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Nitrogen Fluxes through Unsaturated Zones in Five Agricultural Settings across the United States

^a US Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025
^b US Geological Survey, 160 N Stephanie St., Henderson, NV 89074

View larger version (89K): [in this window] [in a new window]   Fig. 1. Aerial maps showing study locations and modeled recharge source locations (Green et al., 2008) for (A) California, (B) Indiana, (C) Maryland, (D) Nebraska, and (E) Washington. The Indiana site was not studied by Green et al. (2008), and particle tracking results of recharge source locations are not available.

View larger version (26K): [in this window] [in a new window]   Fig. 2. Distribution of concentrations of N species in samples from lysimeter sites and ground water sites. Gray boxes are shallow ground water samples. Black boxes are pore water samples collected from lysimeters. The points inside the boxes show the medians, the boxes show the interquartile ranges, and the whiskers show the total range. Locations C20, C21, N21, W20, W21, W23, and W24 have ground water wells with no lysimeters. All other locations have ground water wells and lysimeters.

View larger version (21K): [in this window] [in a new window]   Fig. 3. Relation between concentration enrichment and the fraction of applied water lost to evapotranspiration (ET) (from values in Table 1) for the agricultural profiles at California (CA), Indiana (IN), Maryland (MD), Nebraska (NE), and Washington (WA) and for the NE riparian zone site. Concentration enrichment is defined as Cobs/Capp, where Cobs is the observed unsaturated zone concentration based on the average of median NO3––N values from all lysimeters at each site and Capp is the applied concentration based on the total applied N divided by the total water inputs. Applied N rates were taken from local estimates (filled circles) and county estimates (empty circles) as indicated in Table 1. The curves show the expected increase of Cobs/Capp with increasing ET for various leached fractions of N (N leached below the root zone/N applied).

View larger version (21K): [in this window] [in a new window]   Fig. 4. Depth profiles of total N and NO3––N concentrations in the unsaturated zone and shallow ground water (deepest sample points) for each study site. For total N, bold lines connect the medians, boxes show interquartile range, and whiskers show the total range. For NO3––N, fine lines connect the medians, shown as points. The width between the two lines corresponds to the concentration of organic N + NH4+–N + NO2––N, which consists primarily of organic N at most locations.

View larger version (21K): [in this window] [in a new window]   Fig. 5. Time series of NO3––N concentrations (mg L–1) for shallow lysimeters (solid lines) and deep lysimeters near the water table (dashed lines) for (A) California, (B) Indiana, (C) Maryland, (D) Nebraska riparian zone, (E) Nebraska corn field, and (F) Washington. Shallow lysimeters are between 1 and 2 m below ground surface, except N22b, which is 7 m below ground surface. The bracket at the top of each plot marks the 2004 growing season for that site. For California, the brackets span the interval between the first and last irrigation events. For all other sites, the brackets show the time between planting and harvesting.

View larger version (8K): [in this window] [in a new window]   Fig. 6. Depth profiles of stable isotopes and gas concentrations in unsaturated zone air for the Nebraska site (N22). Error bars show two SD of replicate analyses. Nitrous oxide (N2O) values are given as aqueous concentrations.

View larger version (13K): [in this window] [in a new window]   Fig. 7. Correlation plot of (A) fraction of total N as NO3––N (NO3––N/total N) versus Fe and (B) total N versus Fe in unsaturated zone lysimeter samples.

View larger version (12K): [in this window] [in a new window]   Fig. 8. Cumulative NO3––N load to ground water at each study site based on individual recharge events using Eq. [2] (lines) and using Eq. [3] based on median concentrations (arrows), quartiles (boxes), and minimum and maximum concentrations (whiskers) of samples collected during 2004.

View larger version (18K): [in this window] [in a new window]   Fig. 9. Distribution of NO3––N fluxes (top of figure) and median NO3––N concentration and vertical water flux (bottom of figure) for selected unsaturated zone and ground water sites. For NO3––N, fluxes at each study site, the left box shows local unsaturated zone flux estimates based on median, quartile, and minimum and maximum concentrations of all samples collected from the deepest lysimeter (C22f, I32b, M22d, N22c, and W22l). Columns marked with "G.W." show the flux estimates based on reconstructed NO3––N source concentrations for ground water (Green et al., 2008) and vertical ground water flux estimates based on age profiles. Ground waters older than 25 yr since recharge are excluded. Rectangles at the bottom of the figure are sized according to the median NO3––N concentration for each site (width) and the recharge (height) based on the median vertical ground water fluxes and water table fluctuation estimates.