The goal of our work is to investigate the responses of soil carbon to anthropogenic nitrogen deposition, and to understand mechanisms controlling those responses.  We are testing the hypothesis that nitrogen deposition to northeastern forests is accelerating decomposition of labile substrates within the soil carbon reservoir, while inhibiting decomposition of recalcitrant substrates.

We are incubating soil from six forested sites in the northeastern U.S. with previously established nitrogen fertilization experiments.   The soils represent a range of labile and recalcitrant carbon content.  We are incubating ambient and nitrogen-fertilized soil from all sites, and we have applied a standardized laboratory nitrogen fertilization treatment to the incubating soils, in a fully factorial design.  We are measuring the response of CO₂ and extracellular enzyme activity to the nitrogen fertilization treatments.

Results/Conclusions

Our preliminary data support our hypothesis.  The data indicate that nitrogen fertilization stimulates soil CO₂ production where labile carbon is abundant, while fertilization inhibits CO₂ production where recalcitrant substrates are dominant.  Also, nitrogen fertilization is associated with a stimulation of enzymes targeting labile carbon substrates and an inhibition of enzymes targeting recalcitrant substrates.  Based on our results-to-date, we believe that nitrogen fertilization stimulates CO₂ production in some soils by stimulating microbial decomposition of labile substrates, while nitrogen fertilization inhibits CO₂ production in other soils by inhibiting the microbial decomposition of recalcitrant substrates.