McHale, Michael, R. 1999. Hydrologic controls of nitrogen cycling in an Adirondack watershed.  Ph.D. Dissertation, SUNY-ESF, 230 pp.


Abstract:
There has been a significant amount of research during the last decade concerning nitrogen (N) cycling in small watersheds including evaluation of “N saturation.”  Predictive models of N cycling have met with limited success due to limitations in our understanding of the linkages between watershed N biogeochemistry and hydrology.  The research described in my dissertation evaluates some of these linkages.  Three hypothesis were tested: (1) NO3- is the dominant form of stream water N flux in the Arbutus Lake watershed, (2) near-stream wetlands control the form and amount of N in stream water, and (3) Archer creek stream water NO3- loss is supplied by flushing of shallow soil NO3-.

The hydrologic controls of N cycling were evaluated from January 1, 1995 to December 31, 1996 in the Arbutus Lake watershed in the central Adirondack Mountains of New York State, USA.  At the Arbutus Lake outlet dissolved organic nitrogen (DON), nitrate (NO3-) and ammonium (NH4+) contributed 61%, 33%, and 6% respectively, to the total dissolved N (TDN) flux (259 mol ha-1 yr-1).  At the lake inlet DON, NO3-, and NH4+ constituted 36%, 61%, and 3% respectively, of TDN flux (349 mol ha-1 yr-1).  Dissolved organic N concentrations were positively related to discharge during both the dormant (R2=0.31; P<0.01) and growing season (R2=0.09; P<0.01).  There was no significant relationship between NO3- concentration and discharge during the dormant season and a negative relationship was observed during the growing season (R2=0.29; P<0.01).  Surface water shifted from a NO3- dominated system to a DON dominated system in Arbutus Lake, emphasizing the need to quantify within lake processes.

Changes in subsurface and in-stream (surface) N water chemistry were measured through two riparian wetlands within the Archer Creek watershed (that contains the main inlet to Arbutus Lake) from March 1 – July 31, 1996.  Changes in the chemistry of surface water through the wetlands were less than changes in subsurface water chemistry through the wetlands.  The change in subsurface N concentration from the wetland perimeter to piezometer nests within the wetland were 55.4, 1.0, and 13.0 mmol 1-1 for NH4+, NO3-, and DON, respectively.  The differences in stream water NH4+, NO3-, and DON concentrations between the inlet to the wetlands and the wetland outlet were -0.1, -18.0, and 8.5 mmol 1-1, respectively (negative changes in concentration represent a loss through the wetlands).  Hydrologic cross-sections demonstrated that little or no wetland groundwater contributed to stream flow during the study period.  Within-stream N transformations through the wetlands had a greater impact on stream water N chemistry than groundwater N transformations because in-stream changes affected a greater volume of water and had an immediate impact on stream water chemistry.  

Nitrate transport within Archer Creek Watershed was investigated to determine how seasonal patterns and antecedent moisture affected NO3- release.  Flushing and drainage of NO3- from watershed soils and groundwater were quantified using isotopic, chemical, and hydrometric techniques.  Antecedent moisture conditions and season had little affect on mean soil water NO3- concentrations before storms, which ranged from 1.1 – 5.1 mmol 1-1.  Results from isotopic hydrograph separations and end-member mixing analysis suggest that stream flow was dominated by soil water and till groundwater during baseflow and for six storms.  Mean stream, soil water, near-stream groundwater, throughfall, and till groundwater NO3- concentrations were 20.1, 2.2, 0.6, 15.1, and 44.0 mmol 1-1, respectively.  Therefore, of potential stream water sources, only till groundwater had NO3- concentrations high enough to account for stream water NO3- concentrations.  A conceptual model of stream flow generation and watershed NO3- release is presented in which hillslope hollows are viewed as principal zones of soil- and groundwater mixing in the watershed with till groundwater serving as the main source of stream water NO3- during both baseflow and storms.  

This research has provided a better link between hillslope N cycling and stream water N chemistry.  It has shown that the dominant N species in stream water is dependent upon season and landscape position, the impact of wetlands on subsurface chemistry may be great, but may not have a significant impact on stream water chemistry, and flushing of NO3- from shallow soil layers is probably not an important mechanism for NO3- release from watersheds with a moderate stand age and moderate levels of N deposition.