Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/15/1993
Publication Date: N/A
Citation: Interpretive Summary: In recent years, increased public concern about water quality has heightened interest in the efficient use of nitrogen (fertilizer) for agricultural production systems. Knowledge of spatial structures and other factors determining residual nitrate distributions in the soil profile under different tillage practices is useful to understand its transport, transformation, and retention pattern. We developed a three- dimensional median-polishing scheme to smooth the measured soil nitrate- nitrogen data affected by directional trends. The cleaned data were subsequently used for determining any effect of soil-tillage on the spatial distribution of residual nitrate-nitrogen. Our study showed better spatial correlation under conventional tillage practice than under no-tillage practice. This will help researchers and agricultrual engineers developing new and improved tillage-based nitrogen management practices to enhance soil and groundwater quality.
Technical Abstract: Soil nitrate-nitrogen (NO3-N) data arranged on a three-dimensional grid were analyzed to compare tillage effect (conventional vs. no till) 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 outlier(s)) approach was developed to polish the spatial located data on soil NO3-N affected by directional trends (nonstationarity in the mean) in three major directions (row, column, and depth) and along the horizontal diagonal directions of the grid. Effect of preferential or nonpreferential path of transport of NO3-N in the vertical direction defined as sample hole effect was also removed to make the data trend-free across holes. In the conventional tillage plot, residual NO3-N concentrations decreased gradually to a depth of 90 cm and increased beyond this depth. The coefficient of variation, however, 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 across 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 semivariograms for no-tillage showed nugget-type behavior, indicating a lack of spatial structure in the residual soil nitrate.