Abiotic and biotic factors in the nutrient solution and filter skin (Schmutzdecke) of slow filters integrated to closed hydroponic greenhouse systems

摘要

Rapid pathogen dispersal by circulating nutrient solutions in closed hydroponic systems poses a major threat to this production method. Slow filtration is very effective against important fungal pathogens such as Phytophthora cryptogea and Pythium aphanidermatum. Filter efficacy against pathogens can partly be predicted by observation of autochthonous microbial communities in slow filters and filter efficacy can be improved by manipulation of these communities. However, data about factors influencing filter efficacy are scarce. Generally, slow filter efficacy is the result of complex interactions between abiotic and biotic attributes of nutrient solution and filter body. Selected abiotic (electric conductivity, pH, oxygen content, chemical oxygen demand, dissolved organic carbon) and biotic factors (general bacterial and fungal microflora, fluorescent pseudomonads, filamentous actinomycetes, Fusariumoxysporum, P. aphanidermatum, biochemical oxygen demand, enzyme activity) were monitored in a two-year study of two distinct commercial hydroponic systems (tomatoes and ornamentals) with integrated slow filters in supernatant, filter skin and effluent. The same filter skin parameters were monitored in six small-scaleexperimental systems with slow filters to establish possible correlations with filter efficacy against F. oxysporum f.sp. cyclaminis. In half these systems, filter skins were enriched with fungal cell wall preparation to enhance biological activity of microfauna. Total polysaccharide and extracellular biofilm polysaccharide content of the filter skin were determined and a method for extracellular biofilm polymer extraction devised. The hydroponic systems studied differed in terms of chemical oxygen demand, dissolved organic carbon, oxygen consumption of slow filters by aerobic autochthonous microorganisms and fungal densities before and after slow filtration. A seasonal influence on xylanase enzyme activity in filter skins was also observed. However, as filter efficacy and other parameters in the experimental systems exhibited no significant differences, no firm conclusions could be drawn. Total polysaccharide content within filter skins increased over time and was a good parameter for demonstrating the effects of organic amendments on filter skin formation. Extracellular biofilm polysaccharide content of filter skins was not influenced by organic matter addition but more research is needed to reveal the influence of this parameter on filter efficacy.