Preventing Nitrogen Loss in Different Topography Positions
Nitrogen losses are primarily caused by nitrate leaching and denitrification. These losses can occur rapidly. There must be nitrate present in the soil for significant leaching and denitrification to occur. Nitrification is the conversion of ammonium to nitrate. If applied nitrogen and mineralized organic matter were to stay in the ammonium form, leaching and denitrification losses would be minimal. Nitrogen in the ammonium form is a positively charged cation. Soils have greater cation exchange capacities than anion exchange capacities, therefore anions such as nitrate are more prone to leaching losses. Nitrate, the negatively charged anion, stays dissolved in the soil solution and as plants absorb water, they absorb the nitrate.
Nitrogen losses occur throughout the entire field, although the cause of nitrogen loss is unique in different topography positions. SWAT MAPS are effective at zoning soils with similar characteristics such as organic matter, soil texture, and water availability. These characteristics contribute to factors that effect nitrogen losses.
Nitrate leaching is the downward movement of nitrate with water percolation through the soil. Anions such as nitrate are retained by a soil's anion exchange capacity. Soil with low organic matter and clay content have a low anion exchange capacity. Organic matter is the dominant soil characteristic that determines if a soil has a low or a high exchange capacity. This means that a sandy or a light soil, which is low in organic matter, commonly a SWAT MAP Zone 1-4, is more prone to leaching.
Denitrification losses occur when a soil becomes saturated with water. A saturated soil has low oxygen present. Microorganisms use the oxygen contained in the nitrate for metabolism which results in nitrogen gases lost to the atmosphere. There must be nitrate present and low soil oxygen supply for denitrification to occur. SWAT MAPS Zones 7-10 are most common to have high water and nitrate availability.
Nitrogen stabilizers aid by slowing the conversion of ammonium to nitrate, which reduces leaching and denitrification. Super U, NServe and eNtrench NXTGEN are all examples of nitrogen stabilizers that can be used. SWAT MAPS are effective at identifying areas that will be prone to nitrogen losses from leaching and denitrification. For example (Figure 2 and 3), this field would be prone to denitrification in Zones 6-10. Zones 6-10 in this field would be high in nitrates and would be water holding. Therefore, in Zones 6-10 we increase the lbs of product per acre that we apply of the nitrification inhibitor Super U and decrease the amount of Urea product we are applying per acre. In this example we are shutting off all nitrogen sources in Zones 9-10 because this field tests remarkably high in nitrogen residual and is saline in Zones 9-10. Using variable rate to apply nitrogen sources and nitrogen stabilizers puts the right product, in the right place, at the right rate, at the right time.
SWAT MAPS can lower overall cost of nitrogen stabilizers and nitrogen sources compared to a flat rate application. This can be done by applying these products to areas where loss is most likely to occur.