Chaoui 2003 Soil Biology and Biochemistry

7/30/2019 Chaoui 2003 Soil Biology and Biochemistry

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Effects of earthworm casts and compost on soil microbial activityand plant nutrient availability

Hala I. Chaoui1, Larry M. Zibilske2, Tsutomu Ohno*

Department of Plant, Soil and Environmental Sciences, 5722 Deering Hall, University of Maine, Orono, ME 04469-5722, USA

Received 9 November 2001; received in revised form 6 September 2002; accepted 26 September 2002

Abstract

Vermicomposting differs from conventional composting because the organic material is processed by the digestive systems of worms. The

egested casts can be used to improve the fertility and physical characteristics of soil and potting media. In this study, the effects of earthworm

casts (EW), conventional compost (CP) and NPK inorganic fertilizer (FT) amendments on N mineralization rates, microbial respiration, and

microbial biomass were investigated in a laboratory incubation study. A bioassay with wheat (Triticum aestivium L.) was also conducted to

assess the amendment effects on plant growth and nutrient uptake and to validate the nutrient release results from the incubation study. Both

microbial respiration and biomass were significantly greater in the CP treatment compared to EW treatment for the initial 35 days of

incubation followed by similar respiration rates and biomass to the end of the study at 70 days of incubation. Soil NO 32 increased rapidly in

the EW and CP treatments in the initial 30 days of incubation, attaining 290 and 400 mg N kg 21 soil, respectively. Nitrate in the EW

treatment then declined to 120 mg N kg21 soil by day 70, while nitrate in the CP treatment remained high. While ammonium levels decreased

in the CP treatment as nitrate level increased with increasing incubation time, a low level of ammonium was maintained in the EW treatment

throughout the incubation. The wheat bioassay study included two additional cast treatments (EW-N and EW2) to have treatments with

higher levels of N input. Plants grown with CP or FT treatment had a lower shoot biomass and higher shoot N content than in EW-N and EW-

2 treatments, and also showed symptoms of salinity stress. Ionic strength and other salinity indicators in the earthworm cast treatments were

much lower than in the CP treatment, indicating a lower risk of salinity stress in casts than in compost. All cast and compost amendments

significantly increased wheat P and K uptake compared to either the non-amended control or the mineral fertilizer treatment. The results

show that casts are an efficient source of plant nutrients and that they are less likely to produce salinity stress in container as compared to

compost and synthetic fertilizers.

Published by Elsevier Science Ltd.

Keywords: Earthworm casts; N mineralization; Plant nutrient uptake; Microbial respiration; Vermicomposting

1. Introduction

Vermicomposting is the digestion of organic materials by

earthworms which produce excreta known as casts.

Edwards (1995) reported that in a Rothamsted study with

25 types of vegetables, fruits or ornamentals, earthworm

casts (EW) performed better than compost or commercial

potting mixture amendments. It was suggested that the

higher crop performance of the cast treatment was due to:better soil physical structure; presence of plant growth

hormones; higher levels of soil enzymes; and greater

microbial populations. The beneficial effects of earthworm

cast utilization in other horticulture settings have also been

reported (Tomati et al., 1987; Hidalgo, 1999; Saciragic and

Dzelilovic, 1986).

EW typically have high N contents which suggests that

they would be good sources of plant N (Parmelee and

Crossley, 1988; Ruz-Jerez et al., 1992). Fresh casts often

contain high ammonium levels, but rapid nitrification results

in stable levels of both nitrogen forms due to organic matter

protection in dry casts (Decaens et al., 1999). Nutrients in

casts are initially physically protected, but this is reduced as

the aggregate structure weakens over time (McInerney and

0038-0717/03/$ - see front matter Published by Elsevier Science Ltd.

PII: S 0 0 3 8 - 0 7 1 7 ( 0 2 ) 0 0 2 7 9 - 1

Soil Biology & Biochemistry 35 (2003) 295302www.elsevier.com/locate/soilbio

1 Department of Agricultural Engineering, The Ohio State University,

250 Agricultural Engineering Building, 590 Woody Hayes Drive,

Columbus, OH 43210, USA.2

USDA-ARS, Kika de la Garza Subtropical Agricultural ResearchCenter, Integrated Farming and Natural Resources Research Unit, 2413

E. Hwy 83, Bldg 201, Weslaco, TX 78596-8344, USA.

* Corresponding author. Tel.:1-207-581-2975; fax:1-207-581-2999.

E-mail addresses: [email protected] (T. Ohno), [email protected]

(H.I. Chaoui), [email protected] (L.M. Zibilske).
http://www.elsevier.com/locate/soilbiohttp://www.elsevier.com/locate/soilbio


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Bolger, 2000). In addition to increased N availability, C, P,

K, Ca and Mg availability in the casts is also greater than in

the starting feed material (Orozco et al., 1996; Daniel and

Anderson, 1992; Lavelle et al., 1992; Basker et al., 1993).

Earthworm cast amendment has been shown to increase

plant dry weight (Edwards, 1995; Lui et al., 1991) and plant

N uptake (Zhao and Huang, 1988; Tomati et al., 1994). The

beneficial effect of EW has been observed in both

horticultural plants (Tomati et al., 1987; Hidalgo, 1999;

Saciragic and Dzelilovic, 1986) and in agronomic crops

(Pashanasi et al., 1996). Cantanazaro et al. (1998) and Cox

(1993) demonstrated the importance of the synchronization

between nutrient release and plant uptake and showed that

slower release fertilizers can increase plant yield and reduce

nutrient leaching. EW could serve as a naturally produced

slow release source of plant nutrients.Traditional composts also have agronomic value, but N

immobilization (Sims, 1990), salinity effects (OBrien and

Barker, 1996), and pathogen levels (Eastman, 1999) may be

problematic. Vinceslas-Akpa and Loquet (1997) compared

the effects of composting and vermicomposting lignocellu-

losic maple waste and reported that the vermicompost

product had a lower C/N ratio, higher protein:organic C

ratio, and higher levels of N, which indicates that the

vermicompost products were more suitable for soil amend-

ment use.

In containerized production systems, EW used as an

alternative soil amendment could help reduce several

problems associated with the use of conventional syntheticfertilizer such as excessive leaching loss of nutrients and

salinity-induced plant stress. In addition EW can improve

soil porosity, and thus provide a better root growth medium.

In this study, the effects of stabilized EW, compost and

synthetic fertilizers on soil fertility and plant growth were

investigated by determining mineralization rates of N, P,

and K; microbial biomass-C levels; and microbial respir-

ation in a laboratory incubation experiment. In addition, a

greenhouse plant growth study with wheat (T. aestivium L.)

was conducted to confirm the results of the incubation

experiment.

2. Materials and methods

2.1. Soils and materials characterization

The surface horizon of a Nicholville (course-silty, mixed,

frigid, Aquic Haplorthod) soil was obtained from the

University of Maine Sustainable Agriculture Research

Farm in Stillwater, Maine, USA. The soil contained 78%

sand, 12% silt and 10% clay fractions as determined by the

hydrometer method (Gee and Bauder, 1986). EW of

Lumbricus rubellus were obtained from the Cape Cod

Worm Farm (Buzzards Bay, Massachusetts, USA). Com-

post produced from cattle manure, leaves, and food scrapswere obtained from the University of Maine Witter

Research Farm. The particular feedstock utilized for cast

production and composting will influence the specific

chemical characteristics of the end products. However, we

believe that the materials used in this study are representa-

tive of typical EW and compost available to growers. The

compost and casts were stored at their native moisture state.

The extractable NH4-N and NO3

2-N in the soil and

amendment materials were determined by KCl extraction.Five g samples were extracted in 50 ml of 1 M KCl, placed

on a reciprocal shaker for 15 min at 200 oscillations min21.

The suspensions were filtered and analyzed for NH4-N and

NO32-N using an autoanalyzer. Nutrient contents of the

amended soil mixtures were determined by extracting 5 g

soil with 20 ml of modified-Morgan extract (1.25 M

ammonium acetate, pH 4.8), shaking for 15 min at

18 oscillations min

21

, and filtration of the suspension(McIntosh, 1969). The P, K, Ca, and Mg content of the

extract was determined by inductively coupled plasma

atomic emission spectrometry (ICP-AES). The total carbon

and nitrogen contents of the soil and amendment materials

were determined using a LECO CN-2000 analyzer (St

Joseph, MI).

2.2. Preparation of soilamendment mixtures

This study was designed to evaluate the effect of

earthworm cast and compost amendment on N release

dynamics as compared to synthetic fertilizer. The treatments

were: soil earthworm cast (EW); soil compost (CP);soil synthetic fertilizer (FT); and soil without amendment

served as a control (CT). The treatment rates utilized weredesigned to equalize the quantity of N that would be

available to the plant in a 28-day growth period. A

preliminary incubation study estimated that the N miner-

alized in 28 days were 1.5% and 1.7 of the total nitrogen

content in compost and casts, respectively. The amendment

rates were calculated taking into account the N content, the

bulk densities of the two amendments and the 530 cm3

volume of the pot. The EW treatment contained 267 g

casts kg21 soil and the CP treatment contained 189 g

compost kg21 soil. This resulted in an addition of 35% of

compost and 36% of casts by volume, which minimized theeffect of different bulk densities in the different treatments.

These amendment rates would be appropriate for contain-

erized production systems. The FT treatment received

63 mg N kg21 soil as (NH4)2SO4; 20 mg P kg21 soil as

NaH2PO4H2O; and 108 mg K kg21 soil as K2SO4. All

amendments were thoroughly mixed with the soil and

placed in three separate units: an incubation jar to determine

nutrient mineralization and microbial biomass, a respiration

flask to determine microbial respiration, and a planted pot to

bioassay N availability. The experimental design was a

complete randomized design with three replications.

Field capacity of the treatment mixtures as determined by

gravimetric draining was 42% for the soil, 55% for the CPtreatment (soil compost), 57% for the EW treatment

H.I. Chaoui et al. / Soil Biology & Biochemistry 35 (2003) 295302296


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(soil EW), 77% for the EW-2 and 79% for the EW-N

treatment. Soil moisture levels were maintained at their

respective field capacity levels by daily watering throughout

the experiment.

2.3. Nitrogen mineralization study

The open mineralization incubation pots were placed in a

growth chamber kept at 70% relative humidity, 16 h of light

at 20 8C and 8 h of dark at 16 8C. The mineralization jars

were sampled on 3, 7, 15, 22, 28, 35, 43, 50, 57, and 70 days

after incorporation by removing 60 g of soil. Nitrogen and

other plant nutrient content was determined as described

above. Microbial biomass was determined in the soils using

a chloroform fumigation and extraction methodology

(Voroney et al., 1991). Biomass C calculations followed

Howarth and Paul (1994) using an extraction efficiency

constant of 0.35.

2.4. Microbial respiration study

The microbial respiration flasks were placed in a 19 8C

incubator. The flasks were sampled after 3, 7, 14, 22, 28, 35,

50, 57, 64, and 72 days of incubation. The alkali trap method

was used to quantify the released CO2 (Landa and Fang,

1978). The stoppered 500 ml Erlenmeyer respiration flasks

contained soil treatments at field capacity and scintillation

vials containing 10 ml of 4 M NaOH. Flasks containing the

alkali traps alone served as controls. The alkali traps were

replaced at each sampling date and titrated with 1 M HCl(Stotzky, 1965). The respired CO2 was derived from

titration data, corrected for the control.

2.5. Plant bioassay study

Seeds of winter wheat (T. aestivium L.) were briefly

rinsed with 0.525% NaOCl solution, rinsed thoroughly with

de-ionized water and germinated in a petri dish at 24 8C for

2 days prior to planting. Two seedlings were planted per pot

and placed in a growth chamber set to identical conditions

used in the incubation study above. Two treatments in

addition to those used in the N mineralization study

described above were added to the plant bioassay study:EW-2 which had an increased casts amendment rate (as

compared to EW) of 491 g casts kg21 soil (44% by volume),

and EW-N at a rate of 330 g casts kg21 soil (36% by

volume) which used a different cast lot which contained a

higher total N content than EW was used (Table 1). On day

28, the plant shoots were harvested and washed. The plantmatter was dried at 70 8C for 72 h, weighed and ground for

elemental analysis. The N content was determined using the

CN analyzer. The P and K contents were determined by dry-

ashing the plant tissue at 450 8C for 5 h and re-dissolving the

ash prior to analysis by ICP-AES.

A saturated paste extract was prepared from soil sampled

after plant harvest to determine soluble plant nutrients and

electrical conductivity of the extract. The electrical

conductivity values were converted to ionic strength using

the regression reported by Griffin and Jurinak (1973).

3. Results

3.1. Amendment materials

Although the compost material has a higher absolute N

and C content than the casts, the C/N ratios were very

similar (Table 1). The compost material had a higher level

of extractable NH4 than the casts, but both contained

comparable amounts of NO32 (Table 1). The lower levels of

NH4 found in the EW are probably due to the high

nitrification rates associated with cast stabilization (Decaens

et al., 1999). The compost contained a greater amount of

extractable K and lower amount of extractable P than theEW casts.

3.2. Microbial respiration and biomass

The average daily CO2 production is shown in Table 2.

The elevated respiration across all treatments at day 3 is

most likely due to the stimulation of the soil microbial

activity by the greater oxygen availability attributable to

physical mixing of the soil and amendments at the start of

the experiment. Respiration in the control was relatively

stable from day 7 to the end of the experiment. As shown in

Table 2, respiration levels for the CP treatment were

significantly higher than in the EW treatment for the initial35 days and they then became statistically equivalent until

Table 1

Total C and N; 0.5 M K2SO4 extractable C; 1 M potassium chloride extractable NH4-N and NO3

2-N; 1.25 M ammonium acetate (pH 4.8) extractable K and P;

and microbial biomass of the amendments and soil used in the study

Material Total N

(g kg21)

Total C

(g kg21)

C/N ratio Extractable C

(mg kg21)

Extractable NH4-N

(mg kg21)

Extractable NO32-N

(mg kg21)

Extractable K

(g kg21)

Extractable P

(g kg21)

Microbial biomass

(mg kg21)

EW 13.9 157 11.3 724 1.7 282 1.93 3.71 658

EW-N 15.8 183 11.6 1510 2.3 328 2.29 4.31 148

CP 19.6 228 11.6 5090 14.7 310 11.4 2.42 3980

Soil 3.0 39 13.0 566 4.8 19 0.27 0.01 433

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the end of the incubation. In general, microbial biomass

increased rapidly during incubation, peaking at 15 days for

both casts-treated (EW) and compost treated (CP) soils

(Table 3). The level of microbial biomass was significantly

lower in the EW treatment than in the CP treatment in the

initial 28 days which was probably due to the initially lower

microbial biomass contributed by the casts (Table 1).

3.3. Nutrient mineralization

The quantity of KCl-extractable NH4-N decreased

steadily during the entire incubation time in the FT

treatment which received (NH4)2SO4 and decreased rapidly

within the initial 15 days in the CP treatment (Fig. 1A). The

KCl-extractable NO32-N levels generally increased with

incubation time which was probably due to the transform-

ation of ammonium to nitrate (Fig. 1B). The Morgan soil

test levels of both P and K did not change during the

incubation which suggests that the content of these elements

in the amendments was sufficient for microbial growth.

Extractable P (mg P kg21 soil) in the treatments were: EW,

535; CP, 245; FT, 6.1; and CT, 5.1. Extractable K (mg

K kg

21

soil) in the treatments were: EW, 810; CP, 2830; FT,1020; and CT, 380.

3.4. Plant bioassay analysis

Plant dry shoot biomass data are shown in Fig. 2. The

EW-N, EW-2 and CP shoot weights were significantly

greater than in the EW treatment which is probably due to

higher levels of plant available N in these treatments. The

EW treatment biomass was statistically equivalent to the

NPK FT treatment and all amendment treatments signifi-

cantly increased biomass over the unamended control

treatment. The shoot N content (dry shoot weight X N

concentration) was the highest for the NPK FT treatment,followed by the CP treatment (Fig. 3). The EW-2 and EW-N

treatments had shoot N contents which were statistically

equivalent followed by the EW treatment which has

significantly lower N uptake than EW-2, but not EW-N

(Fig. 3). Shoot P content in all the EW and CP treatments

were higher than in the FT and CT treatments, demonstrat-

ing that these amendments may be adequate sources of P

(Fig. 4A). The K content results were similar with uptake

from the EW-2, EW-N, and CP treatments being higher than

from the other treatments (Fig. 4B).

4. Discussion

The elemental composition of the EW and compost

materials used suggests that the materials have potential as

alternative plant nutrient sources. The low C/N ratio

indicates that the casts and compost would be effective

sources of N through rapid N mineralization reactions

Table 2

Average soil respiration in microgram of CO2 produced per gram of dry soil per day in the earthworm cast (EW), compost (CP), fertilizer (FT) and unamended

control (CT) treatment soils during the incubation

Days EW (mg CO2 g soil21 day21) CP (mg CO2 g soil

21 day21) FT (mg CO2 g soil21 day21) CT (mg CO2 g soil

21 day21)

3 187 ba 464 a 115 b 138 b

7 63 b 324 a 39 b 35 b

14 86 b 227 a 47 c 52 c

22 44 b 186 a 39 b 48 b

28 77 b 190 a 23 c 32 c

35 82 b 155 a 21 c 25 c

50 117 a 122 a 14 b 16 b

57 71 a 76 a NDb 14 b

64 79 a 89 a 24 b 12 b

72 66 a 77 a 4 b 6 b

a Fishers protected mean separation test was used at the p , 0.05 level within each treatment. Means within a row followed by the same letter are not

significantly different.b ND, not determined.

Table 3

Microbial biomass in microgram of C per gram of dry soil per day in the

earthworm cast (EW), compost (CP), fertilizer (FT), and unamended

control (CT) treatment soils during the incubation

Days EW

(mg C g soil21)

CP

(mg C g soil21)

FT

(mg C g soil21)

CT

(mg C g soil21)

3 350 ba 730 a 140 c 183 c

7 634 b 677 a 157 b 207 b

15 653 b 973 a 404 c 175 d

22 284 bc 639 a 433 abc 179 c

28 345 b 688 a 358 ab 191 b

35 297 ab 495 a 316 ab 166 b

43 404 a 316 a 120 b 283 a

50 326 b 589 a 187 b 357 b

57 424 a 318 ab 178 b 254 b

70 397 b 866 a 121 c 272 bc

a

Fishers protected mean separation test was used at the p,

0.05 levelwithin each treatment. Means within a row followed by the same letter are

not significantly different.



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(Tisdale et al., 1993). The compost material contained a

much higher level of soluble organic C (K2SO4-extractable)

than the cast materials. This is indicative of the lesser degree

of decomposition that the compost has undergone and

suggests that the material is still rich in labile carbon

compounds which can serve as an energy source for

Fig. 1. Potassium chloride (1 mol l21) extractable levels of (A) ammonium

nitrogen and (B) nitrate nitrogen during the incubation period in EW,

compost (CP), synthetic NPK fertilizer (FT), and control (CT) treatments.

Fig. 2. Total potassium chloride (1 mol l

21

) extractable (ammonium plusnitrate nitrogen) levels during the incubation period in EW, compost (CP),

synthetic NPK fertilizer (FT), and control (CT) treatments.

Fig. 3. Dry matter biomass of shoots in the earthworm cast (EW), the EW

cast at higher amendment rate (EW-2), earthworm cast with a higher native

N content (EW-N), compost (CP), synthetic NPK fertilizer (FT), and

control (CT) treatment pots. Fishers protected mean separation test was

used at the p , 0.05 level between each treatment mean. Means within a

row followed by the same letter are not significantly different.

Fig. 4. Plant shoot N uptake in the earthworm cast (EW), the EW cast at

higher amendment rate (EW-2), earthworm cast with a higher native N

content (EW-N), compost (CP), synthetic NPK fertilizer (FT), and control

(CT) treatment pots. Fishers protected mean separation test was used at thep , 0.05 level between each treatment mean. Means within a row followed

by the same letter are not significantly different.



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microbes. Casts are a byproduct of the digestion process so

they would be expected to be lower in soluble organic

compounds which are used as a microbial energy substrate.

Although the nutrient content in casts and composts are

much lower than that found in synthetic fertilizers, they are

comparable in nutrient content to that typically found in

other secondary sources such as animal manure (Troeh and

Thompson, 1993). As with other carbon-rich amendment

materials, casts and compost have the potential to increase

soil organic matter levels and improve soil quality.

The quantity of soluble C was 6.3 times greater in the CP

material than in the EW method and suggests that the initial

microbial activity is linked to the level of soluble C in the

treatments (Table 1). Likewise, the microbial activity in the

later stages of the incubation may have been controlled by

the nearly equivalent amount of total of C added to the soil(CP, 25.1 g C added; EW, 21.0 g C added). There were no

significant differences in respiration rates between the soil

amended with mineral fertilizer (FT) and the control (CT),

suggesting that microbial activity was not limited by

inadequate nutrient levels in the soil. In addition to the C

status in controlling microbial respiration, improved aera-

tion of the EW and CP treatment are also thought to be

involved. The greater pore volume in cast and compost

amended soils increases the availability of both water and

nutrients to microorganisms in soils (Scott et al., 1996).

EW may have reduced levels of microbial biomass due

the earthworm use of microbes as an energy source (Bohlen

and Edwards, 1995). Microbial biomass after 35 days of

incubation was highly variable for the EW and CP

treatments, increasing and decreasing with incubation

time. Microbial biomass in the EW treatments did not

significantly differ from the control (CT) in the four final

(Days 43, 50, 57 and 70) sampling dates (Table 3).

However, soil respiration rates for the EW treatment was

significantly higher than in the control soil in these final four

sampling dates (Table 2). The higher respiration rates in the

EW treatment in conjunction with no difference in biomass

as compared to the control could be due to the presence of

different classes of microorganisms in the casts which might

have a different respiration to biomass ratio than theorganisms found in the soil, such as the fungi observed by

Marinissen and Dexter (1990). Bohlen and Edwards (1995)

and Daniel and Anderson (1992) reported similar differ-

ences between soil microbial biomass and respiration level.

This incubation study suggests that EW could serve as a

naturally produced, slow release source of plant nutrients.

The slope of NO3-N production over time shown in Fig. 1B

corresponds to an average rate of the microbially mediated

nitrification reaction (mg NO32-N kg21 soil day21). Thegreater nitrification in the initial 28 days for the CP

treatment (12.1 ^ 0.7 mg NO32-N kg21 soil day21) than in

the EW treatment (4.7 ^ 0.3 mg NO32-N kg21 soil day21)

which was significantly different using the t-test to evaluate

regression slopes (Zar, 1984) suggests that this microbially

mediated process was higher in CP than in EW treatment.

This may be due to the organic matter being more readily

available to microorganisms in compost than in the casts

since organic matter in casts is thought to be stabilized by

the formation of a clay casing (Chan and Heenan, 1995;

Shipitalo and Protz, 1989).

The release of total extractable N (ammonium nitrate)in the EW and CP treatments compared with the NPK FT

treatment is shown in Fig. 5. The total extractable N

Fig. 5. Plant shoot (A) P content and (B) K content in the earthworm cast

(EW), the EW cast at higher amendment rate (EW-2), earthworm cast witha higher native N content (EW-N), compost (CP), synthetic NPK fertilizer

(FT), and control (CT) treatment pots. Significant differences at the 5%

level using Duncans multiple range test between treatment means are

indicated by differing letters. Means within a row followed by the same

letter are not significantly different.

Table 4

Chemical characterization of the saturated paste extracts of the soils from the plant bioassay study at harvest

Days EW (mmol l21) EW-2 (mmol l21) EW-N (mmol l21) CP (mmol l21) FT (mmol l21)

Nitrate-N 0.87 ca 0.01 d 0.04 d 2.37 b 3.44 a

Phosphorus 0.14 c 0.20 a 0.17 b 0.13 c 0.03 d

Potassium 2.0 c 1.8 c 1.9 c 21.3 a 12.1 b

Sodium 2.3 c 2.2 c 2.1 c 17.2 a 7.9 b

Ionic Strength 18.2 c 15.0 cd 14.6 d 66.1 a 62.5 b

a Fishers protected mean separation test was used at the p , 0.05 level within each treatment. Means within a row followed by the same letter are not

significantly different.



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peaked at 35 days for the EW treatment and at 43 days for

the CP treatment while the FT levels were highest at the

start of the incubation and declined with incubation time.

The decrease in nitrate and in extractable total N after the

first 7 weeks is probably due to denitrification. The levels

of P and K found in the EW and CP treatments reflected

the quantity of the nutrients present in the amendment

materials (Table 1). These results show that both EW and

CP are good supplemental sources of readily available P

and K, as well as for N.

The plant study was conducted to test whether the

results and interpretations of the incubation studies were

supported using a bioassay. There was evidence of

salinity stress in the CP and FT plants with some shoots

displaying leaf tip burn. The saturated paste soil extract

analysis conducted at harvest indicated that the levels ofNO32-N and ionic strength in the CP and FT were much

higher than in the earthworm cast treatments suggests

that the burned leaf tips in the CP and FT plants at

harvest were symptoms of salinity stress (Table 4).

Nitrate-N in the EW-2 and EW-N extracts was less than

2% of that found in CP (Table 4), although the initial

extractable N levels in EW-N and EW-2 were similar to

that of compost (Table 1). This indicates a slower N

release in casts and a possible lower risk of nitrate

leaching with the use of casts as compared to compost.

The CP treatment contained the highest quantity of Kand Na in the saturated paste extract which may have

also contributed to the salinity stress in that treatment(Table 4). This result suggests that casts may be safer

than compost and water-soluble synthetic fertilizers in

containerized systems.

In summary, when sufficient quantities of casts (EW-2)

and casts higher in N content (EW-N) were used to provide

sufficient N to the plant, dry shoot biomass was greater than

the yield obtained with equivalent quantity of NPK fertilizer

and statistically equivalent to a treatment where the N

source was compost. Shoot N content was higher in the CP

and FT treatments than in any of the earthworm cast

treatments and evidence of salinity stress symptoms were

observed. These results suggest that EW in EW-2 and EW-

N resulted in higher plant biomass production due to aslower rate of nitrogen mineralization that was more

synchronized with the plant requirements (Cox, 1993).

Ionic strength and other salinity indicators in earthworm

cast treatment were much lower than in the CP treatment,

indicating that the casts used in this study did not cause

adverse salinity stress. The plant biomass and shoot

elemental content data show that casts are an efficient

source of plant nutrients and that the slower rate of N release

in EW gives it an advantage as compared to compost and

synthetic fertilizers. The nutrient content of the organic

waste fed to earthworms determines the level of nutrients

present in the obtained casts (Lavelle et al., 1992), and

compost is also affected similarly by the raw material used.However, comparable results are expected regardless of the

specific source of casts with respect to the physical structure

resulting from the gut digestive processes which is

responsible for the general slow nutrient release character-

istic of EW (Shipitalo and Protz, 1986).

Acknowledgements

Support for this work was provided by Hatch funds from

the Maine Agricultural and Forest Experiment Station.

MAFES Journal Publication 2603.

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Chaoui 2003 Soil Biology and Biochemistry