IMPLICATIONS OF TEMPORAL AND SPATIAL VARIABILITY IN NITRATE-N CONCENTRATIONS DERIVED FROM SOIL-WATER SAMPLERS AND SOIL CORES IN THE VADOSE ZONE OF SOUTH-CENTRAL KANSAS

Field studies in south-central Kansas have shown that nitrate concentrations (reported as nitrogen) in pore water and soil vary temporally and spatially. Temporal and spatial variability of soil nitrate affects the rate of nitrate movement within and below the root zone. Temporal and statistical analysis of core data indicates changes in nitrate concentration below the root zone are most strongly influenced by: 1) growing season fertilizer application rates, 2) nitrate concentrations below the root zone in the spring prior to fertilizer application, 3) nitrate concentrations in the root zone prior to fertilizer application, and 4) growing season precipitation rates.

However, nitrate extracted from cores measures the nitrate concentrations in the vadose zone at specific points in fields at specific times. Due to the temporal and spatial variability in nitrate concentrations derived from soil water samplers and field scale multi-core sampling, these values are probably not effective indicators of the field scale nitrate flux through the vadose zone. For example, concentrations of nitrate from soil-water samplers were generally higher than those determined from soil cores. In sandy soils, the movement of nitrate occurred rapidly in response to precipitation events and soil cores underestimated the amount of nitrate moving through the soil.

Spatial variability of nitrate was most strongly influenced by the variability of soil properties, vadose zone stratigraphy, and topography. The range of nitrate from 19 samples in the upper 75 cm of soil from a dry-land site with two distinct landforms was 25 to 68 kg/ha with a median value of 32 kg/ha. The range of nitrate-N in the upper 75 cm of soil from an irrigated site with less pronounced surface landforms was 13 to 58 kg/ha with a median value of 25 kg/ha. The range of concentrations at the two sites suggests that spatial variability of soil properties, stratigraphy, and topography need to be considered when attempting to determine nitrate flux through the vadose zone.