The health and productivity of some temperate forests appear to be suffering from cation depletion. Our understanding of forest responses to acidic deposition and changes in soil chemistry would benefit from better knowledge of limitations to soil microorganisms, given their critical role mediating the supply of essential nutrients to plants. Surprisingly, soil microbial activity was not stimulated by recent calcium additions to the base-poor forest ecosystem at the Hubbard Brook Experimental Forest, NH, despite an increase in forest floor pH of approximately 1 unit. We tested two hypotheses that might account for the lack of pH response: 1) that microorganisms are only secondarily limited by low pH because of a stronger limitation by phosphorus availability, and 2) that the availability of labile C to microorganisms decreases at higher soil Ca availability, owing to Ca-C polyvalent bonding that limits microbial access and metabolism of C. We conducted a field nutrient addition experiment with five treatments (control, low Ca, high Ca, P, and low Ca + P) applied to 2 x 2 m plots. After 1 ½ years of nutrient additions, we incubated soils in the laboratory to test treatment effects on microbial respiration and respiratory response to added carbon substrate (ground leaf litter).
Results/Conclusions:

Basal respiration did not differ among treatments, and we detected no effect of Ca on the metabolism of added C substrate. However, leaf litter addition elicited a substantially greater respiratory response in P-treated soils compared to controls or Ca-treated soils. Growth of decomposer organisms appears strongly limited by P in these forest soils, with or without added Ca.