Soil microbes mediate major biogeochemical processes in forest ecosystems. Soil pH is considered a "master variable" controlling these processes and many biogeochemical processes in turn in uence pH. Long-term measurements of soil pH, microbial biomass carbon (C) and nitrogen (N) content, microbial respiration, potential net N mineralization and nitri cation rates, denitri cation potential, and soil nitrate (NO 3 -) and ammonium (NH + 4 ) concentrations have occurred since 1994 at the Hubbard Brook Experimental Forest (HBEF), a northern hardwood forest in New Hampshire, USA.Naturally acidic soils at the HBEF have been further acidi ed by anthropogenic acid deposition. Soils in a HBEF watershed have been experimentally treated with calcium silicate to ameliorate acid deposition. While we expected to observe strong positive relationships between soil pH and microbial biomass/activity, weak and/or curvilinear relationships between microbial biomass and activity and soil pH were evident, with peaks at unexpectedly low values (~ 4.5) and values decreased at higher pH values, especially in the calcium-treated soils. It is likely that complexities in plant: microbial interactions inhibit and/or mask microbial response to changes in pH in these acidic soils. These resultsraise fundamental questions about pH as a controller of microbial processes and how ecosystems recover in response to decreases in acid deposition.

Soil microbes mediate major biogeochemical processes in forest ecosystems. Soil pH is considered a “master variable” controlling these processes and many biogeochemical processes in turn influence pH. Long-term measurements of soil pH, microbial biomass carbon (C) and nitrogen (N) content, microbial respiration, potential net N mineralization and nitrification rates, denitrification potential, and soil nitrate (NO3-) and ammonium (NH+ 4) concentrations have occurred since 1994 at the Hubbard Brook Experimental Forest (HBEF), a northern hardwood forest in New Hampshire, USA. Naturally acidic soils at the HBEF have been further acidified by anthropogenic acid deposition. Soils in a HBEF watershed have been experimentally treated with calcium silicate to ameliorate acid deposition. While we expected to observe strong positive relationships between soil pH and microbial biomass/activity, weak and/or curvilinear relationships between microbial biomass and activity and soil pH were evident, with peaks at unexpectedly low values (~ 4.5) and values decreased at higher pH values, especially in the calcium-treated soils. It is likely that complexities in plant: microbial interactions inhibit and/or mask microbial response to changes in pH in these acidic soils. These results raise fundamental questions about pH as a controller of microbial processes and how ecosystems recover in response to decreases in acid deposition.