fNƯ Joum al of Science, Earth Sciences 24 (2008) 202-212
Effects o f biosolids application on soil chemical propertieties in peri-urban agricultural systems
Nguyen Manh K hai1'*, Pham Quang Ha2, N guyen Cong Vinh3, Jon Petter Gustafsson4, Ingrid Ồ bom 5
1 College o f Science, VNU
2 Insitute fo r Agricultural Eĩtvironment, Phu Do, Tu Liem, Hanoi, Vietnam (IAE) 3 National Instiíute fo r Soils and Fertiỉừers, Hanoi, Vietnam (NISF)
4 Department o f Land and Water Resources Engineering, Royal Institute o f Technology (KTH) ) 5 Dept. Crop Production Ecology, Swedish ưniversity o f Agricuỉtural Sciences (SLU)
Received 5 November 2008; received in revised form 26 November 2008.
Abstract. The application of biosolids as a fertilizer in agricultural cultivation are common
1
practices in many countries. This study investigates the effects of such practices in íĩeld 1 experiments on Fluvisol and Acrisol soils in peri-urban Hanoi City. We compared biosolid- fertiỉized vegetable/rice-dominated systems (biosolids applied as chicken manure (Acrisol in Vinh
1
Phuc Province) or composted pig manure mixed with rice sứaw (Fluvisol in Ha Tay Province).
The biosolids were applied at six diíĩerent rates representing from 0 to 450% of the normal annual l dose used by local farmers. The application of biosolids had highly significant positive eíTects on
1
organic carbon (TOC%) and total nitrogen (Ntot%), when the six điíTerent ưeatments of composted l manure and chicken manure werc compared. The soil reverse aqua rểg/j-cxữactable (Rev Aq Reg) ) Zn increased linearly with biosolids application rate at boứi sites and the linear regression showed l Zn (mg kg'1) = 112.5 + 13.25xl0*3xcomposted manure (t ha'1) ( t ^ . 5 8 ) or Zn (mg kg*1) = 43.77 + 35.04xl0'3xchicken manure (t ha*1) (^=0.73), whereas Cu (Rev Aq Reg) only increased l signiíĩcantly at the Vinh Phuc site. The Cd and Pb (Rev Aq Reg) concentration is not clearly / diíĩerent from ửie conừol after short-term (one time) application of biosolids. The application of biosolids increased the EDTA-exừactable fraction of Cd, Cu and Zn, but had no eíĩect on I NH4N03-extractable fractions of ứiese elements.
Keywords: Chicken manurc; Composted manure; Biosolids; Heavy metals; Trace elements.
. IntroductioD
There is a growing concem about the risk o f ontamination o f waters, soils and agricultural
►roducts, in the rapid urbanizing areas in ứie
Corresponding auứior. Tel.: 84-4-35583306 E-mail: [email protected]
South-East Asia due to the heavy, ry9 or inappropriate, use o f organic wastes, fertilizerilizers, pesticides, and poor quality ừrigation wat water [13]. Urbanization and industrializati(ization processes always lead to increased productiduction o f waste, i.c. wastewater and solid wastwaste.
Industìial, agricultural and domestic eữluentluents, such as biosolids and wastewater, are eith eitìier
N M . Khai et aỉ. / V N U Ịournaỉ o f Science, Earth Sciences 24 (2008) 202-212 203
dumpe<mped on land or used for irrigation and fertilizrtilization purposes, what creates both opportiportunities and problems [33].
Th The advantages o f reusing waste are that it providovides a convenient disposal o f waste p ro d u o d u cts and has the beneíìcial aspects o f addingding valuable plant nutrients and organic m atteratters to soil [10]. Biosolids is a beneíìcial soil aril amendment, especially for arable soils o f inherenerently low organic m atter content, as it may improvprove many soil properties, such as pH and the COE contents o f organic m atter and nutrients [10, 23, 29, 29]. However, as vvastes are Products o f hum anm an society, generally enhanced concenncentrations o f potential toxic substances includcluding trace metals, which may limit the long-ttng-term use o f effluents for agricultural purpoarposes due to the likelihood o f phytotoxicity, health alth and environmental effects [17, 33]. Even after ater a short-term application o f biosolids, the level vel o f trace metals in soils can increase considinsiderably [21, 24].
If If the content o f trace metals increases above>ove a certain critical concentration due to their aeir accumulation in soil, it can have negative enviroivironmental effects, which can include negathgative eíĩects on soil biota and hence on microlicrobial and faunal activity [7]. Furthermore, trace race metals can affect crop grovvth and quality, and th d thus pose risks for human health [4, 16].
Therehereíore, the risk o f contam ination by ừace metalsetals must be considered when biosolids are applieoplied and the understanding o f the behaviour o f mef metals in the soil is essential for assessing enviroivironmental risks when the wastes are applieoplied in agro-ecosystems.
TI The m ain objective o f this paper was to quantuantiíy the effects o f reuse o f biosolids (in the form orm o f animal m anure) as nutrient sources by:
(i) ữ) investigating the effects o f biosolids applicaplication on soil pH, EC, organic carbon, total ntal nilrogen and trace metals (cadmium (Cd), coppeDpper (Cu), lead (Pb) and zinc (Zn)); (ii) investivestigating the eíĩects o f application o f
biosolids especially as regards trace metal accumulation and solubility.
2. M aterials an d m ethods
2.1. Location o f the research areas
Soil samples were collected íroTT) peri-urban areas o f Hanoi City including Ha Tay and Vinh Phuc provinces (Table 1). The sampled areas are located in delta and lowland areas with a tropical monsoon climate. The annual rainíall is 1500-2000 mm, more than 50% o f which are concentrated during the period from June to August. The mean monthly temperature varies between 17 and 2 9 ° c , with the warmest period from June to August and the coldest during December and January.
2.2. Biosolids application
Organic fertilizers (biosolids) have been used in agriculture in Vietnam for a long time.
In this study, íĩeld experiments with different rates o f biosolids application to agricultural soil were set up in a collaboration between the National Institute for Soils and Fertilizers (NISF) and CSIRO Land and W ater Australia within an ACIAR project in Ha Tay and Vinh Phuc provinces [26]. Biosolids in the form o f chicken or pig manures co-composted with rice straw (composted manure) were applied at six diíĩerent levels ranging from 0% to 450% of what local íarmers normally apply per year: 20 tones h a 1 (100%) for chicken manure and 14 tones h a ' (100%) for composted manure (Table
1). The experiments had a randomized block design with triplicates o f the treatments. The biosolids were characterized prior to the application (Table 2). The total organic carbon content in chicken manure was higher than that in composted manure. Cadmium, Cu and Zn concentrations in chicken manure and composted manure were higher than ‘total’
(reverse aqua regia) concentrations o f these metals in the experimental soils [8].
204 N .M . Khai et al. / V N U Ịoum al o f Science, Earth Sciences 24 (2008) 202-212
In Ha Tay Province, the soil type is a Cambic Fluvisol [5] and the composted manure was applied in February 2003. The crop here was rice (Oryza sativa L.). At the Vinh Phuc site, where the soil type is a Haplic Acrisol [5],
the chicken manure was applied in Novem ber 2002. Vegetables were cultivated in the experiment, mainly cabbage (Brassica oleacea L.) and squash (Benicasa hispida L.). At both sites, soil sampling was carried out ÚI June 2004.
Table 1. Description of biosolids treatments (randomized block design with three replicates) in experiments on Fluvisols and Acrisols in the peri-urban areas of Hanoi City
No. Locatìon / soil Treatments Geographic Name Crop Time of
types coordinates experiment
1 Ha Tay/ Composted N: 21°6.02'
Fluvisols manure* E: 105°40.78’
0 1
ha'1
BoDpl Rice 17 months7 th a
'1
BoDp2 Rice 17 months14 th a
'1
BoDp3 Rice 17 months21
tha-' BoDp4 Rice 17 months42 t h a ' BoDp5 Rice 17 months
63 t h a ' BoDpó Rice 17 months
2 Vinh Phuc / Chicken manure* N: 21°9.02'
Acrisols E: 105°45.07’
0 1
ha'1
BoMll Cab/squb20
months10
t ha'1
BoM12 Cab/squ20
months201
h a ' BoM13 Cab/squ20
months3 0 1 h a ' BoM14 Cab/squ
20
months60 t ha
'1
BoM15 Cab/squ20
months9 0 1 ha
'1
B0
MI6
Cab/squ20
months'Fresh weight Farmers usually apply biosolids at the rate of 141 ha
'1
yr‘1
for chicken manure. Experimental design included only one application bcab/squ: Cabbagc (Brassica oleacea L.) and squash (Benicasa hispida
for composted manure and
20
t ha'1
yr'1
at the beginning of the experiment.
L.) 2.3. Soil sampling strategy and sample
preparation
For assessment o f the impact o f biosolids on agricultural soils, 3 to 5 sub-samples were collected within a circle o f 2 m diameter in all treatments and then were mixed to obtain a bulk sample for the plot. After air drying at the room temperature, the soil samples were ground and sieved to remove particles >2 mm, and then stored in plastic bags. The soil samples were analyzed at Swedish University o f Agricultural Sciences.
2.4. Soil analysis
Total N (Ntot) and total organic carbon (TOC) were determined on íĩnely ground samples on a LECO CHN analyzer (Leco
CHN®CHN 932 analyzer). Prior to the analyses, the samples were ừeated by 4M HC1 (1:1 soil:solution ratio) for dissolution o f carbonates.
The soil EC and pH were measuređ in deionized H20 (1:5 soil:solution ratio), and pH c,cL2 was determined after adding 0.5M CaCl2 [27]. The soil samples were extracted with IM NH
4
NO3
for 2 hours (1:2.5 soil:solution ratio) to quantiíy the exchangeable and specifícally adsorbed fraction o f trace metals (i.e. Cd, Cu, Pb, Zn) [2, 22]. Potentially dissolved metals were extracted with 0.025 M Na2H2EDTA (1:10 soil:solution ratio) for 1.5 hours [28]. The reverse aqua regia (3:1 H N 0 3:HC1 ratio)-digestible fraction (Rev Aq Reg) o f Cd, Cu, Pb and Zn was extracted by using a meứiod described by Stevens et al. [27].
After centriíugation, filtration and dilution (if
N.M. Khai et al. / VN U Ịournal o f Science, Earth Sciences 24 (2008) 202-212 205
necessary), the metal concenừations were determ ined by using inductively coupled plasma-mass spectrometry (ICP-MS, Perkin Elm er ELAN 6100).
2.5. Biosolids sampling and analyses
Biosolids samples (one sample o f chicken manure, and one sample o f composted manure) were sampled at the time o f application and analyzed for dry matter content, total organic carbon, N, p, K and trace metals (Cd, Cu, Pb, Zn) b y NISF (Table 2) [8]. Total organic carbon (TOC) was determined by the Walkley-Black
method [30], total N (NtoO was determined by using the Kjeldahl procedure [18], while concentrated H N 0 3 and H2SO4 digestion [14]
was used for total p and K. Digests were neutralized by adding NH4OH (10%), p was determined colorimetrically [3] and K - by ílame emission spectrometry. Trace metal concentrations (Cd, Cu, Pb, Zn) o f biosolids were determined after digestion using a Rev Aq Reg procedure [27]. Copper, Pb and Zn were determined on íĩltered digest samples using ílame alomic absorption (AAS, Perkin Elmer 3300) and Cd with a graphite íum ace AAS [8].
T a b le 2. C h ara cteristics o f th e b io so lid s (c h ick en m anure and co m p o ste d m an u re) used in the ex p erim en ts in V in h P huc an d H a T a y p ro v in ce s [8]
N o. P ara m e ters U nits C h ick en m anure C o m p o sted manure*
1 M o istu re % 57 53
2 T o tal n itro g en (N,o,) % , dw 1.6 1.3
3 T o ta l p h o sp h o ru s (P,o,) % , d w 1.1 1.2
4 P o ta ssiu m (K ) % , dw 1.6 2.3
5 T o tal o rg a n ic c a rb o n (T O C ) % , dw 31.4 18.2
6 C o p p e r (C u) m g k g '1, dw 48.5 45 .0
7 Z ir c (Z n) m g k g '1, dw 2 6 3 .0 190.0
8 C a d m iu m (C d) m g k g '1, dw 3.4 3.5
9 L ead (P b ) m g k g '1, dw 25.3 15.9
' C o m p o ste d m an u re = co m p o sted m ixture o f pig m an u re and rice straw . dw = d ry w eight.
2.6. Statistical analysis
Data from the experiments were analyzed using the General Linear Model (GLM) procedure o f Minitab Soíhvare version 14.0 [19].
Treatment means which showed significant differences at the probability level o f p<0.05 were compared using Tukey's painvise comparison procedure, while the biosolids applicatìon was used as factor in the models. The statistical model used was yjj = ụ + (Xị + dị, where JU is the mean value for all ừeatments, Oíị is the different between mean vaỉue o f treatment / with overall mean, and eiị is a random error.
The results were also analyzed by regression analysis to assess the relationship betvveen concentrations o f elements in the soiỉ (TOC, Ntot, ừace metals) and the amount of
biosolids applied. The statistical regression model was: yiị = a + bxi + e,ị, where y is the concentration o f elements, a is the intercept, b is the slope o f yi against the corresponding value o f y» Xi the biosolids dose, and e\ị is the random error effect.
3. Results
3.1. The ẹffects o f applying biosoỉids on soil p H and electrical conductivity
The application o f biosolids showed a tendency to increase o f soil pH, an eíĩect that was signiíĩcant at the higher applicatíon rates (Table 3). Signiíĩcant effects on EC were found for the high biosolid applicatìon rates o f 63 t ha'1
206 N .M . Khai et al. / V N U Ịoum al o f Science, Earth Sâences 24 (2008) 202-212
(tons per hectare) and 60 t h a '1 for composted and EC might is due to increased soil organic manure and chicken manure respectively (450% matter (see below ) and alkali-metals at higher and 300% o f normal application rate application rates o f biosolids.
respectively). The reason o f increasing soil pH
Table 3. Electric conductivity (EC, cm'1), pH, exchangeable Ca, Mg, Na, K (IM NH4NO3 extractable; g kg*1) in topsoil (0-20 cm) samples from experiments. Diíĩerent supcrscript letters indicate signiíĩcant diíĩerences
between ưeatments at the same site (P<0.05)
Site EC' pH'n!0 pH2c«cu Exchangeable
K Na Ca Mg
Ha Tay site
BoDpl 62.78* 6.02' 5.23 0.07 0.02 1.36 0.17
BoDp2 61.85* 6.05* 5.35 0.07 0.02 1.36 0.18
BoDp3 63.69' 5.99* 5.23 0.07 0.02 1.33 0.17
BoDp4 67.32* 6.08b 5.31 0.08 0.02 1.39 0.18
BoDp5 66.20' 6.13b 5.37 0.06 0.02 1.42 0.18
BoDpó 71.39” 6.13b 5.35 0.06 0.02 1.43 0.19
Vinh Phuc site
BoMll 39.32* 6.08* 5.37* 0.02' 0.01* 0.521 0.03*
B
0
MI2
49.25* 6.36* 6.43* 0.02* 0.01* 0.71* 0.06*BoM13 38.61* 6.25* 5.54* 0.02* 0.01* 0.55* 0.04*
BoM14 41.39* 6.17* 5.35' 0.02* 0.01' 0.55* 0.05*
BoM15 80.12b 7.65b 7.13b 0.05b 0.03b 0.86b 0.10b B
0
MI6
98.25b 7.96b 7.32b 0.09b 0.05b 0.93b 0.20b 1 pH in H20 , ratio so il: water = 1:52 pH in 0.05 M CaCl2, ratio soil: solution = 1:5
Biosollds / 1 ha"1 Blosotìds / 1 ha-1
Fig. 1. Correlation between total organic carbon (TOC), total niừogen (Nkx) in soil and biosolids application rate
ừi the H a T a y a n d V in h P h u c exp erim en ts. T h e re g ressio n equatio n s for H a T a y w ere T O C (% ) = 2 .3 9 + 3.38 X
10‘31 ha'1 compasted manure (r2 = 0.63) and Nt* (%) = 0.28 + 0.19 X 10‘3 1 ha'1 composted manure (r2 = 0.84).
The regression equations for Vinh Phuc were TOC (%) = 0.90 + 4.33 X 10'3 1 ha’1 chicken manure (r2 = 0.89) and Ntot(%) = 0.12 + 0.25 X 10 3 1 ha' 1 chicken manure (r2 = 0 .72).
N .M . Khai et nì. / V N U Ịournal o f Science, Earth Sciences 24 (2008) 202-212 207
3.2. The ejfects o f applying biosolids on soil orgarỉic carbort and total nitrogen contents
The application o f biosolids for agriculture caused a signiíicant increase o f TOC and Ntoi contents at all experimental sites (Fig. 1). The increase was linearly related to the am ount o f biosolids applied. The linear regression indicated that the slope of the relationship was higher for the Vinh Phuc site in comparison to the Ha Tay site. This was probably due to the concenữation o f both TOC and Nto, being much higher in the chicken manure than in the composted manure (Table 2)
3.3. The effects o f applying biosolids on trace metal concentrations in soiỉ
The concentrations o f Rev Aq Reg- digestible Zn in soils were increased significantly by application o f biosolids at both experimental sites. Copper concenừations were
increased signiíìcantly by application of biosolids only at the Vinh Phuc site. There was a higher increase in Zn concentration at Vinh Phuc compared wiứi Ha Tay. This was probably due to the concentration of Zn being higher in the chicken manure than in the composted manure. There were no signiỉĩcant effects o f biosolids application on concentration of Rev Aq Reg Cd and Pb in treated soils (Fig. 2).
The potentially dissolved Cd, Cu, Pb and Zn (EDTA-extractable) increased linearly as a íunction o f biosolids application, except for Pb in the Ha Tay experiment (Table 4 and Fig. 3).
The N H 4N 0 3-extracted ữactions of Cd, Cu, Pb and Zn constituteđ only a small proportion o f the EDTA-extracted fractions. There were no significant diíĩerences between different biosolids and application rates. The reason for this lack o f signifícance might be the low concentrations in combination vvith a variation between the replicates.
Table 4. Efĩect of biosoliđs application on 0.025 M EDTA (mg kg
'1
dw) and IM NH4
NO3
exữactable (mg kg'1
dw) trace metals. Different letters indicate signiíicant dífferences betwecn ữeatments at the same site (P<0.05)
Site EDTA-exữactable NH<N03-extractable
Cu Zn Cd Pb Cu Zn Cd Pb
Ha Tay site
BoDpl 17.19* 4.26* 0.189' 23.28 0.013 0.494 0.0287 0.075
BoDp2 17.48,b 4.13' 0.194* 23.00
0.011
0.374 0.0241 0.055BoDp3 17.72‘b 4.62“ 0.206* 23.55 0.017 0.552 0.0328 0.077 BoDp4 17 75*b 5.08*b 0.209*b 23.45 0.014 0.524 0.0258 0.064 BoDpS 18.44b 5.04*b 0.215,b 24.07 0.013 0.457 0.0246 0.050 BoDpó 19.71° 6.16b
0
.222
b 23.510.011
0.5310.0211
0.041Vinh Phuc site
BoMll 3.89* 9.51* 0.037* 3.56* 0.031 1.884 0.0048
0.012
B
0
MI2
4.50‘b 13.27*b 0.046* 4.05* 0.057 0.2160.0010
0.001 BoM13 4.05,b 11.02* 0.038' 3.83*b 0.037 1.407 0.0029 0.006 BoM14 4.04*t' 11.93*** 0.039* 3.83*b 0.039 2.088 0.0032 0.007 BoM15 52 0 1* 24.09bc 0.049,b 4.50bc 0.118 0.059 0.0004 <0.001
B
0
MI6
6.10c 35.28c 0.066b 4.86c 0.153 0.072 0.0003 <0.001
208 N .M . Khai et a i / VNƯ Ịoum al o f Science, Earth Sciences 24 (2008) 202-212
Biosolids / 1 ha-1
20 40
Bk>solids /t ha"
100
Biosolids/tha', BỉosoNdt / 1 ha*1
Fig. 2. Correlation between Cd, Cu, Pb and Zn (reverse aqua reg/a-extractable) m soil and biosolids application rate in the Ha Tay and Vinh Phuc experiments. Only signiíìcant results are shown wiứi regression line. The regression equation for Ha Tay was Zn (mg kg'1) = 112.51 + 13.25 X 10‘31 ha'1 composted manure (r2 = 0.58).
The regression equations for Vinh Phuc were Cu (mg k g ‘) = 9.48 + 6.32 X 10*3 1 ha'1 chicken manure (r2 = 0.68) and Zn (mg kg'1) = 43.77 + 35.04 X 10’31 ha‘l chicken manure (r2 = 0.73).
Điosoiids /1h a 1 Điosolids /1h a ’
N .M . Khai et aỉ. / V N U Ịournaỉ o f Science, Earth Sciences 24 (2008) 202-212 209
Biosotids 11 ha ’ Biosolids / 1 ha 1
Fig. 3. Correlation between Cd, Cu, Pb and Zn (EDTA-exữactable) in soil and biosolids application rate in the Ha Tay and Vinh Phuc experiments. Only signiíìcant results are shown with regression line. The regression
eq u a tio n s fo r H a T a y w ere C u = 17.2 + 0 .0 3 7 t h a '1 co m p o sted m an u re (R 2 = 0 .7 8 ); Z n = 4 .1 6 + 0.029 t h a '1
composted manure (R2 = 0.61); Cd = 0.19 + 0.0005 t ha'1 composted manure (R = 0.55). The regression equations for Vinh Phuc were Cu = 3.80 + 0.024 t ha'1 chicken manure (R2 = 0.67); Zn = 7.45 + 0.286 t ha'1 chicken manure (R2 = 0.77); Cd = 0.035 + 0.0003 t ha"1 chicken manure (R2 = 0.51); Pb = 3.63 + 0.014 t ha'1
chicken manure (R2 = 0.73).
4. Discussion
The use o f biosoliđs as a fertilizer significantly increased TOC and Niot in the soils. The organic m atter in biosolids increases the ability o f soil to retain water. The biosolids treatment involved application o f 7-63 t ha'1 of composted m anure or 10-90 t h a '1 o f chicken manure incorporated into the top 20 cm o f the soil. The topsoil bulk density was 1.2 t rrf3 for the Ha Tay site and 1.3 t m‘3 for the Vinh Phuc site. The theoretical estimations showed that soil organic carbon content (TOC, % ) wou!d have initially increased by a factor o f 3.3 X 10~3 and 5.5 X 10‘31'1 biosolids for Ha Tay and Vinh Phuc sites, respectively, as a result o f the biosolids application. The íĩeld experimental data showed that the organic carbon increase for the com posted manure was in agreement with the calculated value. However, for the chicken manure, the increase in measured carbon content w as lower than estímated (Fig.
2). This was probably due to the decomposition o f chicken m anure that occuưed during the experimental period, and that was less pronounced in the composted manure, which hađ probably already decomposed during the
composting process. An increase in total organic carbon and nitrogen through application o f biosolids Has also been íòund in the previous studies [12, 31, 32]. However, Garrido et al.
(2005) did not find a signiíicant increase in organic matter and total nitrogen, possibly because a lower biosolids rate (4.5 t ha'1) was used in that study [27].
Although biosolids ha ve been demonstrated to be an useíul nutrient source for agricultural soils, the beneíĩcial properties o f biosolids can, depenđing on their origin, be limited by theừ contents o f potentially harmful substances. The biosolids applied in the experiments in this study did not originate from domestic waste but from animal manure. The same practice has been observed in ứie studies o f nutrient íluxes in peri-urban vegetable production in the Asia, where the animal manure is purchased from villages specializing in animal production and transported to peri-urban areas, where it is applied in intensive agricultural production systems, in particular to vegetable crops [11, 13]. The soil amended with biosolids in the present study had higher "total" (Rev Aq Reg) concentratìons o f Cu (only for the Vinh Phuc site) and Zn than the conừol soils. Ogiyama et
210 N.M. Khai et al. / V N U Ịoum al o f Science, Earth Sáences 24 (2008) 202-212
al. (2006) reported accumulation o f Zn in soils due to animal manure application [20]. Sloan et al. (1997) reported that biosolids application signiíicantly increased total concentrations o f Cu and Zn in the soils studied, although the differences were less than the initial increase anticipated for the application o f biosolids [25].
This may be due to plant trace metal uptake.
Furthennore, the increase in trace metal contents in soil may be dependent on the biosolids application rate [6]. In the present study, there was a signiíicant (P<0.05) increase in "total" Zn for the application rates greater than 21 t ha'1 for the composted manure and 30 t ha’1 for the chicken manure.
Although the "total" concentrations o f Cd and Pb in the chicken manure and Cd in the composted manure were higher than in the experimental soils, the concentrations o f Cd and Pb in biosolids-amended soils were not signiíìcantly higher, although there was an increasing trend. This was probably due to the concentrations o f Cd and Pb in biosolids were not being sufficiently high to give signiíĩcant effects on concentrations o f these metals (Rev Aq Reg) in experimental soils. The content o f Pb in composted manure was lower than that in experimental soil (Ha Tay site). In addition, the short-term nature (one time) o f biosolids application in these experiments may have contributed to the lack o f a signiíìcant effect on these elements.
Trace metals in biosolids are generally sứongly sorbed to the biosolids matrix. Thus, trace metals added to soil with biosolids are less phytoavailable than those added as simple inorganic salts [15]. There was no significant effect on the NH4N 0 3-extractable fraction o f trace metals compared with the level in control soils. However, the potential dissolved Cd, Cu and Zn was significantly higher than in the control soils, but not in the case o f Pb. This indicates that adding biosolids to agricultural sóils can increase binding sites or even act as a sink for trace metals already present in soil,
reducing metal concentration in ứie soil solution, despite the biosolids having higher metal concentrations than the soil itselí [1, 9, 16].
Cripps et al. (1992), who found that application o f biosolids at a rate o f 11 t ha 1 increased availability o f Cu in soil, but that neither Cu nor Zn was leached from surface soil [1].
5. C onclusions
Application o f biosolids as fertilizer sources has become a common practice in Vietnam, especially in the peri-urban areas. The reuse of these nutrients had some beneíĩcial effecís on soil fertility, such as increased total organic carbon and nitrogen. This study found that both organic carbon content and total nitrogen were improved in soils treated with biosolids. However, these beneíĩts were limited by the presence of some potential toxic trace metals in biosolids.
The addition o f biosolids, here in the form o f chicken manure and composted manure, also increased the soil concentration o f EC. The total concentrations o f Zn and Cu (for the Vinh Phuc site) and potential dissolved Cd, Cu, and Zn were signiíĩcantly higher in soils treated with biosolids, whereas the total concentrations o f Cd and Pb were not clearly different from the con troi. This was probably due to the short- term nature o f biosolids application (one occasion) and the relatively low concentrations of these trace metals in the biosolids.
A cknow ledgem ents
The soil sampled was collected from biosolids íĩeld experiment o f the ACIAR Project "Impact o f trace metals on sustainability o f fertilization and waste recycling in peri- urban and intensive agriculture in south-east Asia" in Ha Tay and Vinh Phuc provinces, carried out in a collaboration between c s m o and NISF. We would like to kindly thank the
N .M . Khai et a l Ị V N U Ịournaỉ o f Science, Earth Sciences 24 (2008) 202-212 211
staff at NISF for their help in soil sampling and sample preparation. Finally, Gunilla Hallberg and Gunilla Lundberg are acknowledged for carrying out some laboratory analyses at SLƯ.
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