December 13, 2006
Biological Quality of Potting Media Based on MSW Composts: A Comparative Study
By Moldes, A; Cendn, Y; Lpez, E; Barral, M T
Municipal solid waste (MSW) compost from aerobic or anaerobic bioprocesses was evaluated as components of substrates for potted plant production. Experiments were conducted with potted media consisting of MSW compost mixed with other conventional substrates (peat or composted pine bark). Spring barley (Hordeum vulgare L.) and cress (Lepidium sativum L.) were used to evaluate the biological quality of composts. Higher germination rates of spring barley were obtained when MSW compost from aerobic treatment was employed as compared with MSW compost from the anaerobic bioprocess. Improved biological indices were observed when MSW composts were mixed with composted pine bark rather than with peat. Mixtures of 75% aerobic MSW compost and 25% composted pine bark were more favorable for cress growth than peat as sole substrate.
In the last 20 years, the demand for inexpensive substrates which are more available than peat have increased considerably in the European farming sector, due to the expansion of the soilless production of various plant species. The primary reasons for substituting peat with other renewable substrates is the high cost of quality peat, and since in some areas the availability of Sphagnum peat is reduced due to government restrictions. Several authors have proposed MSW compost as an alternative substrate to peat (Siminis et al. 1990; Roe et al. 1997; Ribeiro et al. 2000; Sanchez-Monedero et al. 2004). Lopez et al. (1998) reported that the market price in Spain of peat-based potting media was almost twice that of compostbased media. Castillo et al. (2004) conducted several experiments with tomato (Lycopersicum esculentum L.), using mixtures of several substrates consisting of old peat, white peat, MSW compost, coco fibre, and perlite. They reported that the growth and development of tomato seedlings in mixtures of old peat and MSW compost were similar to that obtained in standard mixtures consisting of old peat and white peat, although when MSW compost was used alone or mixed with coco fibre, highly irregular seedling growth indices were obtained. Ribeiro et al. (2000) evaluated the use of MSW compost as a fertilizer for potted geranium (Pelargonium x hortorum), mixing MSW compost with a peat-based media. They observed that rates of MSW compost of 10 %-20 % promoted the highest plant growth.
The main limiting factor of MSW compost for its use as substrate is the high salt concentration. Sanchez-Monedero et al. (2004) tested two composts with high salt content (8.5-13.2 dS/m) as a constituent of growing media for vegetable transplant production, and reported that either the composts or mixtures with a commercial substrate or peat in a rate of up to 67 % did not result in any detrimental effect on plant growth.
MSW composts present different properties due to the heterogeneous composition of urban waste and to differences in the production process. Currently, some European composting systems employ anaerobic processes but the composts obtained by the aerobic curing of the digestate from the anaerobic step, are still not well recognized.
Biological tests are considered to be the best methods to evaluate the tolerance of plants to potential substrate components. Not all the contaminants that could restrict plant growth are determined in conventional substrate analyses. As a result, biological tests are the most realistic and complete method of analysing compatibility with plants. The ability of MSW compost based substrates to support plant growth was evaluated by means of biological tests using spring barley (Hordeum vulgare L.) and cress (Lepidium sativum L.). Cress is commonly used in these tests as it is sensitive to salts, metals and organic contaminants. However, spring barley was also selected and subjected to biological tests as a representative of salt-tolerant species. The objective of this investigation was to evaluate MSW compost as an alternative to peat in the preparation of growing media.
Materials and Methods
Aerobic MSW compost (MSWC1) was supplied by Albada, A Coruna (Spain) and anaerobic MSW compost (MSWC2) was provided by the Regional Recycling and Composting Facility of urban solid waste from Serra do Barbanza, Galicia (Spain). MSWC1 was separated at origin and processed through an aerobic bioprocess for 15 days in composting tunnels, followed by 3 months of curing. MSWC2 was obtained by a first step of anaerobic fermentation, after source separation, followed by an aerobic curing step to stabilize the incompletely digested residue. Commercial Sphagnum peat was supplied by MIKSSKAAR AS (Estonia) and composted pine bark (CPB) was produced by Cortina, Lugo (Spain) by aerobic composting in windrows.
The physical and chemical parameters of the substrates were characterized following the Spanish version (AENOR 2001, 2002) of European (EN) standards. Analyses were conducted in triplicate. For all analyses fresh substrates as received were screened to
Six substrate combinations were prepared following a randomized complete block design by mixing 25, 50, 75 % MSW compost from aerobic or anaerobic treatments with 75,50,25 % of composted pine bark or peat (Table 1), and two pot experiments were conducted to test their ability to support plant growth of cress (Lepidium sativum L.) and spring barley (Hordeum vulgare L.). Peat or composted pine bark substrates at 100 % were used as controls.
Biological tests were conducted in triplicate following the procedure of German standards for compost analysis (FCQAO, 1994). Basins of 12 cm diameter and 7 cm height were loosely filled with different substrates (Table 1), nearly to the rim and watered. After a short period for surplus water to run off, 1g of cress seeds or 50 seeds of spring barley were sown and the basins were covered with a glass plate to reduce evaporative water losses. Cress and spring barley were fertilized with multinutrient solution (Welgro standard plus, Comercial Qumica Masso S.A., Barcelona, Spain) in order to obtain 220 mg N/L substrate, 388 mg P/L and 194 mg potassium (K)/L. Multinutrient is constituted by: 17 % N, 30 % P, 15 % K, 0.13 % iron (Fe), 0.052 % manganese (Mn), 0.06 % zinc (Zn), 0.02 % boron (B) and 0.005 % molybdenum (Mo). Plant basins remained in the greenhouse for 7 days (cress) or 10 days (spring barley) at 20 C with a luminous strength of 2150 lux for 12 h d^sup -1^ photoperiod. The germination percentage was recorded at 3, 7 and 10 days in spring barley experiments. Glass plates were removed when the germinated plants touched them. Plants were cut off exactly between the root and stalk and the fresh weights of shoots were recorded.
Composition of the substrates obtained by mixing in different proportions: compost from aerobic treatment of municipal solid waste (MSWC1), compost from anaerobic treatment of municipal solid waste (MSWC2), composted pine bark (CPB) and commercial peat
Plants shoots were then dried at 105 C and the dry weights were recorded.
In order to evaluate the influence of salt content of MSWC in plant growth, MSW composts were washed in triplicate with varying amounts of water, between (200-500 mL) using a liquid/solid ratio between 1.54.0 g water/g MSW compost. MSW composts were washed using a funnel and filtered through Whatman paper n^sup o^ 4. After washing, pH, CE, and Na^sup +^ were analyzed, following the methods described in the previous section of materials and methods. Biological test with cress and spring barley were conducted with leached 100 % MSW compost, following the previous procedure described by the (FCQAO, 1994). Shoot fresh weights, shoot dry weights and shoot water content were measure at 7 days (cress) and 10 days (spring barley) after seeding.
Data were subjected to an analysis of variance by using SPSS statistical software package and significant treatment differences were separated by Turkey's multiple range test \at p
Results And Discussion
Physicochemical variables used to establish the quality of compost are presented in Table 2. While nearly no differences were observed between MSWC1 and MSWC2, these clearly differ from peat or CPB. Peat and CPB are mainly constituted by OM, which only accounts for 40-49% of MSW composts. The basic pH of the MSW composts contrasts with the acidic pH of peat or with the near neutral pH of CPB. MSW composts had higher concentrations of total P than peat and CPB. The bulk density values of all the substrate components are in the range proposed by Noguera et al. (2003) as an optimum value (
General physical and chemical variables of four growing media1
The high content of soluble salts of many MSW composts is one of the mayor obstacles to their acceptance as components of substrates for nurseries, since it may have a negative influence on seedling growth. MSW composts evaluated in this study have high EC (1-3.5 dSm^sup -1^ range), where plant growth restrictions have been reported by several authors (Gajdos, 1997; Lemaire et al, 1985; Wright, 1986). However, the negative influence of soluble salts depending on the salt tolerance of the plant species. Sanchez- Monedero et al. (2004) tested two composts with high salt content (8.513.2 dS/m) observing that media prepared with either of the composts did not have any detrimntal effect on plant growth of broccoli (Brassica oleracea L.).
Physicochemical parameters are determined by expensive and time consuming analytical processes to provide a predictive correlation for determining if the component is in an acceptable range for subsequent plant growth. Furthermore, there are no analytical procedures that can measure the cumulative or synergistic effects of phytotoxins and other contaminants. Therefore, there has been considerable interest in the development of bioassays to overcome these limitations, either by means of seed germination or plant growth assays. Cress (L. sativum L.) has been commonly used in these tests, due to its ease of handling and rapid germination and growth within a few days. In 1981, Zucconi et al. described a germination test or index using cress that has been frequently adopted in compost characterization. Germination indices (GI) higher than 80% indicate the disappearance of phytotoxins in mature compost (Zucconi et al. 1981). Nevertheless, other species have also been tested. For instance, Garglio et al. (2002) used lettuce (Lactuca sativa L.) as an indicator of biological quality; Fauci et al. (2002) used pinto beans (Phaseolus vulgaris L.) and tomatoes (Lycopersicum sculentum L.) in a plant growth bioassay, while Emino and Warman (2004) proposed the use of seeds of 14 species, including cress, to detect differences between mature and immature MSW compost. Radish (Raphanus sativus L.) is used for the determination of plant toxicity in the Australian Standard Compost for Potting Mixes (ASC, 1999), while cress and spring barley tests are proposed by the Federal Compost Quality Assurance Organization (FCQAO, 1994) for the determination of plant tolerance of compost in Germany. In this study, cress has been selected to evaluate the suitability of compost for the growth of salt-sensitive plants or as a component of seedbed mixes, while barley test provides information on other less salt sensitive applications in potting mix elaboration.
The germination percentage of spring barley at 3, 7 and 10 days, using potting media based on MSW compost from aerobic or anaerobic treatments, respectively is illustrated in Figure 1a and Figure 1b. In all the cases, MSW composts mixed with CPB had higher germination percentages than substrates based on MSW composts and peat (Fig. 1a and 1b). Most of substrates based on MSWC1 from aerobic treatment present better germinations percentages than media based on anaerobic MSWC2 compost. Mixtures of 25% or 50% aerobic MSWC1 with CPB or peat had germination percentages between 96-98%, which were comparable to germination percentages in peat or CPB (Figure 1a), thus indicating that toxins are not present in the substrates in concentrations that could restrict plant growth. Substrates consisting of 75 % aerobic MSWC1 mixed with peat or CPB had germination percentages of 59% and 75%, respectively. For MSWC2 mixtures, there were evident germination restrictions for compost concentrations of 50%, while no germination was observed in mixtures with 75% MSWC2 and peat. When 75% of MSWC2 was mixed with 25% peat or CPB, germination percentages of 0% and 70 % were obtained, respectively. As both composts have similar water extractable Na^sup +^ contents, the poor behavior of MSWC2 can be speculated to be higher in its NH concentration (Table 2) or to the presence of phytotoxic products resulting from the incomplete degradation of organic matter.
FIGURE 1. Germination percentage of H. vulgare at 3, 7 and 10 days, after seeding, using potting media based on MSW compost from aerobic (Figure 1a) or anaerobic treatments (Figure 1b).
Plant dry weight, fresh weight and organic matter were measured for spring barley and cress, in media based on MSWC1 or MSWC2 (Table 3). Substrates 13 and 14 correspond to the controls, based on 100 % P or 100 % CPB, respectively. Organic matter concentrations had minimal variations among compost.
Concerning the biological properties media based on MSWC were more favorable for cress, than for spring barley, and that MSWC1 from aerobic treatment was more satisfactory than MSWC2 from anaerobic treatment, for both species studied. On the basis of cress dry weight, 75 % of MSWC1 and 25 % of CPB allowed comparable results to 100 % CPB and improved results than 100 % peat. The optimum value for cress dry weight in media based on MSWC2 was reached using 25 % MSWC2 and 75 % CPB. With respect to the growth of spring barley, only concentrations of MSWC2 around 25 % or lower, mixed with CPB, provided comparable results to spring barley growing on 100 % Peat. These results are in accordance with those found in the literature. Castillo et al. (2004) reported that the growth and development of tomato (Lycopersicum esculentum L.) seedlings in mixtures of 65 % peat, 30 % MSW compost and 5 % perlite, were similar to those obtained with standard mixtures consisting of 65 % old peat, 30 % white peat and 5 % perlite. Ribeiro et al. (2000) reported that 10 % and 20 % of MSWC mixed with peat promoted the highest plant growth of potted geranium (Pelargonium x hortorum).
Biological parameters of H. vulgare and L. sativum growing on media based on MSWC1 or MSWC2(1)
Ideally, the properties of the substrates have to remain in acceptable range during the period of plant growth. To test this aspect, some relevant physicochemical parameters of the mixtures were evaluated at the beginning and at the end of the biological test (Table 4 and 5). The pH increased, primarily in those cases where peat was employed as a substrate constituent. Increased pH (above the recommended values 5.3-6.5) could also restrict plant growth. The addition of multinutrient solution did not produce any detrimental increase in salinity of substrates, as in most cases, EC and Na^sup +^ decreased, after the biological test, when peat was part of the substrate. Only small changes in EC and Na^sup +^ were produced, when CPB was employed as potting media constituent.
Physicochemical analysis of potting substrates, based on MSWC1, at the beginning (B) and at the end (E) of biological test1
Physicochemical analysis of potting substrates, based on MSWC2 at the beginning (B) and at the end (E) of biological test1
Taking into account the results obtained in the biological tests, and in order to formulate potting media based on high proportions of MSW compost, these were subjected to leaching with water. After washing the MSW composts, the pH did not change but the salt content in the substrate was reduced, since after washing the MSWC with 500 mL of water, the EC and Na^sup +^ concentration decreased by more than 78 % in both composts (Table 6).
Once the MSWC was washed, the biological test with cress was conducted again to test if a reduction of toxicity has been achieved. The more favourable biological parameters were obtained when MSWC1 was washed with increasing amounts of water (Table 7). Whereas, the same experiment conducted with MSWC2 did not produce any growth (data not shown), and this can be due to the presence of inhibitors other than Na^sup +^. Potential toxicity due to the high concentrations of NH^sub 4^-N was checked by heating the MSWC2 at 30 C for 24 h in order to eliminate N as volatile NH^sub 3^ from the alkaline compost. Although the concentrations of NH^sub 4^-N decrease from 900 mg/kg to 467 mg/kg, cress did not grow in the treated MSWC2. Toxicity must therefore be attributed to some other factor such as possibly the phytotoxic organic compounds resulting from incomplete degradation of organic matter, which remain in immature composts. Therefore, to be used as potting media, MSWC2 must be necessarily diluted with other substrates or the curing process should go on longer.
Chemical analysis of MSW compost after the leaching with increasing amount of water1
Biological parameters of Lepidium sativum growing on 100 % MSWC 1 after the leaching with increasing amounts of water1
The growth of cress in containers with potting media composed by various fractions of MSW compost and composted pine bark has been equal to or superior to the plant growth with peat as thesole substrate, whenever the mixes contained 50% or less MSW compost. As the percentage of MSW compost in the potting mix increases above 50 %, the growth suppression also increases, mainly in spring barley growing in MSW compost from anaerobic treatment. Composted pine bark had the best biological quality, as deduced from the germination and growth of spring barley and cress cultivation in potting media, either mixed with MSW compost or as a sole substrate. Consequently, MSW compost from aerobic or anaerobic treatment and composted pine bark could be used as an alternative to traditional peat, since peat availability has diminished and the cost of high quality peat has increased, particularly in countries without local Sphagnum peat resources. The use of MSW compost or composted pine bark in substrates can contribute to the conservation of nonrenewable natural resources and to environmental protection.
This study was supported by the Ministry of Science and Technology of Spain (Project AGL 2003-08958). The authors gratefully acknowledge the analytical assistance of Eva Otero.
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A. Moldes1,2, Y. Cendn1, E. Lpez3 and M.T. Barral1
1. Departamento de Edafologa y Qumica Agrcola, Facultad de Farmacia,
Universidad de Santiago Santiago de Compostela, Spain
2. Departamento de Ingeniera Qumica, Facultad de Ciencias, Universidad de Vigo, Ourense, Spain
3. Departamento de Produccin vegetal, Escuela Politcnica Superior, Lugo, Spain
Copyright J.G. Press Inc. Autumn 2006
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