The impact of an acidifying fertilizer on litter decomposition, root mycorrhizae, and soil and tree chemistry was assessed in two hardwood forests in central Ontario, Canada. Soil beneath mature sugar maple (Acer saccharumMarsh.) trees was treated with (NH4)2SO4granules at application rates of 0, 250, 500, and 1000 kg/ha in May of each year between 1993 and 1994 at Dorset and between 1993 and 1995 at Loring. The fertilizer treatments did not cause visual symptoms of forest decline. At Dorset, SO4and cation concentrations in soil leachate increased, but no difference in soil pH between treatments was found. An increase in foliar N was recorded at Dorset in treated plots, although there were no other differences in foliar or tree ring chemistry between treatments. At Loring, a decrease in soil pH and an increase in SO4in soil leachate were found in treated plots. Mycorrhizal infection and litter decomposition were also reduced in plots treated with (NH4)2SO4. Fertilizer application at Loring led to increases in foliar concentrations of N, Mg, Mn, and Cd, along with increases in concentrations of trace metals (As, Co, Cr, Mn, Ni, and Zn) in tree rings formed during fertilizer application. Significant positive correlations (p< 0.05) were found between wood (rings formed between 1993 and 1995) and foliage (mean of 1993-1995 values) for Mg, Mn, and Al. Deliberate acidification of surface soil resulted in changes in wood and foliar chemistry, although it remains to be established whether acidification of surface soil is responsible for the reported decline in sugar maple.