SPATIAL VARIABILITY OF RESIDUAL NITRATE-NITROGEN UNDER 2 TILLAGE SYSTEMS IN CENTRAL IOWA - A COMPOSITE 3-DIMENSIONAL RESISTANT AND EXPLORATORY APPROACH

Soil nitrate-nitrogen (NO3-N) data arranged on a three-dimensional gri d were analyzed to compare tillage effect on the spatial distribution of residual NO3-N in the soil profile of agricultural plots drained by subsurface tiles. A three-dimensional median-based resistant (to outl ier(s)) approach was developed to polish the spatially located data on soil NO3-N affected by directional trends (nonstationarity in the mea n) in three major directions (row, column, and depth) and along the ho rizontal diagonal directions of the grid. Effect of preferential or no npreferential path of transport of NO3-N in the vertical direction def ined as sample hole effect was also removed to make the data trend-fre e across holes. Composite three-dimensional semivariogram models (alon g horizontal and vertical directions) were used to describe the spatia l structure of residual soil nitrate distribution. Two plots in the sa me field, one under each tillage system (conventional tillage and no t illage), were studied. In each plot, soil samples were collected at fi ve depths (30, 60, 90, 120, and 150 cm) from 35 sites (holes) arranged on a 7 x 5 regular grid of 7.6 x 7.6 m. In the conventional tillage p lot, residual NO3-N concentrations decreased gradually to a depth of 9 0 cm and increased beyond this depth. The coefficient of variation, ho wever, became gradually smaller, showing more uniform distribution for greater depths. In the no-tillage plot, trends were similar to those in the conventional tillage system, but were spatially more stable acr oss the profile. Structural analyses indicated that under conventional tillage, the semivariogram of residual soil nitrate distribution was linear in the horizontal and vertical directions. In contrast, the sem ivariograms for no-tillage showed nugget-type behavior, indicating a l ack of spatial structure in the residual soil nitrate.