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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