Distribution of microorganisms, biomass ATP, and enzyme activities in organic and mineral particles of a long-term wastewater irrigated soil

The aim of the study was to elucidate the spatial distribution of soil micr oorganisms and enyzme activities in a long-term wastewater treated soil. So il was sampled from a plough layer of the Ah horizon of a sandy Haplic Luvi sol which was either (1) irrigated with municipal wastewater for almost 100 years, or (2) no more irrigated since 20 years, or (3) never received wast ewater. The samples were fractionated by wet sieving to obtain seven size f ractions of organic and mineral soil particles, and a separate silt+clay fr action. The individual soil samples contained between 1.2% (never irrigated ) and 4.1% (long-term irrigated) organic particles by weight, but these par ticles harboured up to 47.8% of the total soil carbon and 41.7% of nitrogen , and thus represented an important storage of energy and nutrient for micr oorganisms. In total, however, the highest C and N amounts were accumulated in the silt+clay fraction, whereas coarser mineral particles which dominan ted by weight in the Haplic Luvisol were low in C and N. The highest number s of bacteria, actinomycetes and fungi per gram of the individual soil frac tions were found in organic particles of the long-term irrigated soil. Less nutrient-dependent oligotrophic bacteria were for the most part associated with the silt+clay fraction, irrespective of the soil treatment with waste water. Similar to microbial counts, also the ATP content, as a measure of a ctive microbial biomass, and the activities of beta-glucosidase, beta-acety lglucosaminidase, and proteinase were higher in the long-term irrigated soi l than in that which was never irrigated. In most cases slightly enhanced v alues of microbiological and biochemical parameters were still detectable 2 0 years after the wastewater irrigation was terminated. The values of the i ndividual parameters decreased in all soil samples under testing in general gradually with decrease in size of the organic soil particles. In conclusi on, the coarse soil organic particles > 5 mm and the silt+clay fraction < 0 .05 mm represent the sites with the highest microbial inhabitance, ATP cont ents and enzyme activities in the Ah horizon of an Haplic Luvisol. Long-ter m wastewater irrigation resulted in an increase of microbial counts, total biomass and soil enzyme activities.