Experimental soil acidification and recovery at the Bear Brook Watershed in Maine

The response and subsequent recovery of soils (Typic Haplorthods) to acidification by dilute H2SO4 (2000 and 4000 mol(c) ha(-1) yr(-1) of SO4), HNO3 (2000 and 4000 mol(c) ha(-1) yr(-1) of NO3), and combined H2SO4-HNO3 (2000 mol(c) ha(-1) yr(-1) each of SO4 and NO3) were studied at the Bear Brook Watershed in Maine. Treatments were delivered to eighteen 15 by 15 m plots (three plots per treatment) during four field seasons from 1988-1991. Recovery was monitored for the ensuing two years. Changes in soil and soil solution chemistry were evaluated using tension lysimeters and buried mineral soil bags. Soil solutions responded rapidly to all treatments with increased concentrations of SO4 (four to eight times ambient) and NO3 (39 to 88 times ambient), depending on treatment, accompanied by increased concentrations of Ca and Mg (three to five times ambient) from the upper B horizon. As solutions passed through the upper 25 cm of the pedon, concentrations of SO4 and the sum of base cations decreased significantly. Results from buried mineral soil bags located directly below the forest Boor indicated that a relatively reversible SO4 sorption mechanism was responsible for attenuating most SO4 losses from these soils. Following 4 yr of acid inputs and 2 yr of recovery, soil and soil solution chemistry were remarkably unchanged, demonstrating that this hardwood forest soil was not irreversibly altered and could quickly recover.