New paper about how forests lose nitrogen through shallow groundwater flowpaths


This weir, a place in a stream where a wall controls the flow of water, measures stream flow and dissolved chemical exports from Watershed 3 at the Hubbard Brook Experimental Forest in New Hampshire, where the research took place (photo credit: JP Gannon).

In a paper published in the Proceedings of the National Academy of Sciences, McGuire working with a team from the Hubbard Brook Ecosystem Study found clear evidence of nitrogen loss through denitrification in isolated shallow groundwater patches in a small watershed. The findings were somewhat of a surprise because denitrification has been so difficult to measure at Hubbard Brook even though deposition and export relationships suggested that denitrification may have been a factor in the decline of nitrate concentrations observed in streamwater. The paper significance statement and abstract are below. The paper is also available now in PNAS early edition.


Denitrification is the most poorly understood process in the terrestrial N cycle. As a result, terrestrial N budgets are wildly unbalanced and our ability to address global nitrogen pollution is fundamentally constrained. Denitrification is controlled by multiple factors, often exhibiting extraordinary variation in time and space, especially in terrestrial environments. Temperate forests regularly receive much larger inputs of precipitation N than they export in streamwater. The fate of the rest has been elusive. We present stable isotope measurements revealing extensive evidence of denitrification from temper- ate-forest shallow groundwater in midsummer, even as concurrent measurements of streamwater show little sign of denitrification. These measurements support the importance of a disputed nitrogen removal process and its occurrence at a previously missed time and location.


Despite decades of measurements, the nitrogen balance of tem- perate forest catchments remains poorly understood. Atmospheric nitrogen deposition often greatly exceeds streamwater nitrogen losses; the fate of the remaining nitrogen is highly uncertain. Gaseous losses of nitrogen to denitrification are especially poorly documented and are often ignored. Here, we provide isotopic evidence (δ15NNO3 and δ18ONO3) from shallow groundwater at the Hubbard Brook Experimental Forest indicating extensive denitrification during midsummer, when transient, perched patches of saturation developed in hillslopes, with poor hydrological connectivity to the stream, while streamwater showed no isotopic evidence of denitrification. During small rain events, precipitation directly contributed up to 34% of streamwater nitrate, which was otherwise produced by nitrification. Together, these measurements reveal the importance of denitrification in hydrologically disconnected patches of shallow groundwater during midsummer as largely overlooked control points for nitrogen loss from temperate forest catchments.

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The Cornell Chronicle

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Welcome Ray!

Ray Lee

A new Ph.D. student, Ray Lee, joined our lab this week.  Ray recently completed his M.S. at San Diego State University in geography and studied environmental and resource economics and philosophy during his undergraduate program at UC-Davis.  Ray will be working on a project based at the Coweeta Hydrologic Lab in North Carolina.  Welcome to Blacksburg!

Posted in Lab news

New paper by Gannon on organizing groundwater regimes and response thresholds by soils

JP Gannon had a paper accepted this week in Water Resources Research titled “Organizing groundwater regimes and response thresholds by soils: A framework for understanding runoff generation in a headwater catchment.”

Here’s the abstract: A network of shallow groundwater wells in a headwater catchment at the Hubbard Brook Experimental Forest in New Hampshire, USA was used to investigate the hydrologic behavior of five distinct soil morphological units. The soil morphological units were hypothesized to be indicative of distinct water table regimes. Water table fluctuations in the wells were characterized by their median and interquartile range of depth, proportion of time water table was present in the solum, and storage-discharge behavior of subsurface flow. Statistically significant differences in median, interquartile range, and presence of water table were detected among soil units. Threshold responses were identified in storage-discharge relationships of subsurface flow, with thresholds varying among soil units. These results suggest that soil horizonation is indicative of distinct groundwater flow regimes. The spatial distribution of water table across the catchment showed variably connected/disconnected active areas of runoff generation in the solum. The spatial distribution of water table and therefore areas contributing to stormflow is complex and changes depending on catchment storage.

Key points:

  • Soil horizonation is indicative of distinct water table fluctuations
  • Threshold storage-discharge relationships vary among hydropedological soil units
  • Spatial patterns of runoff generation are linked to soil horizonation and thresholds in catchment storage
Posted in Lab news, Research paper Tagged with: , , , ,

Variable source area concept…Hewlett 1961

I recently obtained a digital copy of Hewlett’s 1961 description of the variable source area concept.  This document, which is part of an annual report of the US Forest Service Southeastern Forest Experiment Station and at the time was based out of Asheville, NC, is regarded as the first publication of the concept in English.  There were simultaneous works published in France (Cappus 1960) and Japan (Tsukamoto 1961), but Hewlett’s version is often credited as the first.  While this 1961 report is not my favorite description of the variable source area concept, it is probably the most difficult to find and that is why I am posting it here (I think copyright is not an issue since it is a federal report).  I prefer the description by Tom Dunne from Kirkby’s Hillslope Hydrology book, but apparently Hewlett did not agree that version.  Anyway, it’s a shame that Hewlett (or Cappus or Tsukamoto) did not explicitly publish his ideas in a peer-review journal in the early 1960s.

Posted in Research paper

Welcome Carrie and Amy

We have two new students in the lab!  Carrie Jensen and Amy Gondran.


Posted in Lab news

New paper on urban soil rehabilitation

STE_coverYujuan Chen just had her third paper published from her dissertation.  Yujuan is now with the Food and Agriculture Organization in Rome Italy.

Chen, Y., Day, S.D., Wick, A.F., McGuire, K.J. 2014. Influence of urban land development and subsequent soil rehabilitation on soil aggregates, carbon, and hydraulic conductivity, Science of The Total Environment, 494-495: 329–336, 10.1016/j.scitotenv.2014.06.099.

Paper highlights:

  • Urban land development reduces soil macroaggregates and permeability.
  • Can subsurface soil rehabilitation with compost mitigate these effects?
  • Soil rehabilitation does not measurably enhance aggregate formation within 5 years.
  • Soil rehabilitation does improve subsurface hydraulic conductivity.
  • Urban soil ecosystem service provision is strongly management dependent.


Posted in Research paper Tagged with: , ,

Repost from Water Underground on John Selker’s “Fellows Speak” letter

Words of advice from John Selker.  This is worth reading.

The link above is a repost from Tom Gleeson’s blog “Water Underground” and originally appeared in the AGU Hydrology Section July 2014 newsletter.

Posted in ReBlog, Science Tagged with: , , ,

50th Anniversary of Water Resources Research Act and Establishment of the VA Water Center


50 years old today, President Lyndon B. Johnson signed into law the Water Resources Research Act, which established a Water Resources Research Institute in each state.  The Virginia Water Resources Research Center, which is my home appointment at Virginia Tech, is Virginia’s mandated WRRI.  Let’s hope the WRRA and the institutes continue to “enlist the intellectual power of universities and research institutes in a nationwide effort to conserve and utilize our water resources for the common benefit” for another 50 years!  The WRRI’s are responsible for training future water scientists and engineers, and transferring the results of sponsored research to water managers and the public.

For more information, see the USGS news release on this anniversary.

Posted in News, Policy Tagged with: ,

Keep your hat on. We may end up miles from here.



Until fairly recently, it seemed like this was impossibly far away.

I cleaned my desk, have check marks on all the right forms, and my things are all in boxes. It is time to move on! I am VERY excited to say that I will be starting a position as assistant professor in the Geosciences and Natural Resources department at Western Carolina University. It will be an exciting opportunity to work on getting undergraduate students excited about science while continuing to work on questions that excite me at the school’s research watershed.

A very very heartfelt thank you to the VT watershed hydrology group. I honestly can’t think of one thing I would change about my graduate school experience in this group. The support, friendship, and talent is second to none. If I had to do it all over again, I would choose this group again in a heartbeat. I could go on and on, but I’ll keep it short. THANK YOU.

With that, I have some time to kill before August. Time to go on a bike ride.

Check out the WCU logo… maroon and orange is great, but that purple!!


Posted in Lab news

Congratulations Kris!

Kris Brown successfully defended his dissertation yesterday. Congratulations to Kris.

The title of his dissertation was “Sediment delivery from reopened forest roads at stream crossings in the Virginia Piedmont physiographic region, USA”


Posted in Lab news

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