WINTER SURVIVAL OF MICROBIAL CONTAMINANTS IN SOIL

The aim of this study was to evaluate, at site scale, the influence of freezing and freeze/thaw cycles on the survival of fecal coliforms and fecal enterococci in soil, in a climate change perspective. Viable cells of both fecal indicators were counted in ten different soil profiles and at different depths, before and after a winter season. Before the winter period and during grazing, viable cells of fecal coliforms and fecal enterococci were detected only in the first 10 cm below ground, while, after the winter period and before the new seasonal grazing, a lower number of viable cells of both fecal indicators was detected only in some of the soil profiles, and within the first 5 cm. Thus, a significant decrease in viable cells was observed in all soil profiles, due to cold shock, freezing, prolonged exposure to subzero temperatures, and alternating freezing and thawing periods, even though this decrease was not uniform at site scale. Taking into consideration the results of specific investigations, we hypothesised that the non-uniform spatial distribution of grass roots within the studied soil ccould play an important role in influencing this phenomenon, while several abiotic factors, such as organic matter, grain size and specific weight of particles, total porosity, and relative density, did not play any significant role. In a climate change perspective, taking into account also the local trend in air temperature, a different distribution of microbial pollution over time is expected in spring waters, and a higher risk of transmission of infections is expected throughout the year. The progressive increase in air temperature, with emphasis on the minimum one, will cause a progressive decrease in freezing and freeze/thawing at higher altitudes, therefore minimising cold shocks on microbial cells, and causing spring water pollution also during winter, differently from the actual observations.