Initiation of Irrigation Effects on Temporal Nitrate Leaching

it more susceptible to leaching (Albus and Knighton, 1998). Higher moisture contents will also raise microbial Groundwater and surface water are significant resources for rural activity including mineralization (Skopp et al., 1990). water supplies, and certain agricultural practices may have substantial effects on these resources. An 11-yr study was started in 1989 near The increase in mineralization rates has been shown to Oakes, ND that continuously monitored NO3–N concentrations in directly affect nutrient leaching (Beck, 1983; Doran, subsurface water of a field that was converted from dryland to center1980). Albus and Knighton (1998) found that the initiapivot irrigation in 1989. The vadose zone was monitored with four tion of irrigation caused a flush of NO3–N to the shallow disturbed and 16 undisturbed-profile lysimeters, and the groundwater groundwater, which raised concentrations to 40 mg L 1 of the surficial aquifer was monitored with 18 sets of nested wells, from background levels. The background concentrawhich sampled shallow, intermediate, and deep depths. The depth to tions were near the USEA’s drinking water Maximum water table of the surficial aquifer was approximately 3 m and the Contamination Level (MCL) of 10 mg L 1. Albus and saturated thickness extended to a depth of 7 m. Also, NO3–N levels Knighton (1998) found that the groundwater NO3–N from two subsurface drains were monitored. The time series NO3–N concentration remained elevated for nearly 4 yr. concentration data from each of the monitoring locations exhibited the similar three-phase trend where NO3–N concentrations first increased, In addition to groundwater degradation by NO3–N, then decreased, and finally reached a steady-state level that was mainthere is a risk of contamination to surface waters. Nitained. The first and second phases of this trend were shorter ( 3 yr trate-contaminated water from subsurface drained watotal) for the lysimeters and increased as the depth of observation tersheds is the primary source of NO3–N loading to increased (5 and 8 yr total for shallow and intermediate wells, respecsurface water within the Midwest (David et al., 1997). tively). Also, the peak NO3–N concentration decreased as the observaThe excessive NO3–N levels are not only costly to treat tion went deeper into the profile (ranging from 150 mg L 1 in lysimefor human consumption, but they have been implicated ters, to 50 mg L 1 in shallow wells, and to 40 mg L 1 in intermediate in the formation of a hypoxic zone in the Gulf of Mexico wells). The NO3–N levels in the deep wells averaged 0.48 mg L 1, (Rabalais et al., 1996). had a maximum of 1.59 mg L 1, and exhibited a slight increase through Although there has been research on corn (Zea mays time. The subsurface drainage NO3–N levels were an average of 77% lower than the groundwater concentrations, which may have been L.) yield and N fertilizer application rates for nearly caused by biotic and abiotic reduction. The increase in NO3–N concenhalf a century (Krantz and Chandler, 1954), there has trations in subsurface waters as a result of the initiation of irrigation been much less focus on the relationship between mancan be partially explained by the residual N in the soil from dryland agement practices and NO3–N leaching (Angle et al., agriculture. As soil moisture increased, the availability and mobility 1993; Rasse et al., 1999). By managing N fertilizer, Baker of nitrogen increased, which attributed to the flush of NO3–N through and Johnson (1981) found that increasing the rate from the soil profile. 100 to 250 kg ha 1 on corn grown in rotation with either soybean [Glycine max (L.) Merr.] or oat (Avena sativa L.) would double the NO3–N concentration in subsurO hundred thirty-five million people in the face drainage from 20 to 40 mg L 1. Albus and KnighUSA, including 99% of its rural population, rely on ton (1998) showed that through proper N management, groundwater for their drinking water (USEPA, 2000). high N concentrations in shallow groundwater can be It is necessary to understand the impact of irrigation on significantly lowered. Steele et al. (2000) also demongroundwater quality in rural areas where irrigated agristrated that irrigation water management can be used culture is significant. The initiation of irrigation may reto optimize corn yield, which can decrease the amount sult in larger quantities of water moving through the soil of NO3–N leached by improving N uptake by corn. Niprofile. Concerns about subsurface water quality grow trogen management practices can be developed to minias more water moves through the soil profile because mize adverse impacts on surface and subsurface water, advective transport of pollutants (e.g., agricultural chemiwhile maintaining sufficient yields. cals) may also increase. Applying excess nutrient fertilizThe objective of this research was to quantify trends ers will directly affect subsurface water quality espein NO3–N concentration in the root-zone, groundwater, cially for NO3–N, which is highly mobile. and subsurface drain flow leaving a field after initiation The increase in soil moisture that results from the of sprinkler-irrigated agriculture. Another objective of initiation of irrigation dissolves excess NO3–N present this research was to observe the effects of best managein the soil profile from dryland agriculture and makes ment practices on the subsurface water quality. This research was part of a field-scale study of best manageF.X.M. Casey and N.E. Derby, Dep. of Soil Science; D.D. Steele ment practices associated with N fertilizer management and E.C. Stegman, Dep. of Agriculture and Biosystems Engineering, on irrigable soils representative of the Garrison DiverNorth Dakota State University, Fargo, ND 58105; R.E. Knighton, sion Unit in southeastern North Dakota, USA. Several USDA-CSREES-Natural Resources and Environment, 1400 Independence Ave., SWSTOP 2210, Washington, DC 20250-2210. Received Abbreviations: BMP, Best Management Practices [site]; MCL, maxi25 Feb. 2002. *Corresponding author ([email protected]). mum contaminant level; MSEA, Management Systems Evaluation Area; OITA, Oakes Irrigation Test Area. Published in Vadose Zone Journal 1:300–309 (2002).