Climate Change and the Fate of Nitrogen on Northern Temperate Forests

Abstract:

Nitrogen is important in lakes – it is critical for ecosystem productivity, and too little or too much can impair ecosystem services in northern temperate forests. Climate change and climate-driven feedbacks on catchment hydrology and biogeochemistry have the potential to alter the atmospheric vs. aquatic fate of nitrogen in these forests. Warmer conditions over the past 30 years (i.e., 2°C warmer over the past three decades, y = 0.06x - 113.26, r²=0.205, p< 0.05) have resulted in lower hydrologic flows from catchments. However, these warmer conditions are accompanied by increased frequencies of extreme precipitation events that interact with geomorphology to influence the physical-chemical drivers of nitrogen transformations (including soil moisture, temperature, reduction-oxidation potential) within the catchments. Specifically, summer storm events result in hydrological flows over topographic depressions and flats, areas where nitrate-N fluxes are significantly higher than the average in the catchment, triggering the formation of redox conditions, which in turn, intercept the conversion of nitrate-N flowing to the stream by transforming it to nitrous oxide (N₂O) and nitrogen (N₂) gas (y = 19.90x^1.20, r²=0.744, p<0.05). An evaluation of the annual return period of summer storms that trigger significant N₂O+N₂ efflux events reveals that, for example, a 100 mm storm event that generates 100 g N₂O+N₂/ha/d has a return period of about 75 years under current climate scenarios but will have a much shorter return period of 25 years by 2100. A consequence of warmer temperatures and a higher frequency of extreme precipitation events is the shift from an aquatic to atmospheric fate for nitrogen, which in turn creates feedbacks for even warmer conditions due to increased effluxes of potent greenhouse gases as well as a reduction of nitrogen export to surface waters, which may promote N₂-fixing cyanobacteria blooms.