Human activities have increased nitrogen deposition to northeastern forests. Most of the added nitrogen appears to be retained in soil organic matter, though we do not fully understand the consequences of nitrogen deposition for the soil carbon cycle.
The goal of our research is to quantify the magnitude and cause of changes in soil carbon in response to nitrogen fertilization. We collected soil from six long-term nitrogen fertilization experiments in northeastern forests for laboratory experiments to understand the impacts of nitrogen fertilization on soil carbon. We incubated ambient and nitrogen-fertilized soils from Fernow Forest (WV), Cary Institute of Ecosystem Studies (NY), Harvard Forest Pine (MA), Harvard Forest Hardwood (MA), Mt. Ascutney (VT), and Bear Brook (ME), and measured carbon dioxide.
Long-term nitrogen fertilization decreased carbon dioxide produced over time in four of the six northeastern forest soils in our study. Less carbon dioxide was produced by nitrogen fertilized soils than ambient soils from Fernow Forest, Cary Institute of Ecosystem Studies, Harvard Forest Pine, and Harvard Forest Hardwood. In soils from Mt. Ascutney and Bear Brook, we did not observe any difference in carbon dioxide production between ambient and nitrogen fertilized soils. We are currently estimating the magnitude of the impact of long-term nitrogen fertilization on decomposition. We are also studying microbial mechanisms underlying decreased decomposition with long-term nitrogen fertilization.