Dr Peter M Groffman
There is great interest in evaluating, and possibly ameliorating, the effects of forest soil base cation depletion caused by acidic atmospheric deposition in the northeastern U.S. We added 850 kg Ca ha-1 as wollastonite (CaSiO3) to an 11 ha watershed at the Hubbard Brook Experimental Forest (HBEF), a northern hardwood dominated forest in New Hampshire, USA, in fall 1999 to replace Ca leached from the ecosystem by acidic deposition over the past several decades. Soil microbial biomass C and N concentrations, potential net N mineralization and nitrification rates, soil solution and stream chemistry, soil:atmosphere trace gas (CO2, N2O, CH4) fluxes, and foliar, root and litter N concentrations have been monitored in the treated watershed and in reference areas at 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 2002, soil pH was elevated by a full unit in the Oie and by nearly 0.5 units in the Oa soil horizons. 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 and soil solution concentrations of N showed no response to treatment. The lack of stimulation of N cycling by Ca additions suggests that microbes may be adapted to the acidic soil conditions at HBEF or that other factors (e.g., phosphorus, Ca binding of labile organic matter) may constrain the capacity of microbes to respond to increased pH in the treated watershed. The decreases in soil N availability and microbial biomass N suggest that Ca additions have increased the ability of plants to compete with microbes for N. The results raise fundamental questions about the factors regulating microbial biomass and activity in northern hardwood forests that need to be answered before large scale solutions to soil base cation depletion can be developed.