STRUCTURE OF A MICROBIAL COMMUNITY IN SOIL AFTER PROLONGED ADDITION OF LOW-LEVELS OF SIMULATED ACID-RAIN

Humus samples were collected 12 growing seasons after the start of a s imulated acid rain experiment situated in the subarctic environment. T he acid rain was simulated with H2SO4, a combination of H2SO4 and HNO3 , and HNO3 at two levels of moderate acidic loads close to the natural anthropogenic pollution levels of southern Scandinavia, The higher le vels of acid applications resulted in acidification, as defined by hum us chemistry. The concentrations of base cations decreased, while the concentrations of exchangeable H+, Al, and Fe increased. Humus pH decr eased from 3.83 to 3.65. Basal respiration decreased with decreasing h umus pH, and total microbial biomass, measured by substrate-induced re spiration and total amount of phospholipid fatty acids (PLFA), decreas ed slightly. An altered PLFA pattern indicated a change in the microbi al community structure at the higher levels of acid applications, In g eneral, branched fatty acids, typical of gram-positive bacteria, incre ased in the acid plots. PLFA analysis performed on the bacterial commu nity growing on agar plates also showed that the relative amount of PL FA specific for gram-positive bacteria increased due to the acidificat ion, The changed bacterial community was adapted to the more acidic en vironment in the acid-treated plots, even though bacterial growth rate s, estimated by thymidine and leucine incorporation, decreased with pH , Fungal activity (measured as acetate incorporation into ergosterol), vas not affected. This result indicates that bacteria were more affect ed than fungi by the acidification. The capacity of the bacterial comm unity to utilize 95 different carbon sources was variable and only sho wed weak correlations to pH, Differences in the toxicities of H2SO4 an d HNO3 for the microbial community were not found.