Drivers of Carbon Sequestration and Inorganic Nitrogen Export in Temperate Forest Ecosystems

Several temperate forests around the globe act as net carbon sinks and retain significant amounts of nitrogen inputs from atmospheric deposition. However, projected changes in climate, atmospheric deposition, and the functioning of trees and microbes may alter the ability of forests to retain carbon and nitrogen in the future. To evaluate climatic controls on carbon sequestration, we initiated in 2012 the Climate Change Across Seasons Experiment (CCASE) at Hubbard Brook Experimental Forest in New Hampshire, USA, a Long-Term Ecological Research (LTER) site. In this ongoing experiment, we warm soils by five degrees Celsius in the growing season using buried heating cable. We induce soil freeze/thaw events in winter by removing snow to create a 72-hour freeze and then turn on the buried heating cables to create a 72-hour thaw. Results to date from CCASE demonstrate that while growing season warming increases cumulative tree stem biomass carbon, winter soil freeze/thaw cycles caused by a small snowpack offset much of this growing season warming effect and the difference from reference plots is no longer statistically significant. These results demonstrate that Earth system models may overestimate the potential carbon sink in temperate forest ecosystems if they do not incorporate the impact of change in winter climate, such as the shrinking snowpack and greater frequency of soil freeze/thaw events. To evaluate controls on watershed nitrogen export from forest ecosystems, we conducted a meta-analysis of several time series collected through the international LTER Network. Sites included ecosystems across Europe, North America, and East Asia, as well as four climate types (tropical, temperate, Mediterranean, and boreal). We found declining trends in nitrogen deposition, with significant positive relationships between precipitation and stream nitrogen fluxes, with no significant relationships with air temperature. Our long-term data shows that although nitrogen deposition is declining over time, atmospheric nitrogen inputs and precipitation remain important predictors for inorganic nitrogen exported from watersheds.