Response of buried mineral soil bags to experimental acidification of forest ecosystem

At Bear Brook Watershed in Maine, H₂SO₄ and/or HNO₃ was added to 15 by 15 m plots (three replicates per treatment) from 1988 through 1990. Total loading above ambient treatments for the 3 yr was 0 (control), 5100 (low S, low N), 10 200 (high S and N + S), and 11 950 (high N) mol ha^-1 yr^-1. Changes in soil chemistry were evaluated using tension lysimeters and buried mineral soil bags placed below the forest floor before treatments were initiated. Total soil S was highest in the high S treatment (355 mg S kg^-1 soil), intermediate in the low S and N + S treatments (314 and 310 mg S kg^-1 soil, respectively), and lowest in the low N, high N, and control treatments (278, 275, and 270 mg S kg^-1 soil, respectively). Differences in total S were mostly attributed to phosphate-extractable SO₄, although smaller changes in ester sulfate and C-bonded S were also detected. Low pH and elevated SO₄ in solution resulted in increased SO₄ adsorption. There were no differences caused by treatments in N or C constituents in the soil bags, although N was strongly linked with C (r = 0.86). Nitric and sulfuric acid additions lowered soil pH (H₂O) from 4.83 in the control to 4.67 and 4.70, respectively. Sulfuric acid treatments increased exchangeable Ca to 0.32 cmol_c kg^-1 compared with 0.24 cmol_c kg^-1 in the control, whereas HNO₃ treatments decreased Ca to 0.18 cmol_c kg^-1. A strong correlation (r = 0.94) between exchangeable Ca and base saturation reflected the quantitative dominance of exchangeable Ca among base cations. Similarities in effects of the N + S and the low S treatments were due to identical additions of SO₄ and the absence of a marked effect of NO₃ additions on the mineral soil. Changes in the chemistry of the buried mineral soil bags can be explained by both the effects of H₂SO₄ and HNO₃ additions as well as pedogenic processes.