Heterogeneity of bacterial populations and pesticide degradation potentials in the unsaturated zone of loamy and sandy soils

Soil microbial processes play an important role in relation to pesticide po llution of groundwater, and may be strongly influenced by hydrological and geochemical properties. The consequences of such heterogeneous environments on bacterial biomass. enzymatic activities. carbon utilisation patterns, a nd pesticide mineralisation potentials in the unsaturated zone of a sandy l oam and a coarse sandy soil profile were studied. In sandy loam soil profil es the number of bacteria decreased from 10(9) cells g(-1) in the surface l ayers to about 10(7) cells g(-1) at 1.5-5 m depth. Simultaneously, the hydr olysis of fluorescein diacetate and arylsulfatase activity decreased to bel ow the detection limit It about 1.5 m depth, and carbon utilisation pattern s showed that bacterial populations from surface soil were significantly di fferent from those from 4 m depth. Bacterial biomass and activity in macrop ore soil tended to be slightly higher than in matrix soil, and the carbon u tilisation patterns of bacterial populations extracted from macropore soil and from matrix soil seemed to be different. Maximally 3% of C-14-labelled mecoprop and isoproturon was mineralised in soil from the 1-1.5 m depth, an d less than 1.5% was mineralised in soil from the 3.5-4 m depth. The macrop ore soil tended to have a higher degradation potential than the matrix soil . The total number of bacteria in the coarse sandy soil profile decreased f rom about 10(8) in the plough layer to 107 cells g(-1) at 0.4-2 m, The enzy matic activities and the degradation potentials of C-14-labelled mecoprop a nd isoproturon were significantly correlated (r(2) >0.79) and showed a dist inct decrease at about 0.4 m. In addition to the depth variability, a horiz ontal heterogeneity in this soil was observed as horizons or compartments t hat differed in colour, i.e. with different chemical composition and concen trations of Fe and organic matter. Counts of viable bacteria and measuremen ts of fluorescein diacetate hydrolysis and arylsulfatase activity confirmed a high variability of microbial biomass and activity in the sandy soil pro file.