Carbon additions increase nitrogen availability in northern hardwood forest soils

The effects of acetate additions to northern hardwood forest soils on micro bial biomass carbon (C) and nitrogen (N) content, soil inorganic N levels, respirable C and potential net N mineralization and nitrification were eval uated. The experiment was relevant to a potential watershed-scale calcium ( Ca) addition that aims to replace Ca depleted by long-term exposure to acid rain. One option for this addition is to use calcium-magnesium (Mg) acetat e, a compound that is inexpensive and much more readily soluble than the Ca carbonate that is generally used for large-scale liming. Field plots were treated with sodium (NA) acetate, Na bicarbonate or water (control) and wer e sampled (forest floor - Oe and Oa combined) 2, 10 and 58 days following a pplication. It was expected that the addition of C would lead to an increas e in biomass C and N and a decrease in inorganic N. Instead, we observed no effect on biomass C, a decline in biomass N and an increase in N availabil ity. One possible explanation for our surprising results is that the C addi tion stimulated microbial activity but not growth. A second, and more likel y, explanation for our results is that the C addition did stimulate microbi al growth and activity, but there was no increase in microbial biomass due to predation of the new biomass by soil fauna. The results confirm the emer ging realization that the effects of increases in the flow of C to soils, e ither by deliberate addition or from changes in atmospheric CO2, are more c omplex than would be expected from a simple C:N ratio analysis. Evaluations of large-scale manipulations of forest soils to ameliorate effects of atmo spheric deposition or to dispose of wastes should consider microbial and fa unal dynamics in considerable detail.