The effects of different nitrogen fertilizer treatments on soil characteristics, plant properties and economic returns in southeastern Minnesota cornfields

Background/Question/Methods

Nitrogen fertilizers are essential to achieving the crop yields necessary to feed a rapidly growing human population. However, they can have negative environmental and economic effects, especially when applied in excess. In this study, we sought to examine these effects and optimize nitrogen fertilizer application for no-till corn (Zea mays L.) fields in southeastern Minnesota. We examined two corn fields with different rotation histories: Field 1 was planted in the fourth year of continuous corn cultivation, and Field 2 was in its second year of corn following a seven-year alfalfa crop. Several different nitrogen fertilizer levels (45, 86, 101 and 135 kg N ha^-1) were applied to each field, and soil characteristics (nutrient concentrations, bulk density, pH, organic matter, and microbial activity), macroinvertebrate populations, plant properties, yield and economic returns were assessed to evaluate the effects of these treatment levels.

Results/Conclusions

 Soil nitrate (NO₃^--N) concentrations were significantly greater under higher nitrogen treatments, suggesting that they could contribute to pollution through groundwater leaching or runoff. Other soil characteristics that could influence crop productivity were not significantly affected by nitrogen treatment. Similarly, macroinvertebrate abundance and diversity did not differ by nitrogen treatment, although there were significantly more macroinvertebrates found on Field 2 than Field 1, perhaps due to the inclusion of alfalfa in the crop rotation. Higher levels of fertilizer increased corn height up to a threshold above which additional nitrogen did not contribute to the growth of the plant. Nitrate concentrations in corn stalks showed no significant trends, although in Field 2 the highest nitrogen fertilizer treatment yielded excessive levels of stalk nitrates, indicating that more fertilizer was applied than could be used by corn plants. Yield and economic returns did not differ significantly by nitrogen treatment, which indicates that environmentally-conscious farmers could reduce their use of nitrogen without suffering financially. Yield was significantly higher in Field 2, demonstrating the benefits of including alfalfa (a legume) in crop rotation. This on-farm research benefits farmers by providing specific information about their fields and determining optimal management practices on a localized scale.