Water Quality in Walnut Creek Watershed: Nitrate-Nitrogen in Soils, Subsurface Drainage Water, and Shallow Groundwater

Nonpoint source contamination of surface and groundwater resources with nitrate-N (NO3-N) has been linked to agriculture across the midwestern USA. A 4-yr study was conducted to assess the extent of NO3-N leaching in a central Iowa field. Water flow rate was monitored continuously and data were stored on an internal datalogger. Water samples for chemical analysis were collected weekly provided there was sufficient flow. Twelve soil cores were collected in spring, early summer, mid-summer, and after harvest for each of the 4 yr. Nitrate-N concentrations in shallow groundwater exhibited temporal trends and were higher under Clarion soil than under Okoboji or Canisteo soil. Denitrification rates were two times higher in Okoboji surface soil than in Clarion surface soil and the highest denitrificafion potential among subsurface sediments was observed for deep unoxidized loess. Soil profile NO~-N concentrations decreased with depth and were the same below 30 cm for fertilized corn (Zea mays L.) and soybean (Glycine max L. Merr.). Nitrate-N concentrations in subsurface drainage water exceeded 10 mg L-1 for 12 mo and were between 6 and 9 mg L-1 for 32 mo during the 4-yr study. The temporal pattern of NO3-N concentrations in subsurface drainage water was not related to the timing of fertilizer N application or the amount of fertilizer N applied. Total NO~-N losses to subsurface drains were greatest in 1993 (51.3 kg ha-1) and least in 1994 (4.9 kg ha-l). Most of the subsurface drainage water NO~-N was lost when crop plants were not present (November-May), except in 1993. Our results indicate that NO~-N losses to subsurface drainage water occur primarily as a result of asynchronous production and uptake of NO~-N in the soil and the presence of large quantifies of potentially mineralizable N in the soil organic matter. N SOURCE contamination of surfaceand groundwater with NO3-N has been linked to agricultural production in the midwestern USA. This is especially true for surface waters in the upper Midwest due to extensive subsurface draining of the highly productive but poorly drained soils found in this region (Gast et al., 1978). However, the extent to which agriculture contributes to water-quality deterioration is not fully known. In some geographic regions, surface-water NO3-N concentrations in excess of the 10 mg L-1 drinking water standard frequently have been reported (Hallberg, 1986). Keeney and DeLuca (1993) found that NO3-N concentrations in Des Moines river water in central Iowa were above 10 mg L-1 for an average of 14 d per year, generally in the spring. Subsurface drainage water NO3-N concentrations exhibit yearly and seasonal variability (Kladivko et al., 1991). Nitrogen flux to subsurface drains appears to be primarilly a function of precipitation amounts and distribution, and is only slightly affected by crop N upUSDA-ARS, National Soil Tilth Lab., 2150 Pammel Dr., Ames, IA 50011-4420. Received 23 Jan. 1998. *Corresponding author (cindyc@ nstl.gov). Published in J. Environ. Qual. 28:25-34 (1999). take patterns. The concentration of NO3-N in subsurface drainage water is especially dependent on growing season precipitation through the effect of precipitation on subsurface drain flux (Jaynes et al., 1999). When the soil is dry and precipitation is low, infiltration and percolation are negligible and plant uptake is limited. However, mineralization of soil organic N continues to occur and results in an accumulation of inorganic N. When precipitation finally exceeds evapotranspiration, NO3-N concentrations in subsurface drain water can increase very rapidly and result in large NO~-N loading losses to the subsurface drains, which subsequently empty into surface waters. Randall and Iragavarapu (1995) report annual subsurface drain NO~-N loads ranging from 1.4 to 139 kg ha-1, with the lower loading losses occurring during years with less than average growing season precipitation. Annual losses of NO3-N in subsurface drainage water from corn fields ranged between 11 and 107 kg N ha-1 in four different tillage systems in Iowa (Bjorneberg et al., 1996). A large portion of the N loss occurred prior to fertilizer application in early April. Subsurface drainage water NO~-N concentrations, averaged over the growing season, were consistently above 10 mg L-I, and were greater from corn than from soybean. Patni et al. (1996) reported to 30 kg ha-1 loss of NO3-N through subsurface tile drains in fields cropped to corn in southern Ontario. Nitrate-N has been detected in groundwater at levels ranging from 0.17 to 36 mg L-1 in the Midwest Corn Belt (Hallberg, 1986; Burkart and Kolpin, 1993; Spalding and Exner, 1993). Excess concentrations of NO3-N (>3 L-1) are more frequently found in unconsolidated aquifers than in bedrock aquifers (Burkart and Kolpin, 1993). In Iowa, the disitribution of NO~-N contamination in groundwater is not uniform. Contamination occurs most frequently in southwest, northwest, and southcentral portions of the state. North central and central Iowa have the lowest incidence of contamination (Spalding and Exner, 1993). One possible explanation for these differences is higher rates of denitrification in the poorly drained central Iowa soils. In addition, significant amounts of NO~-N are being intercepted by subsurface drains and discharged to surface waters (Power and Schepers, 1989). Our experimental objectives were: (i) to quantify NO~-N concentrations and loadings in subsurface drain water, shallow groundwater beneath the subsurface drains, and in the soil profile over a 4-yr time period, (ii) to quantify denitrification rates in surface soils and subsurface sediments, and (iii) to assess the effect Abbreviations: ANOVA, analysis of variance; C, carbon; DOY, day of year; MCL, maximum contaminant level; N, nitrogen; UAN, urea ammonium nitrate.