By investigating multiple hypotheses, we attribute the recurrent summer peaks in stream NO3- in Pond Branch primarily to changes in net nitrification-denitrification dynamics, in the near-stream zone, though a lack of uptake by vernal riparian vegetation and in- stream biota during the peak of the growing season could further enhance this
seasonality. The geomorphic template and presence of pronounced microtopography of the riparian zone at this site are important controls on the spatial distribution of redox conditions. The physical structure of the near-stream zone determines the spatial extent (and volume) of hydrologically connected portions of the landscape, in which redox sensitive processes can control stream N concentrations. This hydrologically connected and variable redox volume of the near-stream zone evolves over different hydroclimatic conditions. The N source zone will be largest during drier summer conditions when denitrification is limited by high soil oxygen concentrations. The N source zone is also affected by temperature and moisture controls on microbially mediated N
transformations. In terms of transport capacity, mid-summer is typically coincident with largest ET flux, resulting in the largest diel fluctuations in water table. These periods thus have the highest potential to both produce NO3- and transport it from the near- stream source volume through the stream network. Diel transport facilitated by groundwater fluctuations could also transport recently produced NO3- from riparian hollows and stream segments that have been hydrologically disconnected on the surface. Therefore, even as portions of the stream and near-stream environment would be
matter could still serve as a biogeochemical source area through which subsurface transport still occurs.
In terms the dissertation, this Chapter shows that nearly the entire stream N export originates from the riparian zone.
Acknowledgements:
We thank Wil Wollheim and Gopal Mulukutla for help on programming the SUNA sensor and Scott Morford for analyzing rock samples. Thanks to Dan Dillon and many others for field assistance. This work was funded by National Science Foundation grants DEB-1027188 (Baltimore LTER) and DEB-0919047 (Ecosystem Studies).
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Table 3.1. Mean Monthly Discharge at Pond Branch through the growing season
Pond Branch Mean Monthly Discharge (mm)
April May June July August September
2010 2.49 2.08 1.15 0.93 0.71 0.91 2011 0.90 0.68 0.33 0.31 0.77 2.19 Long-‐term (1998-‐2012) mean 1.36 1.15 0.89 0.68 0.63 0.73
Pond Branch Total Monthly Rainfall (mm)
2010 April 70.10 May 63.12 June 22.86 July 72.39 August 77.34 September 148.59
2011 107.82 81.66 28.32 140.21 213.11 237.62
Long-‐term
mean* 76.20 98.81 87.12 97.79 95.00 101.09
Figure 3.1. Conceptual model of summer season nitrate sources. 1_Geochemical weathering of bedrock and saprolite. 2) Delayed transmission of winter precipitation to the stream via fracture flow and groundwater seeps. 3) Senescence of riparian vegetation. 4) Reductions in in-stream uptake. 5) Riparian soils
Figure 3.2. LiDAR derived shaded relief image of the Pond Branch watershed, a 37 ha forested subwatershed in Baltimore County, USA. Sensor locations for O2, TDR probes and riparian groundwater wells are shown.
In Gasline POBR Outlet Lower Transect Upper Transect Upstream Gasline Groundwater Spring Downstream Gasline
Stream Sampling Points Sensor Transects
99
Figure 3.3. Riparian topography and active streams channels in the Pond Branch bottomland. The riparian zone is broader in the upper well transect (upper) and narrower at the lower riparian transect (lower panel) where valley slopes are steeper. Microtopographic variation is pronounced throughout the entire riparian zone.
Figure 3.4. Bi-weekly NO3 concentrations generated from weekly BES grab samples from 1999-2010 reveal recurrent summer peaks with increasing concentrations in late May to early June and declines in September and October. Loads increase over the same period during the growing season.
Figure 3.5. Longitudinal stream samples show increasing loads in total dissolved nitrate, nitrate, and ammonium as upslope accumulating area increases from a large groundwater seep Sampling locations shown in Figure 2.
● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● 0.0 0.1 0.2 0.3 0 200 400 600
Distance from Seep to Outlet (m)
2011 Nitr ate (mg/L) Date ● ● ● ● ● ● ● Oct12 Sep14 Sep2 Aug18 Jul17 Jun7 Apr15 ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●● ●● ●● ●● ●● ● ● 0.0 0.1 0.2 0.3 0 200 400 600
Distance from Seep to Outlet (m)
2011 Ammonium (mg/L) Date ● ● ● ● ● ● ● Oct12 Sep14 Sep2 Aug8 Jul17 Jun7 Apr15
Figure 3.6. Nitrate concentrations collected at a groundwater seep and the outlet for all samples as a function of Julian Day (a), the concentrations at the outlet minus
concentrations at the seep (b), and boxplots for all samples (c) and for samples collected during the summer (d).
Figure 3.7. Stream nitrate and stage measurements at 15-minute resolution over a 4 day period at baseflow fitted with low order trends, and b. residuals of discharge and nitrate from the low order trends.
Figure 3.8. Nitrate concentrations versus day of year in 2010- 2011 in tension lysimeters in riparian and upland locations in the Pond Branch watershed.
0 1 2 3 0 100 200 300 Julian Day N itra te (mg /L ) Location Riparian Uplands
Figure 3.9. Average daily soil oxygen concentrations measured in toeslope, riparian hummock, and riparian hollow landscape positions Dec 18, 2011 – Oct 11, 2012 at the lower riparian transect in the Pond Branch watershed.
10 15 20
Jan Apr Jul Oct
Date Toeslope Soil Oxygen(%) 12.5 15.0 17.5 20.0
Jan Apr Jul Oct
Date Hummock Soil Oxygen(%) 0 1 2 3
Jan Apr Jul Oct
Date
Hollo
w
Figure 3.10. Soil oxygen concentrations measured hourly in toeslope, riparian hummock, and riparian hollow landscape positions from June 18 – July 8, 2011 at the lower (a) and upper (b) riparian transects in the Pond Branch watershed.
(a)
Figure 3.11. A conceptual model of how seasonal and diel (transpiration-induced) water table fluctuations provide the biogeochemical conditions to produce and transport nitrate from the riparian zone to the stream during the growing season. As net production of nitrate proceeds, there is a greater potential for a portion to be transported to/through the stream network. The area of redox variability changes in response to wetter (winter) and drier (summer, day time, extended drought) conditions as hydrologically connected areas decrease. Complete N cycling processes include: 1. N fixation, 2. Mineralization, 3. Nitrification, 4. Desorption, 5. Absorption, 6. Uptake of ammonium (NH4+), 7. leaching and exudation of DON, 8. Sorption and assimilation of organic molecules, 9. Biotic assimilation of NO3-, 10. Leaching and desorption of NO3-, 11.anaerobic oxidation of ammonium (annamox), and 12. Dissimilatory nitrate reduction to ammonium (DNRA). Processes 10-12 have been shown elsewhere, but are not thought to be important in Pond Branch (see Duncan et al., 2013).
CHAPTER 4. RIPARIAN ECOHYDROLOGIC CONTROLS ON PATTERNS OF