Calcium additions and microbial nitrogen cycle processes in a northern hardwood forest

Evaluating, and possibly ameliorating, the effects of base cation depletion in forest soils caused by acid deposition is an important topic in the northeastern United States. We added 850 kg Ca ha^-1 as wollastonite (CaSiO₃) to an 11.8-ha watershed at the Hubbard Brook Experimental Forest (HBEF), a northern hardwood forest in New Hampshire, USA, in fall 1999 to replace calcium (Ca) leached from the ecosystem by acid deposition over the past 6 decades. Soil microbial biomass carbon (C) and nitrogen (N) concentrations, gross and potential net N mineralization and nitrification rates, soil solution and stream chemistry, soil:atmosphere trace gas (CO ₂, N₂O, CH₄) fluxes, and foliar N concentrations have been monitored in the treated watershed and in reference areas at the HBEF before and since the Ca addition. We expected that rates of microbial C and N cycle processes would increase in response to the treatment. By 2000, soil pH was increased by a full unit in the Oie soil horizon, and by 2002 it was increased by nearly 0.5 units in the Oa soil horizon. However, there were declines in the N content of the microbial biomass, potential net and gross N mineralization rates, and soil inorganic N pools in the Oie horizon of the treated watershed. Stream, soil solution, and foliar concentrations of N showed no response to treatment. The lack of stimulation of N cycling by Ca addition suggests that microbes may not be stimulated by increased pH and Ca levels in the naturally acidic soils at the HBEF, or that other factors (for example, phosphorus, or Ca binding of labile organic matter) may constrain the capacity of microbes to respond to increased pH in the treated watershed. Possible fates for the approximately 10 kg N ha^-1 decline in microbial and soil inorganic pools include components of the plant community that we did not measure (for example, seedlings, understory shrubs), increased fluxes of N₂ and/or N storage in soil organic matter. These results raise questions about the factors regulating microbial biomass and activity in northern hardwood forests that should be considered in the context of proposals to mitigate the depletion of nutrient cations in soil.