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

Background/Question/Methods

Soil microbes facilitate major biogeochemical processes in forest ecosystems and soil pH is viewed as a "master variable" controlling these processes. Long-term monitoring 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 has occurred since 1994 at the Hubbard Brook Experimental Forest (HBEF), a northern hardwood forest in New Hampshire, USA. This site has naturally acidic soils that have been further acidified by anthropogenic acid deposition, as well as soils that have been experimentally treated with calcium minerals to ameliorate this deposition.

Results/Conclusions

While we expected to observe strong positive relationships between soil pH and microbial biomass/activity, results show weak and/or curvilinear relationships between microbial biomass and activity and soil pH, with peaks at unexpectedly low values (~4.5) and decreases 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 control over microbial processes and how ecosystems respond to decreases in acid deposition.