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The Ecosystems Center, Marine Biological Laboratory, Woods Hole, Massachusetts 02543
Land uses, such as agriculture and residential development, have greatly influenced the amount of nitrogen (N) transported from coastal watersheds to receiving estuaries. This is a concern to ecologists and management groups in coastal regions such as Cape Cod, Martha’s Vineyard, and Nantucket, where precipitation percolates rapidly through sandy glacial sediments in the vadose zone (unsaturated layer between the soil and aquifer), causing rapid vertical transport of N to the aquifer (saturated layer) and horizontal movement of N to coastal waters (1,2). In many forested watersheds, ammonium (NH4+) and nitrate (NO3-) are transported to receiving estuaries in low amounts relative to dissolved organic N (DON), which includes organic acids and other compounds (3,4). However, in human-altered systems, high amounts of inorganic N, particularly in the form of NO3-, are often transported to aquatic systems, elevating primary production (5). To make management decisions for coastal areas with high anthropogenic N inputs, it is important to study systems in which human influences are minimal so that background N transformations can be identified.
Our goal in this study was to quantify N concentrations and to identify N transformations in groundwater moving along a known flow path in a forested system with a known land-use history, minimal septic inputs, and no overland flow. We measured the relative concentrations of dissolved N species (NH4+, NO3-, and DON) in throughfall, soil solution in the vadose (unsaturated) zone, and groundwater from an oak forest on Job’s Neck peninsula in Edgartown, Massachusetts. We also measured N concentrations at the seepage face of the Edgartown Great Pond estuary which lies roughly 500–1000 m downgradient in the groundwater flowpath from the forest.
We collected throughfall, and water from the vadose zone, aquifer, and seepage face from June 2000 to August 2001 and analyzed samples for NH4+, NO3-, and DON concentrations. We used spatially extensive sampling to capture fine-scale differences in vegetation and topography: 60 throughfall collection units and 50 zero-tension lysimeters installed at 40-cm depth in a stratified random pattern throughout the forest, 40 iron piezometers installed to the water table along the groundwater flow paths, and 34 points for shallow groundwater discharge sampling at the seepage face of Edgartown Great Pond. All water samples were filtered with ashed (2 h at 550 °C) Whatman GF/F filters and frozen in 60-ml polyethylene bottles until analyzed colorimetrically for NH4+, NO3-, and TDN concentrations (TDN was analyzed by persulfate digestion). DON concentrations were calculated by subtracting NH4+ + NO3- from TDN concentrations of each sample. We used a one-way analysis of variance and a Tukey’s post-hoc test to determine statistical differences between means (at 0.05 level of significance). All statistical analyses were performed using SYSTAT (SPSS Inc., 1997, Version 7.0).
TDN increased significantly (P < 0.001) from 31.44 ± 2.71 µM in throughfall to 54.08 ± 3.21 µM in the aquifer (Fig. 1A). DON was the principal component of dissolved N in the vadose zone, aquifer, and at the seepage face (Table 1). These data are consistent with other studies that show dominance of DON in soil solution and groundwater of forested watersheds (3,4). DON increased significantly (P < 0.001) from 9.15 ± 0.76 µM in throughfall inputs to 46.63 ± 2.96 µM in the aquifer (Fig. 1B). Most DON consists of organic acids and other compounds that originate in the upper layers of the forest floor and move to groundwater during periods of heavy precipitation (4,6). There was no significant difference between DON concentrations in the aquifer and at the seepage face, suggesting that further removal or accumulation of DON may not occur as groundwater moves horizontally to receiving waters.
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NO3- decreased significantly (P < 0.001) from 10.33 ± 1.16 µM in throughfall to 0.99 ± 0.08 µM in the aquifer (Fig. 1D). NO3- was about 2% of TDN in the aquifer (Table 1), indicating that very little NO3- moves from the plant-rooting zone to the aquifer. In the aquifer, concentration of NO3- was also lower than NH4+, which suggests low rates of nitrification along the flowpath from soil solution to the aquifer. This pattern is consistent with NH4+ and NO3- concentrations measured in soil solution and groundwater in Cape Cod coastal forests (2,6).
NO3- increased significantly (P < 0.001) from 0.99 ± 0.08 µM in the aquifer to 13.79 ± 5.26 µM at the seepage face (Fig. 1D). NO3- concentrations were highly variable but this overall pattern suggested that NO3- from additional sources was detected at some locations along the Edgartown Great Pond shoreline. There are several possible explanations for this result. Long-distance transport of NO3- from septic discharges farther inland are possible but, we feel, unlikely, given the relative hydrological isolation of Job’s Neck, the west-to-east groundwater movement under the forest, and our measurements of higher NO3- concentrations at the southern (coastal) end of the pond shoreline. It is also possible that increases in NO3- result from zones of oxidation of NH4+ or DON to NO3- within the seepage face, or from inputs of fixed N derived from the N-fixing shrub Myrica pensylvanica, which is present at many places along the pond shoreline.
From these findings, we conclude that: (1) relatively low NH4+ and NO3- and high DON are transported from the forest to the coastal pond, (2) incomplete retention of NH4+ above the aquifer and comparatively low NO3- concentrations in the aquifer suggest that nitrification rates are low in forest soils and in the aquifer, and (3) there is the possibility that in some places the seepage face may contribute a small amount of NO3- to discharging groundwater rather than remove it, because of NH4+ or DON oxidation or N inputs derived from N-fixing species. These findings can serve as a baseline for understanding how N transformations change with increasing human development and a shift toward a greater proportion of NO3- reaching the seepage face from the coastal aquifer.
This research was supported by the Mellon Foundation. We thank Tom Chase and Mike Dunphy of The Nature Conservancy and the Kohlberg family for allowing us to work on their property.
Footnotes
1 The Nature Conservancy, Plymouth, MA 02360. 
Literature Cited
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= Throughfall, 
= Vadose zone, 
= Aquifer, 
= Seepage face. Means represent the average concentrations of samples taken from June 2000–August 2001. Only samples for which all three N analyses were completed are included. Throughfall N = 72, Vadose zone N = 79, Aquifer N = 138, Seepage face N = 62. Bars are ± 1 SE and letters above bars indicate significant difference to the P < 0.001 level.