Unravelling trends in stream nitrogen

Posted on March 27, 2017

Oxides of nitrogen derived from agriculture, industry and emissions from the burning of fossil fuels fall out from the atmosphere and enter terrestrial and freshwater ecosystems. Over the last century the deposition of reactive nitrogen has increased three to five times, and this is reflected in a general global increase in stream nitrogen concentrations. However, at locations well removed from industry and agriculture – such as high-latitude forests – one might still expect to find no historic trend in stream nitrogen concentrations. Given this prediction, it came as a surprise to find, in high-latitude streams in Sweden, Canada and the USA, not only that there was no historic increase, but that there was a long-term trend in the opposite direction – one of decreasing nitrogen concentration. In an attempt to understand this observation, for which there was no clear explanation, researchers examined datasets from 1985 to 2011 for patterns in the terrestrial storage and stream-based export of nitrogen in nine contrasting forested watersheds in northern Sweden. They found a consistent decline in the waterborne export of inorganic N over the entire region that couldn’t be explained by reference to river discharge, which remained relatively constant. However, several other factors did show long-trends over the study period. First, there was a significant increase in tree stem volume and the amount of nitrogen stored in tree biomass, which was probably a result of forest management practices that emphasized biomass production, and thus the trees’ demand for nitrogen. Second, the climate in northern Sweden shifted, with increases in mean annual temperature and the number of growing degree days, and a nine day shift toward earlier peak snowmelt. And third, nitrogen concentrations in the soil increased, a trend that could be linked to both productivity-oriented forest management and the enhancing effects of climate warming on tree growth and nitrogen storage in soil detritus. Taken together, these findings suggest that forest management and climate change can control the storage and flow of nitrogen. In contrast to most regions of the world, where high anthropogenic nitrogen loads contribute to problems linked to excess productivity, it seems that in high-latitude streams and lakes, forest management and climate change may create the opposite problem of nutrient limitation, with negative implications for freshwater communities and food webs.

Reference: Lucas, R.W. et al. 2016. Long- term declines in stream and river inorganic nitrogen (N) export correspond to forest change. Ecological Applications 26(2), 545–556. http://onlinelibrary.wiley.com/doi/10.1890/14-2413/full