``In situ'' vermicomposting of biological sludges and impacts on
soil quality
G. Masciandaro
a, B. Ceccanti
a,*, C. Garcia
ba
CNR, Istituto Chimica del Terreno, Area della Ricerca, Loc. S. Cataldo, Via Al®eri, 1-56127 Ghezzano, Pisa, Italy b
CSIC, Centro de Edafologia y Biologia Aplicada del Segura, Avda. de La Fama s/n Murcia, Spain
Accepted 10 January 2000
Abstract
A laboratory experiment was carried out to study soil quality amelioration through ``in situ'' vermicomposting of biological sludges. The experiment dealt with the stabilization, through the action of worms (Eisenia fetida), of ®ve mixtures containing aerobic and anaerobic biological sludges spread on the soil surface. The results showed that by increasing the percentage of anaerobic sludge in the mixtures, the number of worms which left the sludge and chose the soil as their habitat increased. The chemico-structural changes of the sludges left on the soil surface by worms were evaluated through the technique of pyrolysis-gas chromatography, which showed that the degrees of mineralization and humi®cation of organic matter were dependent on the composition of the sludge mixtures. When the amount of aerobic sludge in the mixtures was higher than 50%, a stimulation of soil microbial metabolism occurred, as demonstrated by the index of metabolic potential (de®ned by dehydrogenase/water soluble carbon ratio). All treatments increased the percentage of soil total shrinkage area, mostly due to the formation of cracks of small±medium size (<1000mm), which represent a favourable site for microbiological and biochemical processes in the soil. A positive statistical correlation between soil dehydrogenase activity, C and N substrates, and cracks of small±medium size was found.72000 Elsevier Science Ltd. All rights reserved.
Keywords: Vermicomposting;Eisenia fetida; Biological sludges; Dehydrogenase activity; Pyrolysis-gas chromatography; Soil metabolism; Soil
cracking
1. Introduction
Sustaining soil productivity has a high priority in European agriculture (Larson et al., 1981). The decline in soil fertility and productivity due to excessive soil erosion, nutrient run-o, and loss of soil organic mat-ter has stimulated inmat-terest in improving overall soil quality by the addition of organic matter (Bastian and Ryan, 1986). To maintain or improve the tilth, ferti-lity, and productivity of agricultural soils, several kinds of wastes can be used (such as solid organic waste, sewage sludge, agricultural waste, animal
man-ure, and some sorts of industrial waste) as source of organic matter. Some of these wastes can be added to the soil without any risk (Lerch et al., 1992; Ayuso et al., 1996), but for some of them (i.e. sewage sludges) careful attention is required since they can produce toxicity and have depressive eects on microbial metabolism (Ayuso et al., 1996). Some authors have suggested that materials should be composted before applying to soil in order to achieve biological trans-formation of the organic matter and avoid potential risks from pathogens and heavy metals (Beloso et al., 1993; Gliotti et al., 1997). Both fresh and composted amendments stimulate soil biological activity; fresh wastes produce an initial burst of biochemical activity (due to high release of easily degradable organic com-pounds) which tends to fall away as time progresses,
0038-0717/00/$ - see front matter72000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 0 7 1 7 ( 0 0 ) 0 0 0 1 1 - 0
www.elsevier.com/locate/soilbio
* Corresponding author. Tel.: 480; fax: +39-050-588-473.
while compost induces lower biochemical activities but more resistance in soils (Pascual et al., 1998, 1999).
An appropriate technology in waste management for producing compost is the use of earthworms. This technology, which refers to both social and environ-mental goals of ``sustainable development'', is largely used in India (Snel, 1999), in Australia and New Zeal-and (Appelhof et al., 1996), in Cuba (Werner Zeal-and Cue-vas, 1996), and in Italy (Ceccanti and Masciandaro, 1999). Worms are being put to use at all levels from home worm bins to large scale composting of munici-pal and industrial biosolids and yard trimmings (Appelhof et al., 1996). The result of the composting process through worms (vermicomposting) is a high quality humic product (earthworm casting) to be used as soil organic amendment (Ceccanti and Mascian-daro, 1999). From the literature it is known that earth-worms accelerate composting process (Riggle and Holmes, 1994), control potential environmental risks, improve soil structure and chemico-physical and bio-logical properties of the soil (Stewart and Scullion, 1988; Allison et al., 1973).
In this study we want to obtain the advantages of using both, fresh and composted organic materials as soil amendments by promoting a sort of composting
of fresh sludges directly on soil (``in situ'') using
Eise-nia fetidaearthworms.
The speci®c objectives were: (1) to evaluate the sludge transformation through earthworms (vermicom-posting) directly on soil, in relation to sludge quality; (2) to investigate the eects of dierent sludge compo-sition on soil chemico-physical and biochemical prop-erties.
``In situ'' vermicomposting of fresh sludges could concur to: (a) cost reduction in waste stabilization, (b) synthesis of humic substances in situ that surely are more pertaining to that particular soil environment
than those prepared in an industrial plant, and (c) add organic matter to control erosion even in soils on steep slopes.
2. Materials and methods
2.1. Soil
A sandy soil located at Peccioli (Pisa) farm
(Central-West of Italy, 850 mm average yearly rainfall, 15.68C
average yearly temperature) was sampled and used for the laboratory experiments. Some characteristics of the soil, before the experiments, are reported in Table 1.
2.2. Sludges
Aerobic and anaerobic sludges were collected from a municipal sewage plant serving residential and paper industry areas. The aerobic sludge was collected from an activated-sludge plant treating municipal waste-waters, while the anaerobic sludge came from a
diges-ter treating sludges obtained from municipal
wastewaters (10%) and paper-mill wastewaters (90%); Their characteristics are reported in Table 1. Before using in the experiment, aerobic and anaerobic sludges, with a dry matter content of about 40%, were mixed and pre-treated (manually aerated every day for 15 days) for reducing the characteristic smell of putresci-ble substances, and for decreasing biotoxic compounds formed under anaerobiosis (Ceccanti and Mascian-daro, 1999).
2.3. Treatments
One kilogram of dry-weight-equivalent soil was
placed into plastic containers (15 106 cm).
Sub-Table 1
Characteristics of soil and sludges used for the experiments of ``in situ'' vermicomposting. The values are expressed on a dry weight basis
Parameters Units Soil Aerobic sludge Anaerobic sludge
sludge
Texture Sandy-loam ± ±
Dehydrogenase activity mg INTF gÿ1hÿ1 8.21 652 813
Electrical conductivity dS mÿ1 0.140 0.436 0.956
sequently, ten adult worms (Eisenia fetida) were added to the pre-treated mixtures of aerobic and anaerobic sewage sludges, having a content of dry matter of about 40%; 0.5 kg (as wet weight) of sludge mixture enriched with worms was spread on the soil surface, corresponding to a layer of about 3 cm (the total weight, soil + sludge mixture, in each pot was 1.5 kg). The following treatments were carried out and the percentages (on a weight basis) represent increasing amounts of anaerobic sludge in the mixtures of aerobic±anaerobic sludges. (Table 2). The soil under treatment, during the experiments, was wetted to main-tain the water content at 60±80% of soil ®eld capacity. This moisture has been found optimum for worm growth and development. The experiments were
car-ried out in triplicate at room temperature (20±258C)
for four months. After one week, the number of worms moving from the sludge above the soil surface to the soil was manually checked and recorded. After four months, the sludge left on the soil surface was removed and analysed to determine the amount of sludge transferred into the soil by the worms. The worms were removed from soil putting a little pile of fresh sludge in a corner of the container which attracted them with the fresh food. One soil sample (made up by mixing ®ve sub-samples) per container
was collected, sieved to <2 mm, and stored at 48C for
biochemical analysis. For chemical and physical ana-lyses, soil samples were dried and stored at room tem-perature. The results, expressed on a dry weight (dw) basis, were the average of three replicates for each treatment.
2.4. Extraction of water-soluble organic fraction from the soils after treatments
The extraction was carried out by shaking soil with
distilled water (soil/water ratio of 1:10 w/v) at 508C
for 1 h. Then the extracts were centrifuged at 15,000 g for 15 minutes (Garcia et al. 1990).
2.5. Chemical analyses
Electrical conductivity (EC) and pH were measured
in 1/10 (w/v) aqueous solution. Total C (Ctot) and
water-extractable organic C (WSC) were determined by dichromate oxidation (Yeomans and Bremner,
1988), and total N (Ntot) by the Kjeldahl method
(Jackson, 1960). Inorganic anions (NO2±N, NO3±N,
SO4±S) were determined on soil water extracts by ionic
chromatography using a DIONEX chromatograph.
2.6. Biochemical assay
Dehydrogenase (DH-ase) activity was determined by
the method of Garcia et al. (1993), using INT (2-p
-iodo-3-nitrophenyl 5-phenyl tetrazolium) chloride as substrate, dark incubation, and determining spectro-photometrically (490 nm) the INTF (iodonitrophenyl tetrazolium formazan) product of the reaction. This method is based on the combination of two earlier reported methods (Bene®eld et al. 1977; Trevors, 1984).
2.7. Phyto-test
An experiment of plant growth, using Petri dishes was carried out in triplicate for 3 weeks, to evaluate the potential phytotoxicity of sludge-treated soils. Fifty grams of soil after the four month study was seeded
with 0.5 g (about 40 seeds) of Lepidium sativum, kept
moist (at 60% of the water retention capacity) and at room temperature. A growth index was calculated as follows:
GI% P
P0
100
where P is the plant weight of treated soil, and P0 is
the plant weight of control soil. Control soil was the soil treated with 0% anaerobic sludge, in order to evaluate the residual toxicity related to the presence of anaerobic sludge (Hartenstein, 1981).
2.8. Physical analysis
Twenty grams of soil (sieved to <2 mm) was mixed with distilled water (20 ml) until it became ¯uid. The
mixture was then poured into a square box (99 cm).
The drying process was carried out at a constant
tem-perature (258C). The optical measurements were
Table 2 Treatments
Anaerobic sludge in the mixture (%) Anaerobic sludge (g) Aerobic sludge (g) Soil (g)
0% 0 500 1000
25% 125 375 1000
50% 250 250 1000
75% 375 125 1000
directly conducted on the soil samples prepared in this way. Cracking measurements were carried out with a Quantimet 570 apparatus using an electro-optical pro-cedure for Image processing and Analysis systems. Brie¯y, the image of the sample is scanned by a televi-sion camera and displayed on a monitor screen. The video signal is passed to a detector where 500,000 pic-ture points (pp) on the image are individually analysed for their grey level. Cracks are measured by setting the instrument to detect the corresponding grey level which is dierent from the clods. The analysis system gives the percent of cracking area with respect to the total area, and cracks can be further subdivided into dierent dimensional classes (in this case, cracks
grouped in three classes: (i) < 500 mm, (ii) 500±1000
mm, and (iii) >1000 mm have been reported) which
correspond to average values of the crack width. All determinations were made in triplicate, with a dier-ence, between the three measurements on the same soil sample, of about 0.1% (Pagliai et al., 1980).
2.9. Chemico-structural analyses of sludge: pyrolysis-gas chromatography (Py-GC)
Py-GC is based on a rapid decomposition of organic matter under a controlled high ¯ash of temperature. A gas chromatograph is used for the separation and quanti®cation of pyrolytic fragments (pyro-chromato-gram) originating from organic matter decomposition. Py-GC was carried out on residual sludge after the separation from the soil, at the end of the experiments. A small quantity of air-dried, ground (<100 mesh) sample was introduced into pyrolysis quarz microtubes in a CDS Pyroprobe 190. The Py-GC instrument con-sists of a platinum coil probe and a quartz sample
holder. Pyrolysis was carried out at 8008C for 10 s,
with a heating rate of 108C/ms (nominal conditions).
The probe was coupled directly to a Carlo Erba 6000 gas chromatograph with a ¯ame ionization detector (FID). Chromatographic conditions were as follows: a
3 m6 mm, 80/100 mesh, SA 1422 (Supelco)
PORO-PAK Q column; the temperature pyrogram was 608C,
increasing to 2408C by 88C/min (Garcia et al., 1992).
The pyrograms were quanti®ed through seven peaks corresponding to volatile fragments (Ceccanti et al.,
1986): acetic acid (K), acetonitrile (E1), benzene (B),
toluene (F3), pyrrole (O), furfural (N), and phenol (Y).
The peak areas were normalised, so that the area under each peak referred to the percentage of the total of the seven peaks (relative abundances). The alpha-betic code used was conventional and has already been employed in earlier papers on natural soils (Alcaniz et al., 1984). Peak purity of the most important frag-ments had previously been checked by coupling the same chromatographic system to a mass-detector in the same operative conditions.
Two ratios, between the relative abundances of some of the above peaks, were determined (Ceccanti et al., 1986):
. N/O: mineralization index: This index expresses the
ratio between furfural, which is the pyrolytic pro-duct arising from polysaccharides, and pyrrole, which is derived from nitrogenous compounds, humi®ed organic matter, and microbial cells. The higher the ratio, the lower the mineralization of the organic matter, that is, a high concentration of poly-saccharides may still be present.
. B/E3: humi®cation index: The higher the ratio, the
higher the humi®cation of the organic matter, because benzene derives mostly from the pyrolytic degradation of condensed aromatic structures, while toluene comes from aromatic uncondensed rings with aliphatic chains (Ceccanti et al., 1986).
In addition, an index of similarity (Sij) between the
relative abundances (I) of the homologous peaks (k)
in two pyro-chromatograms (i and j), was calculated
using the following expression: Sij
S Ii=Ijk=n, withIi<Ij,andnis the number of peaks:
The similarity coecient Sijis a numerical parameter
that permits a comparison between a pair of pyro-grams without discriminating the peaks. The index varies in the range 0±1: the higher the value, the higher the similarity. However, three conventional levels, high (0.75±0.85), middle (0.70±0.75), and low (0.60±0.70), have been suggested for the characterization of hetero-geneous materials, such as soil organic matter (Cec-canti et al., 1986) and compost (Cec(Cec-canti and Garcia, 1994).
2.10. Statistical analysis
All results reported in the text are the means of de-terminations made on three replicates. The means were compared by using least signi®cant dierence values
calculated atp< 0.05 (Tukey's test).
Correlation matrices were calculated with the results of the parameters measured in all treatments. The pro-cedure of correlation analysis generates a matrix of correlation coecients for a set of observed values.
The signi®cance levels reported p<0:05are based on
the Student's distribution.
3. Results and discussion
3.1. Sludges: chemico-structural changes
sludge with respect to the aerobic one brought about an increase in the quantity of sludge left on the soil surface (Table 3).
Many authors have found that fresh anaerobic sludge can be toxic for earthworms (Hartenstein, 1981) in that it could hinder worm growth and activity. This inhibitory eect could be due to (i) oxygen de®ciency caused by low aeration of the compacted growth med-ium, and (ii) the presence of toxic compounds originat-ing from the anerobic process of sludge stabilization at depurator plants.
On this basis, it might be hypothesized that the worms were capable of distinguishing potentially toxic from non-toxic sludges. In fact, the percentage of adult worms moving, after one week, from the surface sludge towards the soil increased with the increase of anaerobic sludge in the mixtures, thus suggesting that the worms had chosen the soil as habitat for their bioactivity (Fig. 1). By comparing pairs of
pyro-chro-matograms of residue sludges (not used by the worms as feed) it was possible to quantify the dierences between two treatments, through an index of similarity
Sij (Table 4). Fresh aerobic and anaerobic sludges
showed a very low Sij index, indicating great
dier-ences between them. The dierdier-ences vanished after four months incubation with soil and worms, as
demonstrated by the fact that the Sij value increased
from 0.61 to 0.71, thus falling in the range of weak similarity (Ceccanti et al., 1986). However, each sludge mixture, when compared with the others and with fully aerobic or anaerobic sludge, showed the highest
Sij values (higher than 0.75) that fell in the range of
great similarity. From this trend, it can be assumed that the mixtures, regardless of their composition, underwent a similar biotransformation during the four month incubation period, while fully aerobic or an-aerobic sludges maintained a certain dierence. Practi-cally, a sort of composting ``in situ'' occurred, which reduced the initial dierences between aerobic and an-aerobic sludges, especially when mixed.
The evolution of the relative abundances of the main pyrolytic fragments (Table 5), which normally
give evidence of micro-structural dierences of
samples, here seemed to support the trend of the
simi-larity index Sij. In fact, signi®cant dierences were
found only for fully aerobic and anaerobic sludges,
Table 3
Quantity of sludge remaining on the soil surface after four months of ``in situ'' vermicomposting, and relative percentages with respect to the in-itial amount of aerobic/anaerobic sludge mixture put on the soil at the beginning of the experiments
Treatments (anaerobic sludge %) Residue of sludge left on the soil surface (g per container) Percentage of the initial amount
0% 20 10
25% 30 15
50% 45 22.5
75% 50 25
100% 74 37
Fig. 1. Distribution of adult worms in grams in sludge and in soil after one week of treatments with dierent mixtures of aerobic±an-aerobic sludges (dierent letters indicate statistically dierent values).
Table 4
Pyrolytic indices of similarity (Sij) between pairs of treatments at the
end of ``in situ'' vermicomposting
Comparison between treatments Sij
0±25% =0.79
0±50% =0.83
0±75% =0.80
100±25% =0.88
100±50% =0.72
100±75% =0.88
25±50% =0.81
25±75% =0.96
50±75% =0.80
0±100% =0.71
0±100% (fresh sludges) =0.61a
a
The bold value represents theSijindex between aerobic and
while the mixtures were very similar. Pyrolytic ratios among the relative abundances N/O (furfural/pyrrole,
mineralization index) and B/E3 (benzene/toluene,
humi®cation index) better enhanced dierences
between composted aerobic, anaerobic sludges and their mixtures. The N/O ratio, for example, is con-sidered an index of mineralization; the higher the value of the ratio, the lower the degree of degradation of the sample. The values obtained for this index con®rmed that the sludge mixtures appeared more degradable when the anaerobic sludge was absent (0%) or present at medium±low concentrations (25±50%). Based on the fact that pyrrole is chemically and microbiologi-cally more stable than furfural, N/O has also been interpreted as an index of organic matter stability; being lower when the organic matter is more evolved (less mineralizable) (Masciandaro et al., 1998). The
index of humi®cation B/E3(benzene/toluene ratio)
rep-resents the degree of ``condensation'' of aromatic rings; the higher the ratio, the higher the degree of or-ganic matter humi®cation. The humi®cation index was highest in the treatment with 50% of anaerobic sludge and lowest in the condition of 100% anaerobic sludge.
From the values of these indices, it can be seen that among the mixtures, the 50% aerobic±50% anaerobic sludge was more mineralised and humi®ed (Table 5), while fully anaerobic sludge was less mineralised and hence less humi®ed than fully aerobic sludge. These results suggest that anaerobic sludge is more recalci-trant for microbial decomposition and, for this reason, it is probably undesirable feed for worms.
3.2. Soil: changes in biochemical and structural properties
The presence of dierent degrees of mineralization and humi®cation on the soil surface of sludges, and the dierent number of soil-colonizing, worms, is expected to modify soil properties. At the end of the four months of incubation, the sludges were removed from the soil surface and the soil was sampled and
analysed. Total soil N and NO3±N showed lower
values when mixtures of aerobic and anaerobic sludges were spread on the soil surface, than when fully aerobic or anaerobic sludge was separately employed
(Table 6). High NO3±N concentration means that
favourable conditions for nitrifying microrganisms were prevalent in the soil medium. Worms, due to their movement in the soil, make channels thus creat-ing aerobic micro-sites in which the nitri®cation pro-cess takes place (Parkin and Berry, 1994). As a result,
NO3±N concentration and the NO3±N/NH4±N ratio
were very high when 100% anaerobic sludge was applied, that is when the highest percentage of worms moved from sludge into the soil.
Usually a positive correlation between soil microbial activity and total organic C or water soluble C has been found to depend on soil quality level (Lynch and Panting, 1980; Garcia et al., 1997). Soil microbial ac-tivity is commonly expressed as dehydrogenase acac-tivity (DH-ase) as was suggested by Trevors (1984), Alef (1991) and Garcia et al. (1994)). For this reason, dehy-drogenase has been proposed as a valid biomarker of soil quality under dierent agronomic practices (Reddy and Faza, 1989; Bolton et al., 1985) and climate (Cec-canti et al., 1994). In this study, as expected, dehydro-genase activity showed decreasing values from 0 to 100% treatment (Fig. 2), thus re¯ecting the decrease in the content of soil total organic C (Table 6) due to the worm action and activity. It has been suggested that DH-ase activity, especially when referred to the ener-getic and immediately available C substrate, as water soluble C (WSC) gives an idea of the metabolic poten-tiality of soil rehabilitation (Masciandaro et al., 1998; Garcia et al., 1997).
The metabolic response of the soil when the concen-tration of anaerobic sludge did not exceed 50% was high, as showed by the DH-ase/WSC index (Fig. 2). The relation between soil metabolic activity and C and N forms was evidenced by the correlation between available C and N substrates and dehydrogenase ac-tivity (Table 7). The availability of mineral and organic soluble nutrients and the enhancement of microbial ac-tivity, created the conditions suitable for plant
germi-Table 5
Pyrolysis-gas chromatography of sludge left on soil at the end of the experiments. Relative abundances (%) of the main pyro-chromatographic peaks calculated by the normalised peak areas: acetonitrile (E1), acetic (K), benzene (B), pyrrole (O), toluene (E3), furfural (N), phenol (Y).
Changes in the indices of mineralization (N/O) and humi®cation (B/E3) a
nation and growth. Tests of plant growth carried out on the soil after the sludge treatments (Fig. 3), showed that no phyto-toxic substances had been introduced into the soil by the worm movement. The growth index (GI%) was high in soil treated with
25±50% of anaerobic sludge, suggesting that the pre-sence of anaerobic sludge at a low concentration (25%) or equally mixed with anaerobic sludge (50% treatment) promoted the chemico-nutritional con-ditions to support plant growth and root
develop-Table 6
Chemical characteristics of the soil after the ``in situ'' vermicomposting experiments. The values are expressed on a dry weight basis. For each parameter, the values followed by the same letter are not signi®cantly dierent p<0:05)
Treatments
Anaerobic sludge (%) pH EC (dS mÿ1) Ctot(g kg ÿ1
) Ntot(g kg ÿ1
) NH4ÿN (mg kg ÿ1
) NO3ÿN (mg kg ÿ1
) NO3ÿN
NH4ÿN SO4ÿS (mg kg
ÿ1
)
0% 7.2 a 0.420 a 41.4 a 4.20 a 9.3 a 280 b 30.1 b 190 a
25% 7.1 a 0.430 a 40.3 a 3.50 b 6.0 b 220 b 36.6 b 160 b
50% 7.2 a 0.370 b 39.8 a 3.30 b 6.0 b 250 b 41.6 b 170 b
75% 7.0 a 0.340 b 36.7 b 3.60 b 5.8 b 255 b 49.1 b 100 c
100% 7.0 a 0.360 b 35.4 b 4.40 a 5.1 c 450 a 88.2 a 110 c
Fig. 2. Dehydrogenase activity (DH-ase), water soluble carbon (WSC), and metabolic potential index (DH-ase/WSC10ÿ2) in treated soil for
each parameter the values followed by the same letter are not signi®cantly dierent p<0:05). Table 7
Area % of surface shrinkage in the soil after the ``in situ'' vermicomposting treatments with respect to the total cracking area and three size classes. In each column dierent letters indicate statistically dierent values p<0:05)
Treatments
(anaerobic sludge %) Total cracking area (%) < 500mm (%) 500±1000mm (%) > 1000mm (%)
Untreated soil 1.60 d 1.07 c 0.43 c 0
0% 22.5 a 15.4 a 6.6 a 0.54 a
25% 20.9 a 15.1 a 5.5 b 0.33 c
50% 19.6 b 13.0 b 6.3 a 0.32 c
75% 19.2 b 12.6 b 6.2 a 0.30 c
ment. This was also favoured by the modi®cation of soil physical structure.
The physical properties of the soil following the ``in situ'' vermicomposting treatments had improved. In fact, as is well known (Stewart and Scullion, 1988), the addition of organic matter sources to a soil improves its physical properties since the organic matter supplies polysaccharides and other polymeric substances which act as aggregating cements; also, worm casts are con-sidered more stable than other soil aggregates (Allison et al., 1973). The physical eects were evaluated through the measurement of surface shrinkage of the soil, at the end of the treatments. The values of soil surface shrinkage are given in Table 8 as a percentage of the total area of the soil samples. All treatments increased the percentage of total shrinkage area, with respect to the untreated soil; shrinkage represented the whole area of cracks belonging to all dimensional
classes ranging between 0 and 2000 mm in diameter.
The increase of the total shrinkage area was mostly
due to the formation of small size cracks (<500 mm,
which are considered important for maintaining opti-mum conditions of humidity) as evidenced by the cor-relation coecients reported in Table 8. It has been
reported that small±medium size cracks (<1000 mm)
constitute a micro-habitat, very suitable for microbial activity (Guidi et al., 1978). In fact, in this soil a highly positive correlation between dehydrogenase enzyme ac-tivity, total shrinkage area and small size cracks was found (Table 8). Smaller size cracks are an index of
Ta
improved soil structure because they are associated with an increase in small aggregates that results in a more open sub-angular blocky structure which is more favourable for plant development (Pagliai et al., 1978). The improvement of soil structure is of great agro-nomic relevance, in that good physical properties favour water retention, oxygen diusion, and nutrient availability which can improve crop yields.
In conclusion, the experiments of ``in situ'' vermi-composting favoured the introduction of good quality organic matter into the soil through the action of worms which represented an agent preventing the accumulation of toxic compounds, as would have occurred with direct sludge incorporation. In this bio-solid sludge land application system, the dierences in initial sludge composition might interact with the mechanisms by which, earthworms aected soil quality. When aerobic and anaerobic sludges were mixed is equal proportions, a stimulation of soil metabolism occurred without adverse eects on plant growth. Soil biotic activity, favoured by the availability of C and N compounds, contributed to soil structural improvement.
Acknowledgements
The authors thank Mr. M. La Marca for technical assistance in the physical measurements.
References
Alcaniz, J.M., SereÁs, A., Gassiot-Matas, M., 1984. DiferenciacioÁn entre humus mull carbonatado y mull evolucionado por Py-GC. In: Proceedings of the I Congreso Nacional de la Ciencia del Suelo, Madrid, vol. 1, 217±228.
Alef, K., Methodenhandbuch Bodenmikrobiologie. Ecomed, Landsberg/Lech, Germany., (1991).
Allison, F.E., 1973. Functions and eects of organic matter in min-eral soils. In: Allison, F.E. (Ed.), Soil Organic Matter and its Role in Crop Production. Elsevier Scienti®c, New York, pp. 277± 378.
Appelhof, M., Webster, K, Bucker®eld, J., 1996. Vermicomposting in Australia and New Zealand. Biocycle 6, 63±66.
Ayuso, M., Pascual, J.A., Garcia, C, Hernandez, T, 1996. Evaluation of urban wastes for agricultural use. Soil Science and Plant Nutrition 42, 105±111.
Bastian, R.K, Ryan, J.A., 1986. Design and management of success-ful land application system. In: Utilization, Treatment and Disposal of Waste on Land. Soil Science Society of America, Madison, WI, USA, pp. 217±234.
Beloso, M.C., Villar, M.C., Cabaneiro, A., Carballas, M., Gonzalez-Pietro, S.S, Carballas, T, 1993. Carbon and nitrogen mineraliz-ation in an acid soil fertilized with composted urban refuses. Bioresource Technology 45, 123±129.
Bene®eld, C.B., Howard, P.J.A, Howard, D.M, 1977. The estimation of dehydrogenase activity in soil. Soil Biology and Biochemistry 6, 67±70.
Bolton, H., Elliott, L.F., Papendick, R.I., Bezdicek, D.F, 1985. Soil microbial biomass and selected soil enzyme activities: eect of
fer-tilization and cropping practice. Soil Biology and Biochemistry 17, 297±302.
Ceccanti, B., Alcaniz, J.M., Gispert, M, Gassiot, M., 1986. Characterization of organic matter from two dierent soils by py-rolysis-gas chromatography and isoelectrofocusing. Soil Science 142, 83±90.
Ceccanti, B., Garcia, C., 1994. Coupled chemical and biochemical methodologies to characterize a composting process and the humic substances. In: Senesi N., Miano T. M. (Eds.). Humic Substances in the Global Environment and Implication on Human Health. pp. 1279±1284.
Ceccanti, B., Pezzarossa, B., Gallardo-Lancho, F.J., Masciandaro, G., 1994. Bio-tests as markers of soil utilization and fertility. Geomicrobiology Journal 11, 309±316.
Ceccanti, B., Masciandaro, G., 1999. Vermicomposting of municipal and papermill sludges. Biocycle, 6, 71±72.
Garcia, C., Hernandez, T., Costa, F., 1990. Study on water extract of sewage sludge compost. Soil Science and Plant Nutrition 37, 399±408.
Garcia, C., Hernandez, T., Costa, F., Ceccanti, B., Calcinai, M., 1992. A chemical-structural study of organic wastes and their humic acids during composting by means of pyrolysis-gas chro-matography. The Science of the Total Environment 119, 157±168. Garcia, C., Hernandez, T., Costa, F., Ceccanti, B., Masciandaro, G., 1993. The dehydrogenase activity in a soil as an ecological mar-ker in process of perturbated system regeneration. In: Eleventh International Symposium on Environmental Biogeochemistry, 27±30 September (summary), Salamanca, Spain.
Garcia, C., Hernandez, T., Costa, F., 1994. Microbial activity in soils under Mediterranean environmental conditions. Soil Biology and Biochemistry 26, 1185±1191.
Garcia, C., Hernandez, T., Costa, F., 1997. Potential use of dehydro-genase activity as an index of microbial activity in degraded soils. Communication in Soil Science and Plant Analyses 28, 123±134. Gliotti, C., Giusquiani, P.L., Businelli, D., Machioni, A., 1997.
Composition changes of dissolved organic matter in a soil amended with municipal waste compost. Soil Science 162, 919± 926.
Guidi, G., Pagliai, M., Petruzzelli, G., 1978. Quantitative size evalu-ation of cracks and clods in arti®cially dried soil samples. Geoderma 19, 105±113.
Hartenstein, R., 1981. Use of Eisenia foetida in organic recycling based on laboratory experiments. Workshop on the role of earth-worms in the stabilization of organic residues 1 155±165. Jackson, M.L., 1960. Soil Chemical Analysis. Prentice-Hall,
Englewood Clis, NJ.
Larson, W.E., Walsh, L.M., Stewart, B.A., Boelter, D.H., 1981. Soil and Water Resources: Research Priorities for the Nation. Soil Science Society of America, Madison, WI, p. 45.
Lerch, R.N., Barbarick, K.A., Sommers, L.E., Westfall, D.G., 1992. Sewage sludge proteins as labile carbon and nitrogen sources. Soil Science Society of America Journal 56, 1470±1476.
Lynch, J.M., Panting, L.M., 1980. Cultivation and the soil biomass. Soil Biology and Biochemistry 12, 29±33.
Masciandaro, G., Ceccanti, B., Gallardo-Lancho, J.F., 1998. Organic matter properties in cultivated versus set-aside arable soils. Agriculture Ecosystems and Environment 67, 267±274.
Pagliai, M., Guidi, G., Petruzzelli, G., Eect of molecular weight on dextran-soil interaction. Modi®cation of Soil Structure. In: Emerson W. W., Bond R.D., Dexter A.R. (Eds.) Vol. 21, pp 175±180.
Pagliai, M., Guidi, G., La Marca, M., 1980. Macro- and micromor-phometric investigation on soil-dextran interactions. Journal of Soil Science 31, 493±504.
Pascual, J.A., Hernandez, T., Garcia, C., Ayuso, M., 1998. Enzymatic activities in arid soil amended with urban organic wastes: laboratory experiment. Bioresource Technology 64, 131± 138.
Pascual, J.A., Garcia, C., Hernandez, T., 1999. Comparison of fresh and composted organic waste in their ecacy for the improve-ment of arid soil quality. Bioresource Technology 68, 255±264. Reddy, G.B., Faza, A., 1989. Dehydrogenase activity in sludge
amended soil. Soil Biology and Biochemistry 21, 327±331. Riggle, D., Holmes, H., 1994. New horizons foor commercial
vermi-culture. Biocycle 10, 58±62.
Snel, M., 1999. Community-based vermicomposting in developing countries. Biocycle 4, 75±76.
Stewart, V.I., Scullion, J., 1988. Earthworms, soil structure and the rehabilitation of former opencast coal-mining land. In: Edwards, C.A, Neuhauser, F. (Eds.), Earthworms in waste and environ-mental management. SPB, The Hague, pp. 263±272.
Trevors, J.T., 1984. Dehydrogenase activity in soil. A comparison between the INT and TTC assay. Soil Biology and Biochemistry 16, 673±674.
Werner, M., Cuevas, J.R., 1996. Vermiculture in Cuba. Biocycle 6, 57±62.
