Nitrogen alters carbon cycling and microbial activity in a recently burned boreal ecosystem

Nitrogen addition often affects soil carbon balance, but the direction and magnitude of this effect are difficult to predict because the mechanisms of soil carbon-nitrogen interactions are poorly understood. These mechanisms could be particularly important for the global carbon cycle if nitrogen deposition and climate warming increase nitrogen availability to ecosystems with large soil carbon stocks, such as boreal forests. In a boreal ecosystem that burned in 1999, we hypothesized that nitrogen fertilization would increase microbial activity and soil carbon fluxes because of increased plant carbon inputs under fertilization. Instead, we found that soil respiration declined by 25% and microbial biomass carbon (by chloroform fumigation) declined by over 50% in response to nitrogen fertilization. Soil carbon stocks also declined by nearly 50% in the O-horizon. Microbial C:N ratios and the activities of carbon-degrading enzymes per unit soil mass did not vary significantly with fertilization, although cellulose- and chitin-degrading enzyme activities nearly doubled with N addition when expressed per unit soil carbon. In contrast, the activity of a protein-degrading enzyme declined by 75% (per unit soil mass). These results suggest that nitrogen addition has reduced the quality or supply of carbon substrates available to soil microbes, despite significant increases in plant productivity. This reduction may have been driven by increased losses of soil carbon and/or reduced soil carbon accumulation since fertilization began. Because of rapid rates of change in plant, soil, and microbial carbon pools, early-successional ecosystems may be particularly sensitive to global changes such as warming and nitrogen deposition.