Potential of utilization of selected organic compounds by the microflora inhabiting the effluent nutrient solution of closed greenhouse irrigation systems

Abstract

Summary The potential for breakdown of organic substrates by the microflora inhabiting the nutrient solution effluent from closed systems was studied in a fruit-bearing tomato cv. Gitana grown in small scale systems adopting nutrient film technique with respect to selected phenolic acids (benzoic acid, ferulic acid, p-hydroxybenzoic acid, salicylic acid, vanillic acid) and eleven substrate groups expected in rhizodeposition (carbohydrates, amino acids, carboxylic acids, polymers, amines, amides, alcohols, aromatic compounds, brominated compounds, esters, phosphorylated compounds). Factors essential for mediating the breakdown (microflora, oxygen content and demand, total organic carbon, pH, electrical conductivity) were described for the studied growing system. Phenolic acids added to the nutrient solution or tap water samples and followed during 54 h, disappeared after a short lag phase when incubated in the colonized effluent, whereas when incubated in nonsterile tap water, the tested phenolic acids were significantly more persistent. Of the substrate groups assessed using Biolog GN-panel, carbohydrates, amino acids, carboxylic acids, polymers and amines were the most consistently utilized substrates during an observation period of 96 h. However, not all of the compounds on the GN-panel included in these classes were metabolized. Remaining six substrate groups were utilized inconsistently and beyond the threshold of 0.5 absorbance units. Viable count of all scrutinized microbial groups (general bacterial and fungal flora, fluorescent pseudomonads, actinomycetes, Trichoderma ) was higher during the second out of two years of observation. Passage through the growing system indicated an increase in pH and electrical conductivity as compared to set values. Oxygen saturation and content decreased from the first to the last sampling whereas the total organic carbon content, the biochemical oxygen demand and the chemical oxygen demand remained unaffected.