Tuesday, November 14, 2006
217-2

Natural Controls and Nitrogen Management Effects on Groundwater Nitrate Spatial and Temporal Patterns and Correlations.

Jeffrey G. White1, Nan Hong2, and Randy Weisz1. (1) North Carolina State Univ, Dept of Soil Science, Campus Box 7619, Raleigh, NC 27695-7619, (2) Univ of Missouri, Div of Plant Sciences, Columbia, MO 65211

To use NO3-N concentration (hereafter, simply "NO3-N") as an indicator of agricultural management effects on shallow groundwater quality requires understanding groundwater NO3-N controls, patterns, and correlations over spatial and temporal scales. Over a 2-yr period, we determined groundwater NO3-N patterns, correlations, and natural controls at various spatial and temporal scales and their associations with uniform and variable-rate N management. We established a 2-yr winter wheat (Triticum aestivum L.)-double-crop soybean [Glycine max (L.) Merr.]-corn (Zea mays L.) rotation in a North Carolina coastal plain field and in a randomized complete block design, applied one VR-N and two uniform-N treatments to wheat and corn. We measured groundwater NO3-N and water table depths every 2 wk at 60 well nests sampling 0.9 to 3.7-m depth. Field-mean groundwater NO3-N exhibited temporal patterns and correlations associated primarily with water table elevation and depth. Mean groundwater NO3-N exhibited two preferred states: high when the water table was shallow and low when the water table was deep. Temporal NO3-N fluctuations greatly exceeded treatment effects. Treatments appeared to affect NO3-N temporal covariance structure. Groundwater NO3-N exhibited distinct spatial patterns and correlations with spatial ranges of 0 to 551 m. Early N treatment effects were minor; groundwater NO3-N spatial patterns and correlations were associated mostly with saturated hydraulic conductivity and water table fluctuations and appeared influenced by subsurface lateral flow. When N treatments became consistently significant later in the study, they overrode natural controls, and NO3-N was spatially uncorrelated or exhibited shorter spatial correlation ranges and patterns associated predominantly with N management treatments.


Handout (.pdf format, 818.0 kb)