IMPACT OF SIMULATED ACID-RAIN ON MICROBIAL RESPIRATION, BIOMASS, AND METABOLIC QUOTIENT IN A MATURE SUGAR MAPLE (ACER-SACCHARUM) FOREST FLOOR

Effects of simulated acid rain on forest floor microbiological process es were investigated in an old-growth sugar maple (Acer saccharum Mars h.) woods at Turkey Lakes Watershed, northern Ontario, Canada. Sulphat e and NO3- in ambient precipitation received on four replicate plots w ere augmented with additions of dilute (0.02 N) H2SO4 and HNO3, applie d atone and in combination periodically during the growing seasons of 1993 and 1994. The equivalent load of SO42- applied in the present exp eriment was ca. 6 times that in ambient precipitation in the H2SO4-onl y treatment and half that in the combined treatment over the study per iod. In situ soil respiration measured during the summer and fall of 1 994 showed no adverse effects of acid treatment. In the laboratory, so il microbial respiration, biomass (as determined by substrate-induced respiration), and microbial biomass carbon : organic carbon (C-mic/C-o rg) ratios were significantly depressed in the L or FH layers of the f orest floor when H2SO4 was applied alone or in combination with HNO3. No effects of HNO3 were detected when applied alone. Microbial respira tion, substrate-induced respiration, and C-mic/C-org ratios were signi ficantly correlated with forest floor pH. Microbial metabolic quotient s (respiration:biomass ratios) were not adversely affected by any of t he treatments. The observed deleterious effects of H2SO4 application u nder experimental conditions suggest the possibility of adverse effect s in the field over the long-term.