11A JAN/FEB 2014—VOL. 69, NO. 1 JOURNAL OF SOIL AND WATER CONSERVATION Jeannie R.B. Barlow and Robert Kröger Nitrogen transport within an agricultural landscape: Insights on how hydrology, biogeochemistry, and the landscape intersect to control the fate and transport of nitrogen in the Mississippi Delta doi:10.2489/jswc.69.1.11A FEATURE HYDROLOGY AND BIOGEOCHEMISTRY AS DETERMINANTS OF NITROGEN OCCURRENCE, TRANSPORT, AND FATE IN AQUATIC ENVIRONMENTS Nitrogen (N) is a ubiquitous contami- nant throughout agricultural landscapes due to both the application of inorganic and organic fertilizers to agricultural fields and the general persistence of nitrate (NO 3 ) in oxygenated aqueous environ- ments (Denver et al. 2010; Domagalski et al. 2008; Green et al. 2008; Coupe 2001; Nolan and Stoner 2000). In order to understand why excess N occurs vari- ous hydrologic systems (environments), it is important to consider potential sources, the locations of these sources in the water- shed, and the timing of the application of sources with respect to the movement of water. To learn how to manage N in a watershed, it is necessary to identify and quantify flow paths and biogeochemical conditions, which ultimately combine to determine transport and fate. If sources, transport mechanisms, and biogeochemi- cal controls were uniformly distributed, it would be possible to manage N uniformly throughout a watershed. However, uni- form conditions are rare to nonexistent in the natural world and in the landscape altered for agricultural production. In order to adjust management activities on the landscape to have the greatest effect, it is important to understand the fate and transport N within the intersection of hydrology and biogeochemistry, that is, to understand the extent and duration of the hydrologic and biogeochemical controls as N is routed through and among each hydrologic compartment. The US Geological Survey’s National Water Quality Assessment Program (NAWQA) recently completed an Agricultural Chemical Transport Study (ACT) in agricultural watersheds across the United States with the objective of gaining a better understanding of the fate and transport of agricultural chemi- cals (Capel et al. 2008). Findings from the NAWQA ACT studies have provided a holistic perspective on the transport and fate of agricultural chemicals, such as N, across diverse hydrologic settings. These results validate the need to consider all hydrologic compartments and their inter- actions in order to better manage the transport of agricultural chemicals (Essaid et al. 2008; Green et al. 2008; Puckett et al. 2008; Barlow and Coupe 2012). One of these NAWQA ACTS stud- ies, located in the northwestern region of Mississippi (referred to as the Delta), provided new insights into the linkage between water quality and water quan- tity, specifically with regards to how N is routed and processed. Nitrogen loads from the Mississippi River Basin have been connected to increases in eutrophi- cation and an expanding hypoxic zone in the northern Gulf of Mexico and are, therefore, an important issue within the Delta due to its proximity to and contri- bution of total N to the Gulf of Mexico (Rabalais et al. 1996; Rabalais and Turner 2001; Goolsby and Battaglin 2001; Alexander et al. 2008; Robertson et al. 2009). Additionally, water quantity in the Delta has been affected by groundwater withdrawals for irrigation. In the process of irrigation, surface water and ground- water flow paths are altered, which can affect hydrological and biogeochemical controls. Understanding such connec- tions can assist in the mitigation of water quality degradation associated with over- abundance of N from the agricultural landscape. This article provides an articu- lated statement about N cycling based on the holistic study of hydrologic systems within the Mississippi Delta agricultural landscape, highlighting how complex interactions within these systems can be viewed and subsequently managed. Hydrology Determines Flow Paths and Flow Paths Drive Chemical Transport. Hydrologic flow paths, or the cycle in which water is routed to and from the landscape, can be thought of as a series of interconnected hydrologic compart- ments (figure 1). All compartments are connected, and anything done on the landscape has the potential to affect all compartments. In a simplified conceptual model, a hydrologic system can be reduced to three primary compartments: surface water, groundwater, and the atmosphere. The intersection of these compartments represents potential flow paths for water and any other constituents, such as N (dis- solved or particulate), transported by water. The rate with which water moves through each compartment is dependent on the amount of resistance present at the com- partmental interface (Brooks et al. 2012). This synthesis focuses on the movement of water and N between the groundwater and surface water compartments to better understand the behavior and transport of N and to improve management strategies on the agricultural landscape. Interactions between groundwater and surface water compartments pro- vide many ecosystem services, such as maintaining baseflow in streams, regu- lating stream temperature regimes for aquatic biota, and buffering the transport of contaminants through the stream- bed interface (Hayashi and Rosenberry 2002; Hester and Gooseff 2010). The ability of groundwater and surface water interaction processes to provide these ecosystem services is dependent upon the hydrologic, physiochemical, and biogeochemical characteristics of each groundwater and surface water system and can be variable over spatial and tem- poral scales. For instance, streamflow alterations due to human influences have resulted in diminished high and low flow regimes in the majority of monitored streams across the United States and are linked to decreased biological integrity of aquatic ecosystems (Carlisle et al. 2010). Hydrologic modifications, such as dams, groundwater and surface water with- drawals, and irrigation return flow, have Jeannie R.B. Barlow is a hydrologist at the US Geological Survey, Mississippi Water Science Center, Jackson, Mississippi. Robert Kröger is an assistant professor in the Wildlife, Fisheries, and Aquaculture Department, Mississippi State University, Mississippi State, Mississippi. Copyright © 2014 Soil and Water Conservation Society. All rights reserved. www.swcs.org 69(1):11A-16A Journal of Soil and Water Conservation