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

Abstract

The main physical and chemical controls on nitrogen (N) fluxes between the root zone and the water table were determined for agricultural sites in California, Indiana, Maryland, Nebraska, and Washington from 2004 to 2005. Sites included irrigated and nonirrigated fields; soil textures ranging from clay to sand; crops including corn, soybeans, almonds, and pasture; and unsaturated zone thicknesses ranging from 1 to 22 m. Chemical analyses of water from lysimeters and shallow wells indicate that advective transport of nitrate is the dominant process affecting the flux of N below the root zone. Vertical profiles of (i) nitrogen species, (ii) stable isotopes of nitrogen and oxygen, and (iii) oxygen, N, and argon in unsaturated zone air and correlations between N and other agricultural chemicals indicate that reactions do not greatly affect N concentrations between the root zone and the capillary fringe. As a result, physical factors, such as N application rate, water inputs, and evapotranspiration, control the differences in concentrations among the sites. Concentrations of N in shallow lysimeters exhibit seasonal variation, whereas concentrations in lysimeters deeper than a few meters are relatively stable. Based on concentration and recharge estimates, fluxes of N through the deep unsaturated zone range from 7 to 99 kg ha(-1) yr(-1). Vertical fluxes of N in ground water are lower due to spatial and historical changes in N inputs. High N fluxes are associated with coarse sediments and high N application rates.

Real-time monitoring of nitrate transport in the deep vadose zone under a crop field – implications for groundwater protection

“…Here, as well as in previous studies in the literature, nitrate fluxes in the unsaturated zone underlying agricultural land uses were associated with nitrogen application rates and soil physical properties (Green et al, 2008;Botros et al, 2012;Turkeltaub et al, 2015b). Therefore, to attenuate nitrate leaching to aquifers, a search should be dedicated to locate the "hot spots" where these conditions prevailed (Liao et al, 2012).…”

“…These factors reduce the potential for transformation processes and plant uptake to occur (Liao et al, 2012). Moreover, various studies conducted under similar conditions (soil types and agriculture land use) to in the current study presented insignificant nitrogen transformation processes and doubt the ability to attenuate nitrate within the deeper vadose zone (Green et al, 2008;Burow et al, 2010;Gautam and Iqbal, 2010;Dann et al, 2013;Zhang et al, 2014;Turkeltaub et al, 2015b). However, other studies sug- gested contrasting conclusions.…”

“…Knowledge of nitrate's fate and transport below the root zone is restricted due to issues such as soil spatial variability and long travel times in the deep vadose zone (Onsoy et al, 2005). Moreover, estimates of cumulative nitrate fluxes in the unsaturated zone have shown significant differences in the timing and magnitude of fluxes derived from different land uses (Green et al, 2008;Turkeltaub et al, 2014Turkeltaub et al, , 2015b. Our understanding of the cumulative effect of nitrate leaching from the root zone through the unsaturated zone on nitrate levels in the groundwater is blurred by mixing and dilution in the aquifer water.…”

“…Our understanding of the cumulative effect of nitrate leaching from the root zone through the unsaturated zone on nitrate levels in the groundwater is blurred by mixing and dilution in the aquifer water. The tendency toward elevated nitrate concentration in aquifer water is thus a relatively slow process (Green et al, 2008). Knowing the time lag between initiation of a pollution process in the unsaturated zone and its final effect on aquifer quality could give decision-makers more time to plan possible backups for alternative water supply (Baram et al, 2014).…”

Using environmental tracers to determine the relative importance of travel times in the unsaturated and saturated zones for the delay of nitrate reduction measures

“…Baillieux et al, 2014). Thus, in most cases, travel times of groundwater, as well as these additional processes affecting nitrate are determined by means of hydrogeological modeling (2D or 3D) and particle tracking, environmental tracers, or a combination of both (Alikhani et al, 2016;Green et al, 2008;Jeffrey Starn et al, 2014;Kaown et al, 2009;MacDonald et al, 2003;McMahon et al, 2006;Osenbrück et al, 2006;Visser et al, 2009;Wang et al, 2012;Zoellmann et al, 2001). Environmental tracers are a complementary tool to hydrogeological models and time series of nitrate or other tracers for determining travel time distributions (or age distributions) of groundwater and dilution (Turnadge and Smerdon, 2014;Visser et al, 2009 Ar,14 C,or 4 He are commonly interpreted with lumped-parameter models (LPM) to obtain travel time distributions: The concentrations of the tracers in a well are calculated as the convolution integral of the input history of the tracers and a relatively simple shape of the age distribution (Zuber and Maloszewski, 2001).…”