While much research over recent decades has focused on the deleterious effects of excess N on forests, recent declines in atmospheric nitrogen deposition and in nitrogen export from these ecosystems in several regions around the world have raised new concerns about nitrogen oligotrophication, limitations of forest productivity, and the capacity for forests to respond dynamically to disturbance and environmental change. In addition to decreases in atmospheric deposition, oligotrophication in forest soils can be driven by increased carbon flow from the atmosphere through soils that stimulates microbial immobilization of nitrogen and decreases available N for plants. Deacidification of soils, which is also occurring in several regions around the world, can also increase carbon flow and stimulate immobilization. In addition to changes in the carbon cycle, oligotrophication is likely driven by increases in the growing season which increase plant demand for N. Here, we examine symptoms of nitrogen oligotrophication in multiple long-term data streams from the Hubbard Brook Long Term Ecological Research site including gaseous and hydrologic nitrogen outputs as well as internal production of inorganic nitrogen via mineralization and nitrification. We also analyze changes in nitrogen cycle responses to disturbance.
Results/Conclusions
We have observed marked declines in gaseous and hydrologic outputs of nitrogen, internal production of inorganic nitrogen via mineralization and nitrification, and response to disturbance at Hubbard Brook over the past 50 years. There have been particularly sharp declines in nitrous oxide emissions from soils and in rates of in situ and potential net mineralization and nitrification since the late 1990s. Responses of multiple nitrogen cycle processes to soil freezing and ice storm disturbances have also declined since the late 1990s. These declines are only partially coincident with declines in atmospheric nitrogen deposition suggesting that multiple factors are contributing to this nitrogen oligotrophication of the northern hardwood forests at this site. A positive feedback loop, where decreased available nitrogen in soils can result in increased nitrogen resorption by trees, which reduces litterfall nitrogen input to soils, further limiting available nitrogen supply and leading to further declines in soil nitrogen availability may also be an important driver of oligotrophication. These results suggest a need to re-evaluate the nature and extent of nitrogen cycling in forests and assess how changing conditions will influence forest ecosystem response to multiple, dynamic stresses of global environmental change.