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Patches of groundwater responsible for nitrogen loss from forests


Water weir This weir is for measuring streamflow and dissolved chemical export from watershed 3 at the Hubbard Brook Experimental Forest in New Hampshire, which is the watershed where the study took place.

Aug. 15, 2015 – Researchers found that even during summer dry spells, isolated patches of soil in forested watersheds remain waterlogged at depth and act as hot spots of microbial activity that remove nitrogen from groundwater and return it to the atmosphere.

Most nitrogen is deposited by rain. Temperate forests receive much larger inputs of nitrogen from the atmosphere than they export to streams. Once nitrogen leaves the forest in streams, it can become a water pollutant.

The process of denitrification — a gaseous loss of nitrogen to the atmosphere — is difficult to measure. Since it removes nitrogen from water, it can improve water quality in downstream lakes and estuaries. However, nitrogen is also an important nutrient for plant growth in the forest, so removals of nitrogen by natural processes can reduce the forest productivity.

The researchers, from Virginia Tech, Cornell University, and the Cary Institute of Ecosystem Studies, measured the presence of nitrate, a form of nitrogen that is highly mobile and reactive in the environment. They then determined whether the nitrate is a result of atmospheric deposition or microbial conversion, and discovered that there is nitrogen loss to the atmosphere by looking at nitrate at the atomic level.

“Some work remains to be done, but the aim is to be able to develop a better sense of where and how nitrogen is processed in the environment and to be in a position to predict how changes in climate affect nitrogen cycling and water quality in forested ecosystems,” said Kevin McGuire, associate director of the Virginia Water Resources Research Center. The team’s findings were reported in the Proceedings of the National Academy of Sciences.

Read the full press release.

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