Environmental and economic issues have increased the need to better understand the role and fate of nitrogen (N) in crop production systems. Nitrogen is the nutrient most often deficient for crop production and its use can result in substantial economic return for farmers.
There are five major factors that influence nitrogen loss from the soil, according to John Lamb, Fabian Fernandez, and Daniel E. Kaiser, Extension Specialists in Nutrient Management at the University of Minnesota.
Leaching. Loss of nitrate by leaching is a physical event. Leaching is the loss of soluble NO3–-N as it moves with soil water, generally excess water, below the root zone. Nitrate-N that moves below the root zone has the potential to enter either groundwater or surface water through tile drainage systems. Coarse-textured soils have a lower water-holding capacity and, therefore, a greater potential to lose nitrate from leaching when compared with fine-textured soils. Some sandy soils, for instance, may retain only 1/2 inch of water per foot of soil while some silt loam or clay loam soils may retain up to 2 inches of water per foot. Nitrate-N can be leached from any soil if rainfall or irrigation moves water through the root zone.
Denitrification. Denitrification can be a major loss mechanism of NO3–-N when soils are saturated with water for 2 or 3 days. Nitrogen in the NH4+-N form is not subject to this loss. Management alternatives are available if denitrification losses are a potential problem.
Volatilization. Significant losses from some surface-applied N sources can occur through the process of volatilization. In this process, N is lost as ammonia (NH3) gas. Nitrogen can be lost in this way from manure and fertilizer products containing urea. Ammonia is an intermediate form of N during the process in which urea is transformed to NH4+-N. Incorporation of these N sources will virtually eliminate volatilization losses. Loss of N from volatilization is greater when soil pH is higher than 7.3, the air temperature is high, the soil surface is moist, and there is a lot of residue on the soil.
Crop Removal. Substantial amounts of N are lost from the soil system through crop removal. A 250 bushel per acre corn crop, for example, removes approximately 175 pounds of N with the grain. Crop removal accounts for a majority of the N that leaves the soil system.
Soil Erosion and Runoff. Nitrogen can be lost from agricultural lands through soil erosion and runoff. Losses through these events do not normally account for a large portion of the soil N budget, but should be considered for surface water quality issues. Incorporation or injection of manure and fertilizer can help to protect against N low through erosion or runoff. Where soils are highly erodible, conservation tillage can reduce soil erosion and runoff, resulting in less surface low of N.