dynamics of carbon, nitrogen and phosphorus in soil amended with irradiated, pasteurized and limed...

Dynamics of carbon, nitrogen and phosphorus in soil amendedwith irradiated, pasteurized and limed biosolids

Olivia Franco-Hern�aandez a, Alba Natalia Mckelligan-Gonzalez a,Ana Maria Lopez-Olguin a, Fabiola Espinosa-Ceron a,

Eleazar Escamilla-Silva b, Luc Dendooven a,*

a Laboratory of Soil Ecology, Department of Biotechnology and Bioengineering, Cinvestav, Av. Instituto Polit�eecnico Nacional 2508,Apartado Postal 14740, C.P. 07000 M�eexico DF, Mexico

b Departamento de Qu�ıımica, Instituto Tecnol�oogico de Celaya, Celaya, Gto., C.P. 38010, Mexico

Received 27 August 2001; received in revised form 1 August 2002; accepted 4 August 2002

Abstract

Sewage biosolids contain high concentrations of pathogens, which limits their use as soil amendment. This study investigated

how application of lime (Ca(OH)2), irradiation, or pasteurization reduced pathogens in biosolids and how its application affected

soil characteristics. A soil sampled outside the canopy of Mesquite trees (Prosopis laevigata) and from a pasture at Lerma (Mexico)

was amended with treated or untreated biosolids, characterized and incubated aerobically while dynamics of carbon (C), nitrogen

(N) and phosphorus (P) were monitored. Heavy metals concentrations in the biosolids were low, so it was of excellent quality

(USEPA). The amount of pathogens in the biosolids made it a class ‘‘B’’ (USEPA) which can be used in forests. Only irradiation

sufficiently reduced faecal coliforms to make it a class ‘‘A’’ biosolids without restrictions in application. C mineralization increased

significantly when biosolids were added, but not concentrations of available P ðP < 0:05Þ. Ammonium (NHþ4 ) concentrations in soil

amended with biosolids were higher compared to unamended soil, but not the concentrations of nitrate (NO�3 ) except when biosolids

treated with Ca(OH)2 was added to the Lerma soil.

� 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Pathogens; Sewage sludge; Irradiation; Pasteurization; Lime; Biosolids; Dynamics of N, C and P

1. Introduction

In recent years industrial wastes and municipal

wastewater biosolids have been applied regularly to soil

(Krogman et al., 1997; Benton and Wester, 1998). Theyhave a large nutrient value, mainly as nitrogen (N),

phosphorous (P) and potassium (K) (Elliot and Demp-

sey, 1991) and their application to land is a way of using

these nutrients as fertilizers. Biosolids, however, may

contain pathogens, heavy metals and toxic organic

compounds, but may also be of high quality, depending

on the initial contaminant levels, treatment processes

applied and their efficiency compared to fertilizers. Ap-

plication rates to soil should be limited so that heavy

metals do not accumulate and end up into the food

chain or groundwater, that pathogens can not infect

people or that concentrations of toxic compounds be-

come dangerous (e.g. Eiceman et al., 1989; Guti�eerrez-Ru�ıız et al., 1995; Benton and Wester, 1998).

Biosolids derived from a wastewater treatment plant

(Empresa para la Prevenci�oon y Control de la Contami-

naci�oon del Agua, Lerma, M�eexico) contain high con-

centrations of pathogens limiting its use as fertilizer.

Different techniques can be applied to reduce pathogen

concentrations such as application of lime (Ca(OH)2),

irradiation or pasteurization. This study investigated (i)how irradiation, application of Ca(OH)2, and pasteur-

ization reduced concentrations of pathogens in the bio-

solids, (ii) how those treatments affected its physical,

chemical and biological characteristics, and (iii) how

dynamics of carbon (C), nitrogen (N) and phosphorus

(P) in two sandy clay loam soils (one under pasture and

Bioresource Technology 87 (2003) 93–102

*Corresponding author. Tel.: +52-5-747-7000x4391; fax: +52-5-747-

7002.

E-mail addresses: [email protected], lucdendo@

prodigy.net.mx (L. Dendooven).

0960-8524/03/$ - see front matter � 2002 Elsevier Science Ltd. All rights reserved.

PII: S0960-8524 (02 )00188-8

mail to: [email protected],

the other from a forest) were affected when the differentforms of biosolids were applied.

2. Methods

2.1. Origin and sampling of the biosolids

Reciclagua (Sistema Ecol�oogico de Regeneraci�oon de

Aguas Residuales Ind., S.A. de C.V.) in Lerma, State of

Mexico (Mexico) treats watewater from different sour-

ces. Ninety percent of the sewage biosolids were from

different industrial origin mainly from textile industries

and the rest from households. The waste from eachcompany must comply with the following guidelines:

biological oxygen demand (BOD) less than 1000 mg l�1,

lipids less than 150 mg l�1 and phenol less than 1 mg l�1.

The wastewater is digested aerobically in a reactor and

the biosolids obtained after the addition of a floculant

are passed trough a belt filter. The watewater is dis-

carded in the river and biosolids are incinerated. Fifty

kg of aerobically digested industrial biosolids weresampled aseptically in plastic bags after it passed

through the belt filter. A ten kg sub-sample of the bio-

solids was irradiated with 60Co (30 kGy) at ININ (In-

stituto Nacional de Irradiaciones Nucleares, Salazar,

State of M�eexico, M�eexico), 10 kg was pasteurized at 60

�C for 30 min, while 10 kg was amended with 1.25 kg

Ca(OH)2 to pH 12.

2.2. Experimental sites and soil sampling

The experimental site is located 20 km east of Dolores

Hidalgo in the state of Guanajuato, Mexico (Northern

Latitude 21� 090 Western Longitude 100� 560). The site�saverage altitude is 1920 m above sea level and it is

characterized by a semi-dry and temperate climate with

a mean annual temperature of 16–18 �C and averageannual precipitation of 500–600 mm (mainly from June

through August) (http://www.inegi.gob.mx).

The sandy clay loam soil was sampled from the 0–10

cm layer by augering with a stony soil auger diameter

seven cm (Eijkelkamp, NL), because this layer normally

shows the largest microbial activity and soil organicmatter content (Alvarez et al., 1998), at three sites: el

Carmen, el Cortijo, and el Plan on 7th of February 1999.

The three sites were less than 10 km from each other.

The sampling took place outside the canopy of three

isolated mesquite trees at 5 and 8 m from the stem in

four perpendicular directions selected at random. The

Mesquite trees were less than 50 m apart and the soil

sampled at each site was pooled and analyzed (Table 1).Details of the experimental site can be found in Reyes-

Reyes et al. (2002).

The second experimental site is located at Lerma

(Northern Latitude 19� 170 Western Longitude 99� 400)in the state of Mexico (Mexico), nearby the treatment

plant and close to the Lerma river. The Mexican Federal

Government (Gobierno Federal, 1988) considered the

area at the end of the 1980s as heavily contaminated dueto the discharge of untreated municipal and industrial

wastes in the river Lerma. Vaca-Paulin et al. (1989)

mentioned heavy metal contamination in this area es-

pecially with nickel (Ni) and chromium (Cr). The

treatment plant installed in Lerma now run by Reci-

clagua reduced further possible contamination in the

area. Its average altitude is 2600 m above sea level and

characterized by a temperate climate with a mean an-nual temperature of 14 �C and average annual precipi-

tation of 734–985 mm. The 0–10 cm layer of a sandy

clay loam soil under pasture was sampled with a stony

soil auger diameter seven cm (Eijkelkamp, NL) from

three adjacent fields on 12th of October 2000 and ana-

lyzed (Table 1).

2.3. Treatments and experimental set-up

The soil was taken to the laboratory (Laboratory of

Soil Ecology, Department of Biotechnology and Bio-engeneering, Centro de Investigaci�oon y Estudios Avan-

zados IPN, Mexico City, Mexico) and treated as follows

(Fig. 1). The soil from each site and each field was

passed separately through a five mm sieve, adjusted to

40% of water holding capacity (WHC) by adding dis-

tilled water and conditioned at 25 �C for seven days in

drums containing a beaker with 100 ml one M sodium

Table 1

Characteristics of the soil sampled outside the canopy of Mesquite, and from a pasture soil in Lerma

Soil pHH2OWHCa

(g kg�1

soil)

Total Carbon Particle size distribution Soil texture

P

(g kg�1 soil)

N

(g kg�1 soil)

Organic

(g kg�1 soil)

Inorganic

(g kg�1 soil)

Sand

(g kg�1 soil)

Silt

(g kg�1 soil)

Clay

(g kg�1 soil)

Mesquite 6.8 589 5:0� 10�3 1.3 6.5 0.47 582 208 211 Sandy clay

loam

Lerma 4.9 910 2:7� 10�3 2.4 8.3 0.26 646 150 203 Sandy clay

loam

aWHC: water holding capacity.

94 O. Franco-Hern�aandez et al. / Bioresource Technology 87 (2003) 93–102

http://www.inegi.gob.mx

hydroxide (NaOH) solution to trap carbon dioxide

(CO2) evolved and a beaker with 100 ml distilled H2O toavoid desiccation of the soil.

Ninety sub-samples of 50 g soil from each experi-

mental site and field were added to 110 ml glass bottles

and 90 sub-samples of ten g of soil were added to 25 ml

plastic beakers. Eighteen sub-samples were amended

with 1.4 g dry biosolids or approximately 50� 103

kg ha�1 for the 0–10 cm layer, 18 with 1.4 g dry irradi-

ated biosolids, 18 with 1.4 g pasteurized dry biosolidskg�1, 18 with 1.4 g dry biosolids treated with Ca(OH)2kg�1, while the remaining 18 were amended with 2.6 ml

distilled water, the amount added with the biosolids, and

served as a control. As such 30 different treatments were

incubated, i.e. two different sites, three fields, the ap-

plication of four different types of biosolids and an un-

amended control. Three glass flasks were selected at

random from each treatment. Ten g of soil was used tomeasure pH while 20 g was extracted for inorganic N

(NHþ4 ), nitrite (NO

�2 ) and nitrate (NO�

3 ) and ninhydrin

N by shaking for 30 min with 200 ml 0.5 M potassium

sulphate (K2SO4) and filtered through Whatman No. 42

paper�. The remaining 20 g soil of each glass flask was

fumigated with ethanol-free chloroform in the dark for

24 h (M€uuller et al., 1992). It was then extracted with 100

ml 0.5 M K2SO4 solution and analysed for ninhydrin N.Three plastic beakers were chosen at random from each

treatment and soil was extracted for inorganic P with

100 ml of 0.5 M sodium bicarbonate (NaHCO3) solu-

tion (pH 8.5), samples were shaken for 30 min, and fil-

tered through Whatman No. 42 paper�. Analyses ofthese samples provided zero-time results.

The glass flasks and the plastic beakers were placed in

940 ml glass jars containing a vessel with 20 ml of a one

M NaOH solution to trap CO2 evolved and a beaker

with ten ml of distilled water. The jars were sealed with

plastic lids and incubated at 22� 2 �C for 70 days. An

additional 15 jars containing a vessel with 10 ml of

distilled H2O and one with 20 ml of 1 M NaOH weresealed and served as controls to account for the CO2

trapped from the air. After 7, 14, 28, 42 and 70 days,

three jars were selected at random from each treatment,

the vessel with 20 ml of a 1 M NaOH solution removed,

air-tight sealed and stored until analyzed for CO2. Un-

fumigated and fumigated soil was analyzed for inor-

ganic, and ninhydrin-N and available P as described

previously.

2.4. Chemical and microbiological analyses

Soil pH was measured in 1:2.5 soil–H2O suspension

using a glass electrode (Thomas, 1996). Total metals

were determined in sub-samples of 0.5 g soil following

microwave digestion (Q Lab 6000, Questron) with 10 ml

nitric acid (HNO3) and 2 ml 10% hydrogen peroxide(H2O2) (USEPA, 1998, method 3051). Total lead (Pb),

manganese (Mn), Ni, cobalt (Co), copper (Cu), Cr, zinc

(Zn), cadmium (Cd) and silver (Ag) were measured by

Fig. 1. Experimental plan to investigate dynamics of C, N and P in two soils amended with biosolids.

O. Franco-Hern�aandez et al. / Bioresource Technology 87 (2003) 93–102 95

flame atomic absorption spectrometry (AA) (VarianSpectrAA220 Fast Sequential). The plastic beakers used

for analysis of metals were new and treated with 2%

HNO3 24 h before use. Total C was determined by

oxidation with potassium dichromate (K2Cr2O7) and

titration of excess dichromate with ammonium ferro-

sulfate ((NH4)2FeSO4) (Kalembasa and Jenkinson,

1973), and inorganic C by adding 5 ml 1 M hydrogen

chloride (HCl) solution to one g air-dried soil andtrapping CO2 evolved in 20 ml one M NaOH. Organic C

was defined as the difference between total and inorganic

C. Total N was measured by the Kjeldhal method

(Bremner, 1996), soil particle size distribution by Bou-

youcos method (Gee and Bauder, 1986) and cation ex-

change capacity (CEC) with the barium acetate method

(Jackson et al., 1986). Total P was measured by aqua

regia digestion with sodium carbonate fusion (Croslandet al., 1995). Available P in the 0.5 M NaHCO3 extract

was determined by the antimony–potassium–tartrate

method (Watenabe and Olsen, 1965), ammonium (NHþ4 )

in the K2SO4 extract by distillation with magnesium

oxide (MgO) (Bremner and Keeney, 1966), and NO�3

and NO�2 colourimetrically. Lipids were soxhlet ex-

tracted with hexane, the hexane was distilled and the

lipids were defined by differences of weight (APHA-AWWA-WPCF, 1989). The CO2 in the 1 M NaOH

was determined by titration with 0.1 M HCl (Jenkinson

and Powlson, 1976). Concentrations of chloride ions

(Cl�) were determined by titration with silver nitrate

(Ag(NO3)) (Frankenberger et al., 1996) and the WHC

was measured on soil samples water-saturated in a

funnel and left to stand overnight.

The ninhydrin N was measured as described byJoergensen and Brookes (1990) in the fumigated and

non-fumigated extracts and microbial biomass C was

calculated as 20.6� [(ninhydrin-N in 0.5 M K2SO4 ex-

tracts of fumigated soil) minus (ninhydrin-N in extracts

of unfumigated soil)]. The biosolids was analyzed for

total and faecal coliform, Salmonella spp., Shigella spp.

and for eggs of helminths (USEPA, 1999, Appendix F,

G, I). Salmonella and Shigella were determined using aserial dilution technique. A sub-sample of 10 g biosolid

was added to 90 ml sterile buffered peptone solution

using an aseptic technique and 10�1, 10�2 and 10�3 di-

lutions were made with sterile 0.8% NaCl solution. 0.1

ml aliquot was plated on two selective media Salmo-

nella–Shigella agar and sulfite bismuth-agar. The second

medium is highly specific for Salmonella. The colonies

were identified by form and color (USEPA, 1999, Ap-pendix G). Fungi, defined as the total number of colony

forming units (CFU), were determined by serial dilution

with a sterilized 1=4 strength Ringer�s solution and

plating on general and selective media (Parkinson,

1994). Rose-bengal agar amended with 0.1 mg strepto-

mycin-sulphate ml�1 was used to enumerate fungi. The

plates were inoculated with l00 ll biosolids suspension

(three plates per suspension kept at 25 �C for three toseven days).

The USEPA method (USEPA, 1999, Appendix 1),

was used to concentrate, detect, and enumerate Ascaris

ova and to determine their viability. Samples were mixed

with buffered water containing a surfactant and large

particles were removed. The solids were allowed to

precipitate and the supernatant was decanted. The sed-

iment was subjected to a density gradient centrifugationusing magnesium sulfate (specific gravity 1.2). Small

particles were removed by a second screening on a small

mesh size screen and proteineous material by an acid

alcohol/ethyl ether extraction. The concentrate was then

incubated at 26 �C for 10 days and microscopically ex-

amined for Ascaris ova on a Sedgwick-Rafter counting

chamber.

2.5. Statistical analysis

Inorganic N concentrations (NHþ4 and NO�

3 ), avail-

able P, microbial biomass C and production of CO2

(dependent variables) were subjected to a one way

analysis of variance to test for significant differences

among the treatments with the independent variablesbeing time and treatments. All analyses were done using

SAS statistical analysis PROC MIXED (SAS Institute

Inc., 1989).

3. Results

The pH in water of the biosolids was 7.1 and in-

creased to 11.9 after the application of Ca(OH)2 and to

8.7 after pasteurization. The conductivity of 2.6 mSm�1

nearly doubled after pasteurization and application of

Ca(OH)2 (Table 2). The concentration of NHþ4 was 220

mg kg�1 in both dry and untreated biosolids. The con-

centration of NHþ4 decreased after treating the biosolids,

with the largest decrease found in the biosolids amended

with Ca(OH)2.

Pasteurization and application of Ca(OH)2 decreased

the CFU of fungi recovered 20-fold while irradiation

decreased the CFU of fungi recovered by 1000-fold

(Table 3). The amount of CFU of total coliforms re-covered was reduced substantially by pasteurization,

application of Ca(OH)2 and irradiation, with the largest

decrease found in the latter. The concentration reduc-

tion in faecal coliforms was lower after pasteurization

and the application of Ca(OH)2 compared to irradia-

tion of the biosolids. No viable eggs of helminthes were

found in the treated biosolids, but 30� 103 were found

in the untreated biosolids. Concentrations of heavymetals recovered from dry biosolids were lower than

100 mg kg�1 except for zinc (Zn) and chromium (Cr)

(Table 4).


pH was significantly higher in soil added with bio-

solids treated with Ca(OH)2, but not when other types

of biosolids were added ðP < 0:05Þ (Fig. 2a and b).

The production of CO2 was significantly higher in the

Mesquite soil than in the Lerma soil and increased

significantly when biosolids were added to both soils

ðP < 0:05Þ (Fig. 3a and b). There was no signifi-

cant difference in the production of CO2 between thedifferent types of biosolids added to soil of Lerma,

but there was in the Mesquite soil. The application

of Ca(OH)2 significantly reduced the production of

CO2 compared to the other types of biosolids ðP <0:05Þ.

The application of biosolids increased the concen-

trations of NHþ4 at day zero, with the least increase

found in the untreated biosolids and the largest in the

pasteurized and irradiated biosolids (Fig. 4a and b). The

concentration of NHþ4 decreased thereafter, except in

the Ca(OH)2 treated biosolids, where it showed a

maximum at day seven. NO�3 concentrations decreased

at day seven in the Lerma soil amended with bio-solids compared to the unamended soil except for the

Ca(OH)2 treatment (Fig. 5a). The concentrations of

NO�3 were similar thereafter except again for the

Ca(OH)2 treatment. No decrease in the concentra-

tion of NO�3 was found in the Ca(OH)2 treatment

Table 2

Physicochemical characteristics of the biosolids

Characteristics on a dry biosolids base Biosolids Biosolids irradiated Biosolids with Ca(OH)2 Biosolids heated at

60 �C, 30 min

pHH2O7.1a 6.9 11.9 8.7

Conductivity (mSm�1) 2.6 2.7 5.9 5.2

Organic carbon (g kg�1) 499 456 111 480

Inorganic C (g kg1) 3.9 4.2 24.5 3.6

Total N (g kg�1) 41 42 40 35

Total P (mgkg�1) 5.10 3.42 3.08 8.18

NHþ4 (mgkg�1) 221 226 57 180

NO�3 (mg kg�1) 29 29 61 28

NO�2 (mg kg�1) 41 40 95 43

Available PO3�4 (mg kg�1) 10.6 10.0 10.5 11.0

Cation exchange capacity (meq/100 g) 16 8 NMb 20

Cl� (g kg�1) 1.67 1.95 0.65 1.19

Ash (g kg�1) 327 334 560 345

Water content (g kg�1) 820 820 690 806

Lipids (g kg�1) 354 340 275 262

aMean of four replicates.bNM: not measured.

Table 3

Fungi and pathogens in the biosolids, and maximum allowed limits of them (USEPA, 1994)

Biosolids Biosolids

irradiated

Biosolids

with

Ca(OH)2

Biosolids

heated at

60 �C, 30 min

Minimum

significant

difference

ðP < 0:05Þ

USEPA (1994) maximum

acceptable limits

Class A Class B

Fungi

(CFUa g�1 dry biosolids)

950b 1 43 48 242 NGc NG

Total coliforms

(CFU g�1 dry biosolids)

66� 103 1 NMd 2100 13,594 NG NG

Faecal coliforms


1200 3 1000 1000 350 <1000 <20� 105

Shigella spp.


NDe ND ND ND ND NG NG

Salmonella spp.


250 1 ND ND 30 <3 <300

Viable eggs of Helminths

(eggs kg�1 dry biosolids)

30� 103 ND ND ND ND <10� 103 <35� 103

aCFU: colony forming units.bMean of four replicates.cNG: not given.dNM: not measured.eND: not detectable.


and the mean concentration was significantly and twice

as large as in the other treatments ðP < 0:05Þ. Concen-trations of NO�

3 in the Mesquite soil were similar and

not significantly different between the treatments (Fig.

5b).

The concentrations of available PO3�4 decreased in all

treatments of the Lerma soil and then increased again

after 14 days with the largest increase found in the

Ca(OH)2 treatment (Fig. 6a). A similar pattern was

found in the Mesquite soil, but mean concentrations of

PO3�4 were significantly lower compared to the Lerma

soil. The amount of P mineralized, however, was similar

Table 4

Concentration of heavy metals in the biosolids and USEPA norms (1994) for excellent and acceptable biosolids

Pb

(mgkg�1

dry

biosolids)

Mn

(mgkg�1

dry

biosolids)

Ni

(mgkg�1

dry

biosolids)

Co

(mgkg�1

dry

biosolids)

Cu

(mgkg�1

dry

biosolids)

Cr

(mgkg�1

dry

biosolids)

Zn

(mgkg�1

dry

biosolids)

Cd

(mgkg�1

dry

biosolids)

Ag

(mgkg�1

dry

biosolids)

Biosolids 19a 13 63 63 29 298 162 8 NDb

USEPA

1994

Norm

Excellent

300 NGc 420 NG 1500 1200 2800 39 NG

USEPA

1994

Norm

Acceptable

800 NG 420 NG 4300 3000 7500 85 NG

aMean of four replicates.bND: not detectable.cNG: not given.

Fig. 2. (a) pH in soil from Lerma (b) and sampled outside the canopy

Mesquite ð�Þ amended with 60 g biosolids kg�1 dry soil ðÞ, withirradiated biosolids ðjÞ, pasteurized biosolids ðMÞ and biosolids amen-ded with Ca(OH)2 to pH 12 ð�Þ incubated aerobically at 22� 1 �C for

70 days. Bars are �1 std of three different fields and three replicates.

Fig. 3. (a) Production of CO2 (mgCkg�1 dry soil) in soil from Lerma

(b) and sampled outside the canopy Mesquite ð�Þ amended with 60 g

biosolids kg�1 dry soil ðÞ, with irradiated biosolids ðjÞ, pasteurizedbiosolids ðMÞ and biosolids amended with Ca(OH)2 to pH 12 ð�Þ in-cubated aerobically at 22� 1 �C for 70 days. Bars are �1 std of three

different fields and three replicates.


in both soils ðP < 0:05Þ (Fig. 6b). The microbial biomassC in soil from Lerma was significantly larger when bio-solids were added compared to the unamended soil

ðP < 0:05Þ (Fig. 7a). The microbial biomass C in the

mesquite soil, however, was not affected by the appli-

cation of biosolids (Fig. 7b).

4. Discussion

The characteristics of the Lerma soil were typical for

soils of the region. Metal concentrations were within the

ranges reported for other soils (Kabata-Pendias, 1995)

(Table 5), but the concentrations of chromium washigher than acceptable intervals for American soils

(Kabata-Pendias, 1995), and concentrations of Cr, Co,

and Cd were higher than those mentioned in Swiss

Government Guidelines (1987). In the soil of Lerma that

could be due to contamination, but not in the Mesquite

soil, because it had never been amended with any fer-

tiliser or biosolids, so the mother material presumably

had large concentration of Cr. The biosolids was ofexcellent quality (USEPA, 1994) so there would be no

problem applying it to those soils considering the con-

centration of its heavy metals.

The amount of CFU of faecal coliform, eggs of

Helminthes and Salmonella spp. were higher than values

required for Class ‘‘A’’ biosolids, but could be classifiedas a Class ‘‘B’’. Class ‘‘A’’ biosolids can be applied

without restrictions while a class ‘‘B’’ biosolids can be

used in forests (USEPA, 1994). Pasteurization and the

application of Ca(OH)2 reduced the CFU of Salmonella

and eggs of Helminthes, but the decrease in faecal col-

iforms was not sufficient to meet class ‘‘A’’ biosolids

criteria (USEPA). Additionally, Eriksen et al. (1996)

recommended storage of the biosolids for at least threemonths while maintaining it at pH above 12 before be-

ing applied to agricultural land. Pasteurization also re-

duced the eggs of Helminthes, but Ahmed and Sorensen

(1997) recommended a double monthly pasteurization

over a year to destroy eggs of Ascaris (a helminth)

and other pathogens. Irradiation at 30 kGy significantly

reduced all pathogens and the irradiation dose was

much higher than values normally recommended todestroy nearly all ova of Ascaris (Capizzi-Banas and

Schwartzbrod, 2001) or faecal coliforms (Rawat et al.,

1998).

The application of biosolids to soil will increase its

organic matter. An increase in organic matter increases

Fig. 4. (a) Concentrations of NHþ4 (mgNkg�1 dry soil) in soil from

Lerma (b) and sampled outside the canopy Mesquite ð�Þ amended

with 60 g biosolids kg�1 dry soil ðÞ, with irradiated biosolids ðjÞ,pasteurized biosolids ðMÞ and biosolids amended with Ca(OH)2 to pH

12 ð�Þ incubated aerobically at 22� 1 �C for 70 days. Bars are �1 stdof three different fields and three replicates.

Fig. 5. (a) Concentrations of NO�3 (mgNkg�1 dry soil) in soil from





infiltration of water, increases CEC, improves soil

structure and prevents erosion. Approximately 28% oforganic C of the biosolids mineralized within 42 days if

no priming effect was considered (Kuzyakov et al.,

2000). Barajas-Aceves and Dendooven (2001) found

that 31% of tannery biosolids mineralized within 70

days. The mineralization of biosolids treated with

Ca(OH)2 when added to the Mesquite soil was only 9%

and the mineralization was inhibited in the first seven

days. It is difficult to postulate which factor(s) mighthave affected the decomposition of the biosolids, but it

appears that it was not due to the treatment(s), because

the reduction in C mineralization effect was absent in the

Lerma soil. The higher pH in the soil at the onset of the

incubation when biosolids treated with Ca(OH)2 was

added might have affected microbial activity directly or

indirectly through the formation of NH3 (Jenkinson,

1981). Wen et al. (1997) reported that irradiationsometimes affected C mineralization, but not always as

in this study.

The large concentration of NHþ4 in the biosolids

provide a valuable soil nutrient. Both soils appeared to

be limited in N, as nearly 40 mg NHþ4 –N could not be

accounted for at the onset of the incubation. No in-

crease in NO�3 appeared and NH3 volatilization could

partially explain losses of N in the Mesquite soil which

has a high pH, but the pH in the Lerma soil is too low to

explain those losses. The addition of Ca(OH)2 ap-

pears to stimulate the N mineralization of the biosolids

when added to soil as the dynamics of NHþ4 were dif-

ferent and the concentrations of NO�3 were higher in the

Lerma soil. Wen et al. (1997) found that sometimes Nmineralization was affected when biosolids were irra-

diated. In our experiment, N mineralization in soil

amended with irradiated or non-irradiated biosolids

were similar. Other characteristics of the biosolids were

not affected, as supported by the work of Rawat et al.

(1998).

Mineralization of P occurred in both soils, but the

application of biosolids did not significantly increasemineralization. The electrolytic conductivity of the bio-

solids was similar to values reported by Pascual et al.

(1997), but it might be worthwhile to follow the salinity

and sodicity in soil when biosolids are applied more than

once. High sodicity and salinity are known to inhibit

plan growth and affect soil processes (e.g. Nelson et al.,

1996; Pathak and Rao, 1998). Application of biosolids

treated with Ca(OH)2 might have the additional effect ofincreasing pH in acid soils, e.g. the Lerma soil, but it

might inhibit mineralization in more alkaline soils (e.g.,

Mesquite soil).

Fig. 6. (a) Concentrations of available PO3�4 (mgPkg�1 dry soil) in

soil from Lerma (b) and sampled outside the canopy Mesquite ð�Þamended with 60 g biosolids kg�1 dry soil ðÞ, with irradiated bio-

solids ðjÞ, pasteurized biosolids ðMÞ and biosolids amended with

Ca(OH)2 to pH 12 ð�Þ incubated aerobically at 22� 1 �C for 70 days.

Bars are �1 std of three different fields and three replicates.

Fig. 7. (a) Microbial biomass C (mgCkg�1 dry soil) in soil from





5. Conclusions

Biosolids heavy metals concentrations generated

from analyses conducted during this study met USEPA

requirements for class A, therefore biosolid amendment

should not adversely affect soil quality with respect to

heavy metal concentrations. Faecal coliform concen-

trations recovered from biosolids required USEPA class

B designation, so that agricultural land applicationshould only be considered following irradiation or

other treatment to reduce potential pathogen transfer to

soils.

N mineralization in the soil types studied did not

change significantly following amendment with un-

treated, pasteurized, or irradiated biosolids, but was

significantly increased following amendment with limed

biosolids. Although the C and N mineralization werenot inhibited after the application of biosolids further

studies are necessary to investigate possible long-term

effects on other soil processes. Possible effects on plant

growth should be investigated too.

Acknowledgements

We thank J. Moreno-Alcantara of the Departamento

de F�ıısica de Radiaciones Instituto Nacional Investigac-

iones Nucleares (ININ, Mexico) for irradiation of the

biosolids and F. Malagony for technical assistance. Theresearch was funded by Reciclagua Sistema Ecol�oogico deRegeneraci�oon de Aguas Residuales Ind., S.A. de C.V.

(Lerma, Estado de M�eexico, M�eexico). O. F-H received

grant-aided support from Consejo Nacional de Ciencia yTecnolog�ııa (CONACyT), M�eexico.

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