Surprising relationships between soil pH and microbial biomass and activity in a northern hardwood forest

Soil microbes mediate major biogeochemical processes in forest ecosystems. Soil pH is considered a “master variable” with a strong positive effect on many biogeochemical processes. To better understand how soil pH influences microbial activity and nitrogen (N) dynamics in forests, we utilized a set of long-term measurements of surface soil pH, N availability, and microbial biomass and respiration from the Hubbard Brook Experimental Forest (HBEF), a northern hardwood forest in New Hampshire, USA. We compared the strengths of these relationships in an unmanipulated watershed, where naturally acidic soils have been further acidified by anthropogenic acid deposition, to those in a nearby watershed, where soils were treated with calcium silicate to ameliorate the effects of acid deposition. While we expected to observe strong positive relationships between soil pH and microbial biomass and activity, we instead found weak and/or curvilinear relationships. In many cases, microbial biomass and activity peaked at unexpectedly low pH values (~ 4.5), and 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 questions about pH as a controller of microbial processes and how ecosystems recover in response to decreases in acid deposition.