Germination and growth of plants in media containing unstable refuse-derived compost

Soil Biol. Biochem. Vol. 26, No. 6, pp. 161-112, 1994 Copyright 0 1994 Elsevier ScienceLtd

Pergamon

003%0717(93)E0012-B

Printed in Great Britain. All rights reserved 0038-07I7/94 57.00+ 0.00

GERMINATION AND GROWTH OF PLANTS CONTAINING UNSTABLE REFUSE-DERIVED

IN MEDIA COMPOST

ALAN A. KEELING,‘* IAN K. PATON’ and JOHN A. J. MULLETT~ ’ Department of Crop Production and Science, Harper Adams Agricultural College, Edgmond, Newport TFlO 8NB, 273 Gartree Road, Oadby, L&ester LE2 2FD and 31ntemational Process Systems, Johnson House, Browells Lane, Feltham TW13 7EQ, England (Accepted 15

October 1993)

Summary-Refuse-derived compost (RDC) was produced by mechanical separation of organic matter from domestic refuse followed by a thermophilic composting phase. Fresh (unstable) compost was used in a variety of plant growth trials. Addition of peat, sand or dolomite limestone substantially improved germination. Extended growth trials showed the slow-nutrient releasing properties of RDC. With ryegrass at 6 months growth, identical total yields were obtained with unamended RDC and 150 kg m3 RDC in a sand-grit substrate. Phytotoxicity was confined to the low molecular weight (mol. wt) fraction, while the high mol. wt fraction possessed slight growth-stimulating properties.

INTRODUCITON Domestic refuse contains ca 30% by weight of vegetable and kitchen waste. It is possible to separate this material mechanically and produce refuse-derived compost (RDC). Composting provides an alternative to landfill or incineration for the processing of a high proportion of mixed waste (both the putrescible and ligno-cellulose fraction). Several studies have shown the potential of RDC as a growing medium component (Chen et al., 1988; Siminis and Manios, 1990; Keeling et al., 1991), and if this potential can be

realized, the waste-stream could be markedly reduced. However, fears have arisen regarding its use because of the difficulty of guaranteeing sanitization and consistency. The process used here to provide the compost involved addition of separated organic matter to bioreactors (Castle Bromwich reclamation plant, Birmingham) and forced aeration from below. After several months, the material was extracted from the reactors and screened. The remaining compost was brown with no identifiable biological components. It had been exposed to thermophilic digestion for an undetermined period, but had not yet stabilized biologically, being odorous and susceptible to mould when bagged. A typical physicochemical composition of the RDC is given in Table 1. There are reports of both glasshouse and field trials using refuse-derived composts. Siminis and Manios (1990) grew rooted cuttings of Ficus benjamina (L.) successfully in substrates of peat with 20% RDC, though no details on patterns of plant growth and development were given. Similar experiments with *Author for correspondence.

F. benjamina (L.) were performed using composted cattle manure or grape mart also suggested that these could be high quality substitutes for peat (Chen et al., 1988). Work by Keeling et al. (1991) showed that 100% RDC could achieve growth results comparable with fertilized peat composts, and that compost stability could considerably influence plant development. It was concluded that the unstable compost acted as an effective slow-release fertilizer. Field trials performed by Avnimelech et al. (1990) showed a complex crop yield response to the application of RDC, and that best results were achieved with superficial application enabling maintenance of aerobic conditions. Hoitink and Fahy (1986) have suggested a role for compost in reducing the effects of soil-borne plant pathogens. Trials have often failed to address the question of compost stability when interpreting the results. Fullystabilized RDC has a fresh soil-like odour and dichloromethane-extractable organic components are absent (Keeling et al., 1994). The stabilization process takes up to 6 months or longer. Our purpose was to test the effects of an unstable RDC, alone and amended, on the growth of a number of species, from germination for up to 6 months in glasshouse trials. MATERIALS AND

METHODS

Plant growth trials

Seven species were used in the trials, AIlium cepa L., Brassica oleruceu L., Brussica oleracea botrytis cuuliforu L., Lactucu saliva L., Lepidum sutivum L., Lolium perenne L. and Lycopersicon esculentum L. (onion, cabbage, cauliflower, lettuce, cress, perennial ryegrass and tomato, respectively). For the vegetable species, the following growing media were used: RDC 767

ALAN A. KEELING et al.

768

alone, RDC + moss peat (1: 1 v/v), RDC + fine horticultural sand (9: 1 v/v), RDC + fine dolomite limestone (9: 1 v/v) and RDC + 1 g 1-l KN03. A high quality conventional fertilized peat based growing medium was used as a control. Twenty seeds of each species (10 for Lycopersicon esculentum L.) were surface sown on the medium in each of three 14 cm dia squat plastic pots, and the trial was replicated in a separate block (i.e. six pots in total). Results are given as the combined mean. Pots were watered regularly to ensure that surface moisture was retained, kept in an unheated greenhouse and the numbers of seedlings recorded for up to 22 days after planting. The time taken for 50% germination was determined from growth curves. At that time, young plants raised in the peat-based medium were transplanted into 14 cm squat plastic pots containing the following media; RDC alone, moss peat, RDC + peat (1: 1 v/v) or the peat-based medium. Eight plants were used for each medium (one per pot) and were maintained for 17 weeks in various positions within an unheated greenhouse (pots free-draining). No artificial fertilizer was applied to any of the pots. Each plant was then harvested by cutting at the base of the stem, and weighed. Mean fresh weight yields for each treatment were determined. The experiment using Lolium perenne L. consisted of 13 treatments with 20 replicates per treatment in a four block random pattern (i.e. four sets of five pots located in different parts of the house). The purpose of this trial was to determine the effects on plant growth of various application rates of RDC in an artificial substrate, compared with peat. The substrate was a mixture of horticultural silver sand with horticultural grit (66: 33 w/w). Thirteen treatments were used as shown in the legend to Fig. 1. Seed (0.5 g) was surface sown in 14 cm squat plastic pots. All pots were maintained in an unheated greenhouse and watered regularly (free-draining). Grass was harTable I Physico-chemical characteristics of refuse-derived compost from mixed domestic waste, produced at the Castle Bromwich recy cling plant Variable Moisture content Bulk density Organic matter PH Conductivity Total N Total P Total K Total Mg C:N ratio Zo Total Zn EDTA extractable Cu Total Cu EDTA extractable Ni Total Ni EDTA extractable Cd Pb

Hg

Cr

Typical result 50% 0.33kgm~’ 55.8% 6.5-7.9 964~s I .62% 0.49% 0.72% 0.2% 16.6: I 374mg kg ’ 46.3 mg kg ’ 270mg kg-’ ll.5mg kg~’ 20mg kg ’ 0.15mg kg ’ 2.05 mg kg ’ 244mg kg ’ 0.65 mg kg ’ 42.5 mg kg-’

vested at 40, 91 and 180 days after sowing and the fresh weigh of plant material in each treatment was determined. The mean result of each treatment was calculated (i.e. the combined mean across blocks). Molecular size distribution of phytotoxic fractions in RDC Samples of RDC (moist; 20 g) were shaken with distilled water (50 ml) for 1 h. The solutions were centrifuged and filtered through Whatman No. 6 filter paper to sediment large particulate matter (fine particulate matter remained). A pre-packed PDlO gel filtration column (Sephadex G25; Pharmacia) was washed extensively with 50 ml distilled water. RDC extract (1 ml) was then added to the surface of the column, and allowed to soak in. The column eluate was collected. A further eleven 1 ml volumes of distilled water were added to the column and eluate fractions collected separately. Each fraction was diluted to 3 ml with distilled water and then placed on Whatman No. 1 filter papers in plastic Petri dishes (8.8 cm dia). Samples of the RDC extracts (3 ml) were placed in each dish. Control plates used distilled water instead of RDC extracts. Twenty seeds of Lepidum sativum L. were placed in each dish. Seeds were kept at ambient laboratory temperatures for 2 days until there was extensive germination and growth in the control plates. Each seedling was then measured from the tip of the root to the base of the seed leaves. The germination index was determined according to the following formula: Total length of shoots in test plates x 100 Total length of shoots in control plates = Germination Index (X). For each experiment, there were two test and two control plates. Combined means were used in the determination of final results. Three separate experiments with different unstable RDC extracts were performed. A solution producing a germination index of < 100% was judged to be phytotoxic, and one producing > 100% growth stimulating. Identification of water-soluble organic components of RDC by GC-MS Extracts of RDC were prepared by adding distilled water (100 ml) to RDC (20 g). Drying was prevented to avoid the loss of volatile organic materials. Solutions were shaken for 1 h, after which most of the particulate matter was removed by centrifugation and filtration. Samples (1 ~1) were introduced into a Hewlett-Packard 5890 series II GC containing a DB5 general purpose column (injector temperature 15OC). The samples were passed through a temperature ramp (30-230°C) into a VG Trio-l MS (electron ionization positive), ion counts being recorded using the VG Lab-Base data system for identification of constituents. Because of excessive background noise with ion detection between 0 and 40 m/z, ion spectra were only quantified between 40 and 650mlz.

Germination and growth of plants in media containing

unstable

refuse-derived

compost

169

Table 2. Percentage germination at 22 days (L. .~fiuum, 16 days) for vegetable species grown in media refuse-derived compost (RDC) compared with conventional peat-based growing medium (PBM). Numbers in parentheses indicate the numbers of days taken to achieve 50% germination. Where no figure

containing

is given.

this is >22

davs. or never achieved. Growing

RDC:

not done1

RDC:

RDC: 10% dolomite lime

RDC: nitrate

PBM

(13)

IO? 13 50?9 (22)

72+7 (II) 93 +6 (3)

52_+I6 (15) 78 k 8 (16) 73+15

5Ok7 (22) 35 ND

88*8 (4) 97 f 5 (3) ND

RDC

50% peat

10% sand

17 * 17 73 +8 (16) 35 + 23

62 + 16 (17) 98 + 6 (6)

47* 12 87 + I I (8)

25 f 5 82? IO

70+ I3 (14) 85 + 13 (IO) 47 + 6 87+6

67 ; 25 (14)

Species A. B. B. L. L. L.

(ND:

medium

cepa oleracea caulipora saliva sotivum

50 * 22 (22) 2.5+9

ND 87 + 5 (13) ND

e.whmml

47Il5

90 *

IO (9)

Identification of components was carried out statistically using the Lab-Base IDENDB programme, which lists the 15 most likely matches for a given peak against a National Bureau of Standards library of mass spectra of known substances (supplied with VG Lab-Base data processing package).

(II)

7*

II

83 + 21 (5)

or lethality for a number of seeds. RDC alone was therefore phytotoxic to seeds. Amendment with 50% (v/v) peat enhanced considerably the total percent germination at 22 days, in several cases giving similar germination to the PBM. Amendment with either 10% (v/v) sand or fine dolomite limestone also enhanced germination. The addition of KNO, did not enhance germination in RDC in any species. The effect of amendments of RDC on time for 50% germination is also shown in Table 2. The shortest germination times were always observed in PBM, and 50% germination was rarely achieved in RDC. Amendment of RDC with peat, sand or dolomite limestone shortened 50% germination times, but

RESULTS

Plant growth trials In RDC alone, the number of seeds germinated at 22 days was substantially lower than for the peatbased growing medium (PBM) (Table 2), but it was unclear whether this was simply a germination delay

20 19

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18

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17

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16

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13

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10

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5 $ M

98 7

-

6

-

5

-

4

-

3

-

2

-

1

-

0 Tl

T2

T3

T4

T5

T6

T7

T8

T9

TlO

DAY

180

Tll

T12

T13

TREATMENTS a

DAY

40

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DAY

91

m

Fig. 1. Growth yields (g fresh weight) over 6 months (three harvests) for perennial ryegrass (Leliumperc~ne L.) in growing media containing RDC and peat. The treatments were as follows: TI-RDC only, T2-Sphagnum moss peat only, T3-sand + grit only (66: 33 w/w). T4--T3 plus 25 kg m-j (0.075 kg kg-r) RDC, T5-T3 plus 50 kg mm3 (0.15 kg kg-’ ) RDC, T6-T3 plus 100 kg mm3 (0.3 kg kg-‘) RDC, T7-T3 plus I50 kg m-j (0.45 kg kg-‘) RDC, T8-T3 plus 200 kg m-j (0.6 kg kg-‘) RDC. T9-T3 plus 25 kg m-j (0.075 kg kg-‘) peat, Tl(rT3 plus 50 kg m-r (0.15 kg kg-‘) peat, TI I-T3 plus 100 kg m-j (0.3 kg kg-‘) peat, TI2-T3 plus I50 kg mm3(0.45 kg kg-‘) peat, TI3-T3 plus 200 kg mm3 (0.6 kg kg-l) peat.

ALANA. KEELINGet al.

770

raised in RDC alone, yields being approximately double the yields in PBM. No additional nutrients were made available to the plants during the 4 month growing period. It was noted at harvest that root development was considerably enhanced with plants grown in the RDC + peat mixture. The effects of RDC on the growth of perennial ryegrass in artificial subsoil are shown in Figs 1 and 2. In RDC alone, increasing yields of grass were obtained with a maximum growth rate between 91 and 180 days after sowing. In other RDC-amended media, growth rates were related to the proportion of RDC present in the medium, initial growth being greatest in the media containing less RDC. However, Fig. 2 shows that above an RDC application rate of 150 kg mm3, the total yield of grass over the 6 months was not increased. In all cases yields of grass in the RDC-amended substrate were substantially greater than in peat-amended substrates, and root growth was also observed to be better developed. served in plants

RDC‘ application

rate ( kglm3)

Fig. 2. Total growth yields over 6 months (g fresh weight) of perennial ryegrass (Lolium perenne L.) in media containing RDC at various application rates.

Molecular size distribution RDC

these were still twice as long as for the PBM. It was noted that while germination and growth of the seedlings was substantially improved in amended RDC, the young plants did not develop initially as rapidly as those raised in PBM. Plant yields were substantially greater in RDC and RDC + peat than in PBM in most cases. This was true even for Lactuca saliva L. and Lycopersicon esculentum L. which are known to be sensitive to phytotoxins. The best growth was most often ob-

of phytotoxic fractions

The pattern of phytotoxicity associated with molecular size fractions of RDC after passage through a PDlO gel filtration column for three separate experiments on different samples of unstable RDC is shown in Fig. 3. High molecular weight materials and particulate matter eluted in fractions 3-5 inclusive (m.w. > 10 kDa), while small molecules and ions eluted in fractions 610. Phytotoxicity was clearly

120

g

110

tl 100

Q

.9 3 3 .-a

& tj

90 80 70 60

1

2

3

4

5

of

6

7

8

9

10

11

12

PDlO fraction No. (ml) Fig. 3. Germination indices of PDlO gel filtration column fractions of RDC water extracts (three separate experiments; symbols represent results of different experiments).

Germination and growth of plants in media containing unstable refuse-derived compost loo

-

771

(A) I

12

6 Retention time (min)

Fig. 4. Typical GC trace of water extracts of RDC. (A) 99 m/z peak (corresponding to 2-piperidinone). (B) Total ion count: major peak corresponding to acetic acid.

associated with the low molecular weight fraction, while the particulate and high molecular weight fraction demonstrated a small but consistent growth stimulating effect. Water-extractable

possessed relatively low concentrations of heavy metals. For the three key phytotoxic metals, Zn, Cu and Ni, the extractable concentrations were at least 5-fold lower than their total concentration, suggesting that no phytotoxicity would have arisen (Winsor and Adams, 1987). The species grown in media containing RDC showed similar germination and growth responses. In unamended RDC, substantially reduced germination was observed for all species, though it is unclear whether this was due to lethal toxicity or a prolonged germination delay. However, amendment with small quantities of either sand or dolomite limestone (10% v/v) substantially increased total germination and reduced the time to achieve 50% germination. This suggested that early seedling development was inhibited by the physical rather than the chemical nature of RDC. Nonetheless, continuing germination delays relative to PBM suggested the action of phytotoxins. Plants grown in RDC containing 50% peat showed much better early development after germination, probably arising from either dilution of phytotoxins or better physical characteristics of the medium. The fact that the addition of nitrate in no way improved

organic substances present in RDC

A typical GC-MS of a water extract of RDC is shown in Fig. 4. In most samples of unstable RDC, there was a single major organic molecular component, identified with the Lab-Base data system as acetic acid. Calibration of the CC-MS with standard solutions of acetic acid suggested that total concentrations of acetic acid in unstable RDC ranged between 100 and 500 mM (data not shown). Another component was occasionally observed at a low concentration with an ion peak at 99 m/z, identified as 2-piperidinone. Other low molecular weight fatty acids were occasionally observed (especially butyric), but at much lower concentrations than acetic acid. DISCUSSION

RDC used in these experiments was unstable (immature), well supplied with a range of nutrients, and

Table 3. Mean fresh weights (g + SD) at 4 months of species harvested from a variety of growing media. Numbers in parentheses represent numbers of surviving plants or numbers used in calculation of means. (NP = no surviving plants). RDC = refuse derived compost; PBM = peatbased growing medium Growing Soecies A. 6. B. L. L.

cepa oleracea caulipora saliva eseulentum

RDC 3.8 * 74.1 + 53.8 f 108.0 + 31.0*

2.1 (7) 13.3 (8) 12.7 (8) 29.5 (8) II.0 (5)

Peat NP (0) 0.5 f 0.3 (3) 2.8 f 1.7 (4) NP (0) 0.4 + 0.1 (2)

medium RDC: oeat (50: 50)

I I.4 66.5 44.8 82.2 39.9

f k f k f

3.8 (8) 13.8 (8) 8.4 (8) 19.7 (6) 8.4 (5)

PBM 4.9 38.4 35.2 32.6 11.1

+ 0. I (6) f 13.9 (8) k 13.4 (8) f 14.1 (5) f3.5 (5)

772

ALANA. KEELINGet al.

germination and early growth indicated that observed phytotoxicity was not related to a shortage of available N. The trial with perennial ryegrass extended experiments by Keeling et al. (1991). The slow nutrient-releasing properties of unstable RDC were shown. Initial growth rate was inversely related to the quantity of RDC contained within the medium, presumably as a result of the presence of phytotoxins or sub-optimal physical conditions. It is unclear whether the results obtained for the 150 kg m-3, 200 kg mm3 and 100% RDC represent the maximum possible yields for ryegrass under the prevailing cultural conditions. This should be tested using additional inorganic fertilizer applications with RDC and control growing media. Further work is necessary to compare the nutrient-releasing properties of RDC with synthetic slow-releasing fertilizers and to assess the extent to which compost can replace them in agricultural or horticultural applications. Gel chromatography of water extracts of unstable RDC followed by cress germination assays confirmed that the phytotoxic components were confined to the low molecular weight fractions. GC-MS also confirmed acetic acid as the most abundant phytotoxin in fresh compost (Lynch, 1977; de Vleeschauwer et al., 1981). However, other substances were detected at much lower concentrations whose phytotoxic effects are unknown (notably 2piperidinone). A much greater range of substances could be extracted into dichloromethane (Keeling et al., 1994). An important observation was the small growth stimulating activity of the high molecular weight fraction of RDC. The cause of this stimulation is uncertain, but it cannot be attributed to the presence of any small nutrient molecules or ions. It could be due to the presence of humic acid-like substances in the high molecular weight fraction; root growth stimulation is a known property of humic substances (Schnitzer and Poabst, 1967). The degree of root stimulation by the high molecular weight fraction may be therefore a useful rapid indicator of the humification of RDC. A consistent observation in these growth trials was the ultimate healthy root development in all media containing RDC.

It is concluded that unstable RDC can be a valuable component of a growing medium, especially when amended to reduce phytotoxicity and shorten germination times. Mixtures with peat appear to provide better growing conditions for seeds and young plants for reasons which are, as yet, unclear. For extended periods, unamended RDC can stimulate healthy development in a wide variety of species without the need for additional nutrients, though some fertiliser additions may be beneficial after several months growth. Acknowledgements-We thank Anna O’Brien for technical assistance, and Drs Mike Hayes, Pat Harvey and Robert Manasse for helpful discussions.

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24, 93-124.

Keeling A. A., Mullett J. A. J. and Paton I. K. (1994) GC-mass spectrometry of refuse-derived composts. Soil Biology & Biochemistry

26, 773-776.

Keeling A. A., Mullet J. A. J., Paton I. K., Bragg N., Chambers B. J., Harvey P. J. and Manasse R. S. (1991) Refuse-derived humus; a plant growth medium. In Advances in Soil Organic Matter Research and rhe Impact on Agriculture and the Environment (W. Wilson, Ed.), pp. 365-375. Royal Society of Chemistry, Cambridge.

Lynch J. M. (1977) Phytotoxicity of acetic acid produced in the anaerobic decomposition of wheat straw. Journal of Applied Bacteriology

42, 8 1-87.

Schnitzer M. and Poabst P. A. (1967) Effects of a soil humic compound on root initiation. Nature 202, 598-599. Siminis H. I. and Manios V. I. (1990) Mixing peat with MSW compost. Biocycle November 1990, 60-61. de Vleeschauwer D., Verdonck 0. and van Assche P. (1981) Phytotoxicity of refuse compost. Biocycle 1, 44-46. Winsor G. and Adams P. (1987) Diagnosis of Mineral Disorders in Plants, Vol. 3. Glasshouse Crops (J. B. D. Robinson, Ed.). Ministry of Agriculture, Fisheries and Food/Agriculture and Fisheries Research Council. HMSO, London. Zucconi F., Forte M., Monaco A. and de Bertholdi M. (1981) Biological evaluation of compost maturity. Biocycle 22, 27-29.