Temporal and spatial patterns of nitrate in a claypan soil

The temporal and spatial patterns of NO3-N were studied on a 35-ha field lo cated in the claypan soil region of north-central Missouri. Soil samples we re 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 we re collected for NO3-N and NH4-N analysis during each surface runoff event. Groundwater samples mere 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 ca pacity (CEC), and soil water content on the spatial distribution of NO3-N c oncentration were also evaluated. Nitrate-N concentration in the 0- to 5-cm soil depth increased in the first few weeks following application, then de creased rapidly and was very low at harvest. During the study period, nitra te 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 co ncentration in groundwater samples decreased by an average of 0.40 mg L-1 y r(-1) from 1992 to 1996, The semivariograms did not exhibit strong spatial dependency except for the samples collected 1 and 4 wk after fertilizer app lications in 1993 and 1995, respectively. Nitrate-N concentration was poorl y correlated to soil water content and depth to claypan and relatively stro ngly correlated to elevation and soil pH.