Every year, about 200,000 tons of nitrogen enter coastal waters globally. Most of this nitrogen is a consequence of human activities: runoff of fertilizer from land, inputs of treated sewage and animal waste, and as a “rain” of nitrogen-laden particles generated by combustion. This nitrogen is removed by linked, natural processes that are currently out of balance, leading to the accumulation of nitrogen. The excess nitrogen wreaks havoc in our coastal waters, contributing to the expansion of “dead zones,” such as that found at the mouth of the Mississippi River. Environmental factors, including water acidity, dissolved oxygen concentration, light inhibition and water temperature, have all been shown to play a role in controlling an important step in the removal process, called nitrification. The diversity and number of factors that seem to be involved, and their covariance, has made it difficult to identify a controlling factor and as a consequence, to model the process accurately.
Now scientists have identified one simple environmental factor, water temperature, as the dominant controlling variable of the first, and rate controlling step, of the biogeochemical pathway. Researchers Sylvia Schaefer and James Hollibaugh made the link during studies of the process in coastal waters off of Georgia. They found that nitrite, an intermediate in the process, accumulates in mid-summer, coinciding with explosive growth of the group of organisms responsible for its production, called Thaumarchaea. Lab experiments linked accumulation to the differential response of Thaumarchaea to temperature over that of a second group that removes nitrite. Schaefer and Hollibaugh then set out to test the generality of this uncoupling by examining environmental monitoring data from 270 locations in the US, France and Bermuda for evidence of nitrite accumulation. They found mid-summer peaks of nitrite in most locations, coinciding with warmer (between 20 and 30 oC) water temperature.
Coastal water temperatures are expected to rise as a consequence of global warming. Higher water temperatures, and especially peak mid-summer temperatures, are thus likely to accelerate the first step of the nitrogen removal process, helping to counteract the impact of excess nitrogen in runoff from land to coastal waters. Unfortunately, the same process will also increase the loss to streams and rivers, and ultimately, to the coastal zone, of fertilizer nitrogen applied to crops and lawns. In addition, the process leading to nitrite production also yields nitrous oxide, a powerful greenhouse gas (>300-fold stronger per molecule than carbon dioxide). Nitrous oxide also destroys atmospheric ozone, leading to increased penetration of the sun’s UV rays into the atmosphere.
The discovery of the environmental controls of nitrification in coastal waters thus has global implications for our understanding of the earth’s nitrogen cycle and its response to global warming.
Schaefer, Sylvia C. and James T. Hollibaugh. 2017. Temperature Decouples Ammonium and Nitrite Oxidation in Coastal Waters. Environmental Science and Technology, in press. http://pubs.acs.org/doi/abs/10.1021/acs.est.6b03483