The work presents investigations of the impact of wastewater irrigation of organic soils (Terric Histosol and Histi-Mollic Gleysol) on their biological activity at a background of some physico-chemical properties and processes being essential under flooding conditions. The results of field experiments were, in accordance with prevision, difficult for unequivocal interpretation. Therefore, model studies under controlled laboratory conditions with soil monoliths and soil samples were performed.

The results showed that periodic soil irrigation with wastewater significantly affected its biological status, and intensity and a trend of these changes strongly depended on soil type, soil depth, plant cover and wastewater dose. Under field conditions, the activity of soil dehydrogenases and catalase was inhibited in peat- muck soil under poplar (Populus nigra and Populus alba) and in mineral-muck soil with grass cover (with domination of Alopecurus pratensis, Phalaris arundinacea and Festuca pratensis), whereas it was stimulated in peat-muck soil planted with willow (Salix americana and Salix viminalis). The persistence of these changes, as determined in the laboratory three years after the irrigation was stopped, depended significantly on the previous soil-plant combination. The inhibition was observed in soil under poplar, and it was unstable in meadow soil where both dehydrogenases and catalase increased down to 50 cm depth. In turn, the stimulation of dehydrogenase activity in soil planted with willow was maintained in surface layer only.

Various indicators used to study the surface soils (0-10 cm) under laboratory conditions revealed a strong transformation in physiological status of microbial community in soils subjected to 4-year irrigation with wastewater (120 cm per year) as compared to the control soils. The deterioration of soil biological properties under poplar was evidenced by a decrease in microbial biomass and biochemical activities which are related to the ability of the native organic matter utilization (dehydrogenase and catalase activities, N2O formation) and confirmed by an increase of metabolic quotient (qCO2) and a decrease of Cmic-to-Corg ratio. However, a high level of potential activity revealed from mineralization of the added glucose, indicated that no detrimental effects in microbial populations of previously irrigated soil took place.

By contrast, the irrigation of soil planted with willow improved its microbial status as measured by an increase of C availability (Cmic-to-Corg ratio), microbial biomass and actual activity (dehydrogenase and catalase activities, N2O evolution) as well as in a decrease of qCO2. The potential activity induced by glucose addition remained higher in the control soil.

The brake of wastewater irrigation of meadow soil caused some beneficial transformations in surface layer (increase in microbial biomass, dehydrogenase activity, N2O formation) with no changes in Cmic-to-Corg, qCO2 index, and activities stimulated by glucose.

Despite of relatively high level of the biochemical activity, the tested soils sampled from both control and irrigated plots showed very high redox resistance. Under prolonged flooding, soil samples showed redox potential Eh > 400 mV, Fe+2 < 20 mg kg–1, high activity of catalase (enzyme specific for aerobic metabolism) and no typical relation between Eh and soil oxidoreductases. By contrast, the intensive redox processes were observed in soil samples amended with glucose and N2O, Fe+2, H2 and CH4 were detected at Eh < 400 mV, < 300 mV, < 200 mV and < –50 mV, respectively. A decrease in redox potential under soil hypoxia was followed by significant increase in the dehydrogenase activity.

All soils irrigated with wastewater showed lower pH, higher contents of Corg, NH4+, NO3–, higher C:N ratios, larger respiration and methanotrophic activities than the control soils. Independently on the irrigation, soils with higher Cmic and Corg showed higher dehydrogenase and respiration activities. The qCO2 and Cmic to-Corg ratio could be useful indexes in the study of the trend of biological activity changes in organic soils irrigated with wastewater.

Despite of some disadvantageous transformations, all soils maintained potential ability for methanotrophic activity, complete denitrification to N2, and redox resistance. These properties emphasized the adaptability of tested organic soils to irrigation with wastewater, however supporting proper soil air-water conditions, as well as low organic matter content in wastewater is necessary.