Change in the Nitrogen Saturation Status of Northern Temperate Forests - Will Lakes Face a More Nitrogen-Limited Future?

English 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. Reductions in atmospheric pollution have also substantially reduced N inputs to the temperate forest biome. Climate change and climate-driven feedbacks on catchment hydrology and biogeochemistry have the potential to alter the atmospheric vs. aquatic fate of nitrogen stored in these forest soils. Warmer conditions over the past 30 years have resulted in lower hydrological fluxes from catchments. However, these warmer conditions are accompanied by increased frequencies of large precipitation events, which interact with geomorphology to influence the physical-chemical drivers of nitrogen transformations by creating variable redox areas that expand and contract in response to precipitation. Snowpack conditions create hydrological conditions that promote denitrification activity throughout the catchment. Summer storm events result in hydrological fluxes over topographic depressions and flats, 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. A consequence of warmer temperatures and a higher frequency of large 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. The combination of climate change and atmospheric pollution may be a shift in the forest from a nitrogen saturation (due to decades of elevated atmospheric pollution) to a trajectory towards nitrogen limitation, which would lead to reduced nitrogen loads to lakes and contribute to the emergence of N₂-fixing cyanobacteria blooms.