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Temporal and Spatial Patterns of Nitrate in a Claypan Soil

Dep. of Biological and Agricultural Engineering, Univ. of Missouri, Columbia, MO 65211;

E. E. Alberts

USDA-ARS, Cropping Systems and Water Quality Research Unit, Columbia, MO 65211.

^* Corresponding author (ghideyf{at}missouri.edu).

ABSTRACT

The temporal and spatial patterns of NO₃-N were studied on a 35-ha field located in the claypan soil region of north-central Missouri. Soil samples were collected from the 0- to 5-, 5- to 10-, and 10- to 15-cm depths and were analyzed for NO₃-N concentrations. Surface water samples from the field were collected for NO₃-N and NH₄-N analysis during each surface runoff event. Groundwater samples were also taken from the field wells four times a year for 5 yr and analyzed for NO₃-N concentrations. The effects of topography, depth to claypan, soil pH, organic matter (OM) content, cation exchange capacity (CEC), and soil water content on the spatial distribution of NO₃-N concentration were also evaluated. Nitrate-N concentration in the 0- to 5-cm soil depth increased in the first few weeks following application, then decreased rapidly and was very low at harvest. During the study period, nitrate movement below the layer of fertilizer application was very low, and <5% of the total N applied in the soil was lost to surface runoff. Nitrate-N concentration in groundwater samples decreased by an average of 0.40 mg L^–1 yr^–1 from 1992 to 1996. The semivariograms did not exhibit strong spatial dependency except for the samples collected 1 and 4 wk after fertilizer applications in 1993 and 1995, respectively. Nitrate-N concentration was poorly correlated to soil water content and depth to claypan and relatively strongly correlated to elevation and soil pH.