amended twice before growing corn(Zea maysL.) during the austral summer and winter (Table 3).The requirement of NPK of Min treatments was calculatedbased on guidelines for corn fertilization in the State of SãoPaulo, Brazil (van Raij and Cantarella 1997).

In all years, corn was grown under standard no-irrigationpractices. After each harvest, crop waste was removed fromthe plots. The biosolids were broadcast applied and incorpo-rated into the soil to a depth of 0.2 m, using a rotary hoe,3 days before the corn was sown. The doses of N were appliedon two separate occasions: a portion during sowing and therest broadcast and incorporated with a cultivator 45 days aftersowing. Doses of K as KCl and P as triple superphosphatewere applied at sowing. Between the 2nd and the 3rd crops,soil acidity was corrected to a pH close to 5.7, using dolomiticlimestone, based on neutralization curves for samples fromeach plot. Bettiol and Ghini (2011) measured the pH of soilsamples in 2002 and did not find significant differencesamong the treatments (5.8 for the control, 5.5 for Min, 5.9for B1, 5.5 for B2, and 5.7 for B3).

During the experiment, from 1999 to 2002, soil compositesamples (from five subsamples) were collected in all plots at

the 0–0.1, 0.1–0.2, and 0.2–0.4-m depth and analyzed(Table 4). Soil samples were air-dried, sieved (2 mm), placedin plastic bags, and stored in the laboratory. Oxalate extract-able P, Al, and Fe (Ox-P, Ox-Al, and Ox-Fe, respectively)were extracted with 0.1 M oxalic acid+0.175 M ammoniumoxalate, pH=3 (Schoumans 2000). Air-dried soil (1.25 g) andextraction solution (25 mL) were shaken in 50-mL tubes in thedark for 4 h. The mixture was then filtered through a 0.45-μmfilter. Mehlich-1 extractable P, Al, and Fe (M1-P, M1-Al, andMl-Fe, respectively) were determined using a soil-to-solutionratio of 1:10 (2.5 g:25 mL; Mehlich 1953), shaken for 5 min,and left to settle overnight in order to avoid the filtration.Then, the supernatant was collected (Embrapa 1997). Oxalateand Mehlich-1 P, Al, and Fe were determined using an ICP-OES.Water-extractable P (WEP) was determined using a soil-to-water ratio of 1:10 (2 g of soil+20 mL of water). Aftershaking for 1 h and centrifugation, supernatants were collect-ed (Self-Davis et al. 2000), and P was determined colorimet-rically (Murphy and Riley 1962).

The degree of phosphorus saturation values were calculat-ed using the oxalate andMehlich-1 extractable P, Al, and Fe inthe following equation: DPS=(P/α [Al+Fe])×100, whereDPS=degree of phosphorus saturation in % (Nair et al.2004); α=0.5 (van der Zee and van Riemsdijk 1988).

Analysis of variancewas performed to compare the mineraltreatment with the control and with the biosolids treatments.Correlations and regressions among the concentrations of P,Al, and Fe extracted by oxalate or Mehlich-1, WEP, and DPSwere evaluated by regression analysis for soil samples collect-ed in the three soil depths (0–0.1, 0.1–0.2, and 0.2–0.4 m).

Results and discussion

Ox-P concentrations were higher than Ml-P concentrations,regardless of treatment and soil depths (Table 5). Concentra-tions of Ox-P higher thanMl-Pwere also found by Elliott et al.(2002) when evaluating the use of waste water treatment for Psolubility and leaching through leaching columns undergreenhouse conditions. The two extractions act through dif-ferent mechanisms and apparent different reaction times.While the Mehlich extract works mainly by dissolution and

Table 1 Chemical characteristics of the Oxisol before amendment with biosolids

Depth (m) pH H2O OCa

(g kg−1)Res-Pb

(mg kg−1)Cac

(mmolckg−1)

Mg c K c Al+3 c CEC Fe2O3d

(g kg−1)Al2O3

d Clay

0–0.1 5.2 26 3.9 27 12 1.4 1.9 68.6 59 167 400

0.1–0.2 5.6 26 2.8 32 14 1.4 2.0 73.0 62 169 460

0.2–0.4 5.3 16 5.9 13 6 1.0 9.1 50.1 62 186 520

aOC organic carbon; bRes-P P extracted with ion exchange resin; cCa, Mg, and Alwere extracted with 1 M KCl, and K extracted with the Mehlich-1solution; dFe2O3 and Al2O3 iron and aluminum oxides extracted with 9 M sulfuric acid

Table 2 Some attributes of the biosolids used during the experiment(1999 to 2002)

Attribute Unit (dry basis) 1999a 1999b 2000 2001 2002

pH 6.3 6.4 5.4 8.9 8.3

Organic carbon g kg−1 305.1 374.0 382.4 370.9 475.4

Total N g kg−1 56.4 67.5 68.2 49.7 57.7

Ammonium N g kg−1 4.7 9.3 10.3 – –

Nitric N mg kg−1 37.0 122 101 – –

Phosphorus g kg−1 16.0 31.3 12.9 13.8 27.3

Potassium g kg−1 1.0 1.0 1.0 1.5 1.0

Calcium g kg−1 29.2 16.8 24.8 13.3 11.5

Magnesium g kg−1 2.2 2.5 2.2 2.7 5.0

Aluminum g kg−1 32.6 33.5 23.3 18.2 21.7

Iron g kg−1 33.8 31.7 24.2 39.4 64.9

Humidity % 83.0 82.4 82.7 74.6 78.5

aMaize–summer;bMaize–winter;

Environ Sci Pollut Res

disequilibrium based on a pH change, oxalate is very good atligand exchange combined with ligand dissolution.

The lower concentrations ofMl-P compared to Ox-P is alsoexplained by the high soil buffering capacity. The initial 1.2pH of the Mehlich-1 solution was rapidly increased andreached values close to the soil pH, as sulfate was rapidlyadsorbed by the soil sites not yet occupied by P, thus causingless P extraction (Novais and Smyth 1999). In addition, bio-solids had high concentrations of Fe and Al oxides as ironsalts (e.g. FeCl3) and aluminum sulfate [Al2(SO4)3] that areadded to the treatment water to remove suspended sedimentand dissolved organic carbon (DOC). The formed flocculentcontains high levels of unutilized amorphous Al or Fe, whichhave the potential to bind soluble P (O’Rourke et al. 2012). Atlow pH, the amount of P bound to these elements increasesand may form Fe and Al phosphates depending on the

concentration of Fe and Al in solution (Liang et al. 2010)and that are easily extracted by oxalate. Higher concentrationsof these elements (mainly Fe) extracted by oxalate as com-pared to Mehlich-1 were also reported by Maguire and Sims(2002) for five US soils.

Concentrations of Ox-P and Ml-P increased linearly withadded P. Ox-P and Ml-P concentrations tended to decreasewith soil depth. In the topsoil (0–0.1 m), Ox-P concentrationsreached 403 mg kg−1, which corresponded to 63 % of the Ox-P extracted from the three soil depths. Extractable Ml-Preached 81 mg kg−1 in the topsoil and represented about65 % of the Ml-P extracted from the three soil depths. Pautlerand Sims (2000) classified as excessive rates >50 mg P kg−1

extracted with Mehlich-1 solution in Mid-Atlantic US soils.The reduced P content of subsurface soil depths is attributed to(i) low mobility of this element due to the high number of Padsorption sites in the surface, (ii) high concentrations of Fe(25.2 g kg−1) and Al (31.4 g kg−1) in the biosolids, and (iii)high concentrations of soil clay and Fe and Al oxides, whichcan adsorb P and reduce leaching (O’Connor et al. 2005;Hoseini and Taleshmikaiel 2013).

The concentrations of Fe and Al extracted by oxalate andby Mehlich-1 increased linearly with the P rates of addedbiosolids (Table 5). Fe and Al contents did not vary markedlywith soil depth. It should be pointed out that the concentra-tions of Fe and Al oxides were naturally high in the clayfraction of this soil (Table 1). Average values of Fe and Alextracted by oxalate may be considered low when comparedto those reported by Penn and Sims (2002) in silty-clayey-loamy and sandy-loamy soils, with medium- and low-OMcontent, respectively. These soils were cropped with alfalfaand corn, wheat, and soybean in rotation and amended with

Table 4 Linear regression for P saturation index calculated from P, Al,and Fe contents extracted with oxalate and Mehlich-1 in soil samples ofan Oxisol amended with P from biosolids and mineral fertilizer

Soil layer(m)

Oxalate extractable Mehlich-1 extractable

0–0.1 y=0.004+0.0005×R2=0.98**

y=0.02+0.0002×R2=0.73**

0.1–0.2 y=0.009+0.0001×R2=0.97**

y=0.013+0.0002×R2=0.97**

0.2–0.4 y=0.003+7E-05×R2=0.84**

y=0.005+1E-04×R2=0.89**

0–0.4 y=0.005+0.0002×R2=0.97**

y=0.013+0.0002×R2=0.98**

**significant at p=0.01

Table 3 Rates of biosolids; mineral P, N, and K2O; total biosolids (BT); and total P (TP) applied

Treatments Biosolids BT TP

1999a 1999b 2000 2001 2002(kg ha−1, dry basis) (kg ha−1)

Control – – – – –

Min -P 35 39 39 31 31 175

-N 18+34 18+72 18+82 20+70 20+80

-K2O 64 72 72 56 70

B1 3,014 3,507 3,766 4,432 4,300 19,019 350

-K2O 28 33 58 96 63

B2 6,068 7,008 7,533 8,863 8,700 38,172 703

-K2O 25 28 45 90 54

B3 12,057 14,017 15,065 17,726 17,400 76,265 1,405

-K2O 17 23 18 75 36

a Initial cultivationb cultivation of second maize crop

Control 0 of P; Minmineral fertilization; B1, B2, and B3 biosolids doses required to provide one, two, and four times the crop N needs, respectively

Environ Sci Pollut Res

biosolids for 4 and 7 years, respectively. Conversely, Elliottet al. (2002) found lower Fe and Al values in sandy soilsamended with biosolids than those obtained in our study.

The concentration of WEP increased linearly with increas-ing P rates in biosolids and mineral fertilizer only in the uppersurface layer (0–0.1 m; Fig. 1). When the biosolids phosphatecomes in contact with the soil, various reactions occurs, andthe dissolution of the biosolids increases the soluble phos-phate in the soil solution, allowing the movement of thedissolved phosphate in a short distance away from thebiosolid's particle. Movement is slow because most of thephosphate will react with the minerals, mainly iron and alu-minum oxides. Wang et al. (2008) also observed increases inWEP concentrations in samples of medium-textured Chinesesoils amended with biosolids. Other authors have also

demonstrated increased P concentrations in soils after theaddition of biosolids (Maguire et al. 2000; Penn and Sims2002) and manure (Nair et al. 2004). Bertol et al. (2010)observed an increase in the total P concentrations in soilsand also an increase of P losses in surface runoff after theapplication of swine manure for 5 years in an Oxisol under ano-till system.

Regardless of the method of P extraction, DPS (Fig. 2)increased linearly with increasing P rates in biosolids andmineral fertilizer. The highest values were found in the upperlayer (0–0.1 m) after the addition of the highest rate of bio-solids. The increase of DPS values with increasing rates of Paddition indicates saturation of P sorption sites. Similar obser-vations have been reported by Iyamuremye et al. (1996),Whalen and Chang (2002), Siddique and Robinson (2004),

Table 5 Contents of P, Al, and Fe extracted with oxalate and Mehlich-1 in samples of an Oxisol amended with P from biosolids and mineral fertilizers

Treatment Oxalate Mehlich-1

P(mg kg−1)

Al(mmol kg−1)

Fe P(mg kg−1)

Al(mmol kg−1)

Fe WSP(mg kg−1)

0–0.1 m

Control 26.2cA 86.9bB 26.8bA 1.6cA 25.0abC 0.7dA 0.6cA

Min 51.6cA 86.0bB 26.7bA 4.8cA 25.3abC 0.8cdA 1.1bcA

B1 121.5bA 89.7abB 27.9abA 10.3bcA 23.0bB 0.9cA 0.7c

B2 164.8bA 90.5abB 29.8abA 17.4bA 27.2aB 1.2bA 1.2b

B3 402.7aA 93.5Aa 31.5aA 80.6aA 25.9abB 1.5 aA 2.9a

Significance

Linear ** ** ** ** ns ** *

Quadratic ** ** ** ** ns ** *

0.1–0.2 m

Control 34.5bA 86.9bB 26.9bA 2.7bA 30.2aB 0.7bA 0.9aA

Min 62.3bA 88.3bB 26.2bA 6.1bA 29.4abB 0.8abA 1.3aA

B1 55.5bB 89.1bB 27.0abA 4.9bB 24.6abB 0.8abA 1.3a

B2 80.6bB 90.2bB 28.5abA 7.7bB 27.4abB 0.9abA 0.8a

B3 164.3aB 97.4aA 29.5aA 33.5aB 24.4bB 1.1aA 1.6a

Significance

Linear ** * ** ** * * ns

Quadratic ** * ** ** * * ns

0.2–0.4 m

Control 15.6bB 95.1aA 25.7bA 1.4bA 37.4aA 0.8aA 0.2aB

Min 25.2abB 99.9aA 27.6abA 2.3bB 35.4aA 0.9aA 0.3aB

B1 29.8abC 96.2aA 27.6abA 2.2bB 33.7abA 0.9 aA 0.1a

B2 25.5abC 99.6aA 27.8abA 2.4bC 34.3abA 0.8 aA 0.2a

B3 76.8aC 98.0aA 29.5aA 10.6aC 30.2aA 1.0aA 0.3a

Significance

Linear ** ns * ** * ns ns

Quadratic ** ns * ** * ns ns

Control0 of P;Minmineral fertilization; B1, B2, and B3biosolids doses required to provide one, two, and four times the crop N needs, respectively. **, *,and ns significant at p=0.01, significant at p=0.05, and nonsignificant, respectively

Means followed by the same letters within each row (lower case letters) and within each column for each depth (capital letters) are not different (p<0.05)

Environ Sci Pollut Res

and Casson et al. (2006). Recently, Abdala et al. (2012) ob-served that the maximum adsorption capacity of phosphorus in

an Ultisol amended with high rates of poultry litter was severe-ly diminished because of the covering of colloid adsorptionsurfaces with organic constituents.

The increase in the DPS may be related to the reduction ofthe number of P adsorption sites because of the action oforganic acids produced by the mineralization of OM. Suchorganic acids compete for sorption sites or form complexeswith Fe, Al, or Ca (Iyamuremye and Dick 1996; Whalen andChang 2002; Sekhon and Bhumbla 2013), reducing the bind-ing energy with P. Large humic molecules can adhere to thesurfaces of clays and metal hydrous oxide particles, maskingthe P-fixation sites and preventing them from interacting withP ions in solution (Wang 2010). In sandy soils, which natu-rally have fewer adsorption sites, high-P concentrations willresult when large amounts of P are applied via organic ormineral fertilizers. Under such conditions, the risk that P willbe leached and enter groundwater is high (Lu and O’Connor2001; Alleoni et al. 2008).

A major pathway for P loss from Oxisols is going to bebound to sediment in surface runoff Shigaki et al. (2006). This

(a)

ox = 0.99 + 0.014x R² = 0.98**

M1 = 1.89 + 0.016x R² = 0.98**

0

5

10

15

20

25

30

0 300 600 900 1200 1500

P (kg ha-1)

DP

S (

%)

(c)

ox = 0.72 + 0.002x R² = 0.85**

M1 = 0.92 + 0.003x R² = 0.89**

0

1

2

3

4

5

6

0 300 600 900 1200 1500

P (kg ha-1)

DP

S (

%)

(b)

ox = 1.97 + 0.0044x R² = 0.95**

M1 = 2.73 + 0.006x R² = 0.96**

0

2

4

6

8

10

12

14

0 300 600 900 1200 1500

P (kg ha-1)

DP

S (

%)

(d)

ox = 1.23 + 0.007xR² = 0.97**

M1 = 1.85 + 0.008x R² = 0.98**

0

2

4

6

8

10

12

14

16

0 300 600 900 1200 1500

P (kg ha-1)

DP

S (

%)

Fig. 2 Relationships between the rates of the total P added to an Oxisolamended with biosolids and mineral fertilizer, and the degree of phos-phorus saturation (DPS) calculated from the P contents extracted either

with oxalate (Ox-P) or Mehlich-1 (M1-P) in the a 0 to 0.1 m, b 0.1 to0.2 m, c 0.2 to 0.4 m, and d 0 to 0.4 m soil depths. Double asterisksdenotes significance at p=0.01

Y = 0,4758 + 0,0016x R² = 0,87**

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

0 300 600 900 1200 1500

P (kg ha-1)

WE

P (

mg

kg-1

)

Fig. 1 Relationship between the rates of the total P added to an Oxisolamendedwith biosolids andmineral fertilizer andwater-extractable phospho-rus (WEP) concentration. Double asterisks denotes significance at p=0.01

Environ Sci Pollut Res

has important implications to the type of measures needed toaddress P-related water quality problems, but it is important topoint out that for some cases and to make environmentalstatements, probably more information is needed. Phosphorusconcentrations in soil solution critical for plant growth rangefrom 0.2 to 0.3 mg L−1 (Shigaki et al. 2006).

Although DPS increased as the applied P rates increased,the values remained relatively low; i.e., they did not seem topose a risk of environmental pollution, except in the topsoil(0–0.1 m) at the rate of 76,265 kg ha−1 of biosolids. At thisrate, DPS reached 21%when calculated from extractions withoxalate and 24 % by Mehlich-1 extraction. These results areclose to those obtained by Abdala et al. (2012) in samplesfrom an Ultisol amended with poultry litter (DPS=23 %).Abdala et al. (2012) suggest that this value could be used to

practically monitor whether tropical soils have reached a levelof P loading to pose an environmental risk of P losses fromsoil to surface and groundwaters. According to Nair et al.(2004), P may have a negative environmental impact on waterquality in soils with DPS values greater than 20 %. Theconcentrations of M1-P, M1-Al, and M1-Fe were positivelycorrelated with that of Ox-P, Ox-Al, and Ox-Fe, respectively(Fig. 3b). The same was observed for DPSOX and DPSM1

(Fig. 3a). Therefore, Mehlich-1 was as effective as oxalate atextracting the P, Al, and Fe used to calculate the DPS in thisOxisol amended with biosolids and mineral fertilizer. Howev-er, it should be taken into account that DPS values obtainedwith Mehlich are conservative since higher values will beobtained with lower P rates. Once the researches regardingthe use of biosolids under tropical conditions are scarce, this

(a)Y = 0.65 + 1.17x

r = 0.99**

0

5

10

15

20

25

30

0 5 10 15 20

DPSOx (%)

DP

SM

1 (%

)

(b)

r = 0.96**

r = 0.47*

r = 0.64**

0

450

900

1350

1800

2250

2700

0 150 300 450 600 750 900 1050

M1-P, M1-Al or M1-Fe (mg kg-1)

Ox-

P,O

x- A

l or

Ox-

Fe

(m

g k

g-1)

P Al Fe

Fig. 3 Correlations between aP, Al, and Fe extracted with oxalate (Ox-P,Ox-Al, and Ox-Fe, respectively) and Mehlich-1 (M1-P, M1-Al, and M1-Fe, respectively) and between b the degree of P saturation calculated by

Mehlich-1 (DPSM1) extraction and oxalate (DPSOx) extraction for anOxisol (0 to 0.4 m) amended with biosolids and mineral fertilizer

(a)Y = 0.89 + 5.12x

r = 0.72**

0

5

10

15

20

25

30

-1 0 1 2 3

WEP (mg kg-1)

DP

SM

1 (%

)

(b)Y = 0.62 + 4.12x

r = 0.68**

0

5

10

15

20

25

-1 0 1 2 3 4

WEP (mg kg-1)

DP

SO

x (%

)

Fig. 4 Correlations between water-extractable P (WEP) and the degree of P saturation calculated from P, Al, and Fe contents extracted by aMehlich-1(DPSM1) and b oxalate (DPSox) for an Oxisol amended with biosolids and mineral fertilizer. Double asterisks denotes significance at p=0.01

Environ Sci Pollut Res

might be good for a more conservative risk assessment on Pmobilization.

The degree of P saturation is a good indicator of potential Ploss in agricultural soils (Hooda et al. 2000; Maguire and Sims2002; Nair et al. 2004; Börling et al. 2004; Wang et al. 2010).This index can be routinely obtained by extracting P, Fe, andAl with Mehlich-1 solution, since this method is alreadycommonly used in most soil analysis laboratories, has lowcost, and is relatively easy compared to oxalate. Ghosh et al.(2011) observed a high correlation (p<0.05) between DPSM1

and M1-P in Brazilian Oxisols suggesting that M1-P couldprovide reasonable estimates of DSPM1 in Oxisols amendedwith manure.

Water-extractable P was positively correlated with DPS(p<0.05, Fig. 4). This is similar to the results reported byMcDowell and Sharpley (2001) and Alleoni et al. (2008).Increasing rates of added P from biosolids and mineral fertil-izer also increased the DPS, as calculated via Mehlich-1 oroxalate extraction (Fig. 2). Highly significant positive corre-lations between WEP and DPS were also observed by Nairet al. (2004) in Entisols and Alfisols, by Casson et al. (2006) inHaplustolls, and by Abdala et al. (2012) in Ultisol that allreceived organic wastes. WEP values above 5.5 and2.0 mg kg−1 for DPSM1 and DPSOx, respectively, indicateincreased risk of P loss. Likewise, Ghosh et al. (2011) evalu-ated the relationship between P soil test (PST), water soluble P(WSP), and DPS in Oxisols of Brazil amended with poultrylitter and observed a DPSM1=16.5 %, which represents a M1-P of 44.5 mg kg−1 of P. Values of DPS above 16.5 % werecorrelated to a rapid increase in WSP (change point), and alikelihood of a negative impact of soil P on surface waterquality was observed.

The correlation between M1-P and DPSOx was highlysignificant, demonstrating the high sensitivity of Mehlich-1compared to oxalate, although the concentration of M1-P wasmuch lower (Fig. 5). In a study examining the DPS of sandysoils that had been amended with manure over several years ornot amended at all, Nair et al. (2004) also observed a positivecorrelation between DPSOx and DPSM1. This positive corre-lation suggests thatM1 can substitute oxalate to calculate DPSand may in fact be more advantageous. The ammoniumoxalate extraction method requires that extraction should takeplace in the dark and that elemental determination be carriedout in less than a week (Schoumans 2000), which makes itdifficult for routine laboratory use.

Conclusions

Amendment with biosolids increased the DPS of an Oxisol.The annual application of biosolids for 5 years, up to threetimes the recommended rate based on N requirements forcorn, led to P saturation levels that were potentially hazardousto the environment in the topsoil surface (0–0.1 m).

The degree of P saturation calculated from P, Al, and Feconcentrations extracted with Mehlich-1 solution was posi-tively correlated with the DPS calculated from P, Al, and Feconcentrations extracted with oxalate in Brazilian clayeyOxisols with high contents of iron oxides. DPS calculatedvia Mehlich-1 may thus offer an alternative for the assess-ments of the environmental risk caused by excess P inOxisols, since Mehlich-1 is used in most soil analysis labora-tories. The concentrations of WEP were low and correlatedpositively with DPS, regardless of the P, Al, and Fe extractionmethod.

Acknowledgments Thanks are to Dr. Wagner Bettiol, for allowing theauthors to collect soil samples in a field experiment at Embrapa Environ-ment, in Jaguariuna, Sao Paulo state, Brazil. Emprapa is the AgriculturalResearch Corporation of the Brazilian Ministry of Agriculture, Livestockand Food Supply.

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