Composted materials have gained a wide acceptance as organic amendments in sustainable agriculture, as they have been shown to provide numerous benefits whereby they increase soil organic matter levels, improve soil physical properties (increased porosity and aggregate stability and reduced bulk density) and modify soil microbial communities [56]. Substantial evidence indicates that the use of compost amendments typically promotes an increase in soil microbial biomass and activity, as reviewed in [56-57]. This enhancing effect may be attributed to the input of microbial biomass as part of the amendments [58]; however, the quantity of organic matter applied with the compost is very small in comparison with the total organic matter present in the soil and, in turn it is believed that the major cause is the activation of the indigenous soil microbiota by the supply of C-rich organic compounds contained in the composting materials [58]. Such effects on microbial communities were reported to be dependent on the feedstocks used in the process [59]. However, other authors did not find significant differences between soil plots that had been amended with four different compost types (green manure compost, organic waste compost, manure compost and sewage sludge compost) over 15 years [60]. Similarly, Ros et al. [61] observed that different types of composts had a similar effect on the fungal community and microbial biomass in soil in a long-term field experiment. This fact suggests that the soil itself influences the community diversity more strongly than the compost treatments. Such discrepancies between the previous findings may be due to differences in soil properties, land-use and compost type (i. e., different starting material and process parameters), frequency and dose of application, length of the experiment and parameters chosen for analysis, among others.

Furthermore, C addition to soil seems to select for specific microbial groups that feed primarily on organic compounds. Therefore, it can be expected that the addition of organic amendments not only increases the size of the microbial community but also changes its composition, as has already been observed in previous experiments [61-63]. As shown by

[1] higher amounts of composts resulted in a more pronounced and faster effect in the structure of microbial communities, as revealed by PLFA analysis, indicating that the compost application rate is a major factor regarding the impact of compost amendments on soil microbiota. Carrera et al. [63] found that soil PLFA profiles were influenced by both the treatment with poultry manure compost and the sampling date. Ros et al. [61] observed that the date of sampling contributed more to modifications in fungal community structure, assessed by PCR-DGGE analysis, than treatment effects. However, in contrast to fungi, the bacterial community structure, both on the universal and the Streptomycetes group-specific level, were influenced by compost amendments, especially the combined compost and mineral fertilisers treatments. This seems reasonable, as bacteria have a much shorter turnover time than fungi and can react faster to the environmental changes in soil. Bacterial growth is often limited by the lack of readily available C substrates, even in soils with a high C/N ratio, and are the first group of microorganisms to assimilate most of the readily available organic substrates after they are added to the soil [64]. CLPP profiles have also been used to evaluate the impact of compost amendments on the potential functional diversity of soil microbiota, as they are considered suitable indicators for detecting soil management changes [65]. As shown by [66] different types of compost (household solid waste compost and manure compost) affected differently the substrate utilization patterns of the soil microbial community relative to unamended control soils. Contrarily, no significant changes in CLPP profiles were found by [59]. Other authors also reported that the sampling date had more weight on CLPP results than compost treatments [63,67]. All these studies together highlight the importance of a multi-parameter approach for determining the influence of compost amendments on the soil microbiota, which is of utmost importance to understand the disease suppressive activity of compost and the mechanisms involved in such suppression [68].

Since the 1980s a large number of experiments have been addressed describing a wide array of pathosystems and composts from a broad variety of raw materials. Interestingly, Noble and Coventry [69] evaluated the suppression of soilborne plant pathogens by compost in both laboratory and field scale experiments. In general, they found that the effects in the field were smaller and more variable than those observed at lab-scale. Termorshuizen et al.

[70] compared the effectiveness of 18 different composts on seven pathosystems and interestingly they found significant disease suppression in 54% of the cases, whereas only 3% of the cases showed significant disease enhancement. They highlighted that the different composts did not affect the pathogens in the same way and that no single compost was found to be effective against all the pathogens. Furthermore, in a study carried out with 100 composts produced from various substrates under various process conditions, it was found that those composts that had undergone some anaerobic phase showed the best results in terms of suppressing plant disease [71].

However, up to now, there is still a general lack of understanding concerning the suppressivity of compost [68], as it depends on a complex range of abiotic and biotic factors. Such factors are reviewed by [72]. Briefly, the main mechanisms by which compost amendments exert their suppressivity effect against soil-borne plant pathogens include hyperparasitism; antibiosis; competition for nutrients (carbon and/or iron); and induced systemic resistance in the host plant [73]. The first three affect the pathogen directly and reduce its survival, whilst the latter one acts indirectly via the plant and affect the disease cycle.

Manure type Bulking agent Composting process Vermicomposting process Duration of experiment Investigated parameters Remarks References Pig manure Vertical continuous- feeding system (E fetida) 36 weeks Microbial biomass C, basal respiration, metabolic quotient (qC02), CLPPs, enzymatic activities Increase in functional microbial diversity with earthworm presence associated with a decrease in qC02, indicative of high metabolic efficiency Increase in cellulase activity with earthworm presence accompanied by greater C losses throughout the process [16-17] Pig manure Vertical continuous — feeding system (E. fetida) 36 weeks PLFAs, basal respiration Decrease in bacterial and fungal biomass, assessed by PLFA biomarkers, associated with an increase in microbial activity in the presence of earthworms [18] Sheep manure Olive waste Turned windrow Vermicomposting bed (£. fetida) 36 weeks for both composting and vermicomposting Enzymatic activities, PCR-DGGE, real time-PCR Higher bacteria] abundance and microbial diversity was found in vermicompost relative to the initial substrate than in compost [19]

Manure type Bulking Composting Vermicomposting Duration of Investigated Remarks References agent process process experiment parameters Cow manure Straw Forced- Vermicomposting bed Composting: 15 d Microbial Lower levels of microbial biomass and [20] ventilation (E. andrei) (thermophilic phase) biomass C, dehydrogenase activity indicative of a basal higher degree of stabilisation were Vermicomposting: respiration, found in the combined treatment 40 d (active phase) enzymatic activities, ergosterol (composting + vermicomposting) Cow manure Agricultural In-vessel Scmicontinuously Composting: 3-4 Substrate- Vermicompost tea composition was [21] plant w aste system vermicomposting system weeks (thermophilic induced influenced by production and storage (E. fetida) phase) respiration, conditions PCR-DGGE, Carbon substrate addition during the tea Vermicomposting: CompoChip production process was identified to not specified be of major importance for obtaining a vermicompost tea with a rich and diverse microbial community Cow Biochar Turned 12 weeks PLFAs Changes in microbial community [221 manure+poultry windrow composition depended on the origin manure of manure composted with biochar . 30 d PCR-DGGE, Ammonia oxidizing archaea were found to [231 Cow manure Sawdust Turned clone libraries be dominant during the composting windrow process probably because they could adapt to increasing temperature and/or nutrient loss Cow manurc+Horse . In-containcr system (E. 30 d (active phase) Basal Species-specific effects of earthworms on [2+1 manure+Pig andrei, E. fetida and P. respiration. microbial community structure and Manure excavatus) microbial growth rates, PLFAs bacterial growth rate

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Two mechanisms of biological control, based on antibiosis, hyperparasitism, competition and induced protection, have been reported for compost amendments. On the one hand, diseases caused by plant pathogens such as Phytophtora spp. and Phytium spp. have been eradicated through a mechanism known as "general suppression", in which the suppressive activity is attributed to a diverse microbial community in the compost rather than to a population of a single defined species augmented to infested soil. Whilst, for Rhizoctonia solani, few microorganisms present in compost are able to eradicate this pathogen and, in turn this type of suppression is referred as "specific suppression". Overall, all of the above reinforces that the activity of microbial communities in composts is a major factor affecting the suppression of soilborne plant pathogens. Indeed, the disease suppressive effect is usually lost following compost sterilization or pasteurization [68]. Better understanding of the microbial behaviour and structure of the antagonistic populations in the compost will provide tools to reduce its variability. In line with this, Danon et al. [32] detected, using PCR-based molecular methods, distinctive community shifts at different stages of prolonged compost curing being Proteobacteria the most abundant phylum in all the stages, whereas Bacteroidetes and Gammproteobacteria were ubiquitous. Actinobacteria were dominant during the mid-curing stage, and no bacterial pathogens were detected even after a year of curing.

The addition of antagonistic microorganisms to compost is also a promising technique to improve its suppressivity. Already in 1983, Nelson et al. [74] increased the suppressive potential of compost by adding selected Trichoderma strains. They found that not only the addition of the antagonist is important, but also the strategy of inoculation of the antagonist in order to efficiently colonize the substrate, as the autochthonous microbial community can inhibit it. Ultimately, predicting disease suppression on the basis of pure compost is expected to be highly advantageous for compost producers. This would enable them to optimise the composting process based on the specific disease jeopardising the target crop. For this purpose, a further step could be the development of quality control criteria based mainly on bioassays designed for a specific pathogen or disease.