Submitted to: Clean Water Clean Environment 21st Century Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 3/8/1995
Publication Date: N/A
Citation: Interpretive Summary: Significant amounts of nitrogen fertilizer are applied in the fall on the fine-textured soils of the northern Corn Belt. However, applying fertilizer to the soil six months in advance of when plants use the nitrogen allows more time for the chemical to be lost to the environment. Soil freezing and thawing are natural processes in the northern Corn Belt. Depending on soil conditions, the amount of plant residue left on the soil surface after harvest, and climatic factors, soils may be subjected to freezing temperatures to a depth of five feet. Previous research at Morris showed that tillage and residue management in the fall can affect the movement of water through the soil profile even though the soil is frozen. Therefore, it may be possible that chemicals are also moving through the soil during the winter. Nitrogen and atrazine herbicide were applied to plots before freeze-up. Soil samples were then taken throughout the winter months and tested for water, nitrogen and atrazine content. Although water moved up to the frozen soil layer, there was a net four inch downward movement of chemicals when soils thawed in the spring. These results show that tillage and residue management practices can be used to help mediate the net vertical movement of soil water and nitrogen during the winter period to prevent contamination of ground water supplies.
Technical Abstract: In the western Corn Belt, over 20% of the N fertilizer is applied in the fall, with southwestern Minnesota reported to be as high as 80%. Economics and timeliness factors are often cited as the dominant reasons. However, in these temperate climates, soil-applied nitrogen is also exposed to soil freezing. The effect of freezing on the fate of nitrogen, pesticides and water quality is not fully understood. Field studies were conducted on a clay loam soil in western Minnesota over three winters. Nitrogen (Urea) and Atrazine were surface broadcast in October of each year at the rates of 130 and 260 kg/ha, and 0 and 3.4 kg a.i./ha, respectively, and tillage incorporated. Soil was incrementally sampled to a depth of 1.5 m during the winter and analyzed for nitrogen and Atrazine concentration, and soil water content. Within the tilled layer, N moved upward from December to January, and downward from February until thaw, with a net downward movement of 0.12 m. Thus, even though most of the fall-applied nitrogen remained within the rooting zone and potentially available for plant uptake, it was also vulnerable for leaching during spring thaw. Snowmelt and rain often collected in surface depressions while the subsoil was still frozen. Subsequent rapid disappearance caused a temporary rise in the water table along with decreased concentrations of ammonium- and nitrate-N due to dilution. Analysis to date indicate at least limited downward movement of Atrazine out of the tilled layer occurred during winter. Tillage and surface residue management can alter depth and duration of soil freezing. Thus, soil management practices are important with respect to controlling ground water quality even during the non-growing season.