A. Pera’s research while affiliated with University of Siena and other places

This paper analyes the more important aspects of the composting process with particular emphasis of the microbiological ones. Some attention is also given to the hygienic and sanitary implications of compost production as well as to the discussion about different plant design options. Moreover, compost is considered in its agricultural role, with particular reference to the effects on plant growth. Problems concerning the evaluation of compost biological maturity and phytotoxicity are finally presented.

Lysimeter trials were carried out to evaluate the effects of compost from the aerobic stabilisation of olive mill wastewater mixed with lignocellulosic residues on soybean yield. The paper is the result of a study aimed at the abatement of the phytotoxic nature of olive mill wastewater by means of composting in order to recycle this effluent, avoid the wastewater depuration costs and give the farmers a high quality humified organic fertilizer.

This paper is an overview of the potential use of composting technology in programmes aimed at organic waste recycling (product-oriented perspective) or decomposition of hazardous materials (process-oriented perspective). This latter approach includes composting as a tool for bioremediation of environmental matrices, such as contaminated soils and sediments. In all above-mentioned cases, biological reactions that characterize composting must be managed carefully to allow putrescible residues to become a humified agricultural fertilizer with no phytotoxic effects, or the degradation of organic pollutants (e.g., aliphatic and aromatic hydrocarbons or halogenated solvents) to proceed to the highest extent with formation of innocuous end products. Thus, the fundamental aspects of controlling the microbial environment in composting matrices are singled out as a means for better evaluating the range of adverse conditions possibly responsible for hindering the correct evolution of the process within different applications.

Vicia faba, in a pot experiment with sandy and clayey soils under greenhouse conditions, was checked for growth response to different amendments with coal alkaline fly ash or cocomposted fly ash mixed with lignocellulosic residues. Soil microbial populations, pH and electrical conductivity as well as heavy metal uptake by plants were monitored. At rates of five and ten percent (on a dry matter basis) in both soils, neither fly ash alone nor cocomposted fly ash exerted any negative effect. Plant biomass production was not influenced in either clayey or sandy soil. Alkaline fly ash did not promote microbial growth when applied alone to the soils. However, cocomposted fly ash generally increased bacterial and actinomycetes counts in both soils. Fungi were not affected by ash. Due to the increase of soil pH by alkaline fly ash or cocomposted fly ash, plant uptake of heavy metals was depressed in the sandy soil. Heavy metal mobility did not cause change in the clayey soil where a high buffering capacity mitigated the effects of fly ash amendments.

In the present study, the effect of humic acids on activity and growth of Nitrosomonas europaea and Nitrobacter agilis was investigated in vitro under axenic conditions. Humates from compost-stabilized vegetable waste or leonardite were added to the chemolithotrophic culturing medium at concentrations of 0, 5, 50 and 100 mg l–1. It was found that both types of humic acids increased either NH4 + or NO2 – oxidation and cell growth of nitrifying bacteria in a dose-independent manner. By combining these results with data from a comparative growth evaluation of N. agilis based on possible utilization of humates or pyruvate in heterotrophic conditions, evidence was obtained that nitrifiers cannot use humic acids as an alternative carbon and energy source. Thus, the stimulating effect of this fraction of soil organic matter on chemolithotrophic ammonia and nitrite oxidizers might be attributed to an increase in microbial membrane permeability favouring a better utilization of nutrients.

Humic acids extracted from composted vegetable residues (green compost) or mined lignite (leonardite) have been tested for their influence on either soil cracking or microaggregation characteristics. This preliminary study has pointed out that humates from green compost, within the range of 1000 to 8000 mg kg−1 of soil, induce small-size clod formation in a dose-independent manner and preserve stability of microaggregates. On the other hand, humic acids from leonardite did not improve soil shrinkage and water-stable microaggregates, but rather they determined deterioration of such physical characteristics of the soil tested.

The responses of chicory plants to amendments with natural and synthetic surface active substances, represented by either potassium humates from compost stabilised green waste or Tween 80, are reported from a pot trial. Results are evaluated in terms of plant biomass production and behaviour of soil microbial populations following different treatments. Amendments with humic acids stimulated vegetative growth of chicory. They also caused significative variations in the numbers of bacterial heterotrophs and autotrophic nitrifiers in the soil. The study suggests that the mechanism through which humic acids affect both plant and soil microbes may chiefly involve enhancement of cell membrane permeability to nutrients.

Growing interest in the market for humic substances with agricultural applications has led to the development of new potential sources of these compounds other than fossil matrices (e.g. different kinds of lignite), which, until now, have represented the main raw material for the extraction of humus-like products. High quality compost (green compost) obtained through the aerobic biostabilization of selected organic residues, such as vegetable waste from source-collection at garden-produce markets, may be considered for this purpose. Beyond the primary need to develop technically and economically reliable procedures for the extraction of humic substances from compost at the industrial scale, importance must be placed on controlling the influence of such compounds on soil-plant systems. Humates from leonardite, representative of the active agents among humus-based commercial preparations, have been compared in pot trials with humic acids, potassium salts, from green compost in order to evaluate their respective effects on soil microbial activity and plant productivity. Differences between pot blocks amended with humic acids have suggested that humus-like substances extracted from compost seem to exert higher stimulative effects on microbial growth and vegetative biomass production than fossil humates.