Nitrogen in the soil: Additions, losses, and the impact on crops
3. A stable, recalcitrant pool of organic materials produced from the decomposition of plant and animal residue is referred to as soil organic matter (SOM). The stable or recalcitrant pool, true soil organic matter, is only slowly available, with perhaps 1 to 2% of the N present becoming available each year. This amounts to about 20 pounds of available N added to the inorganic pool per 1% SOM per year. This 20 pound credit for each one percent of SOM is used by K-State when determining N recommendations for summer crops. But only half that much, or 10 pounds N for each one percent of SOM, is given as a credit for fall or winter crops such as wheat or canola. This is due to the slower rate of SOM decomposition in fall, winter, and early spring, when wheat is actively taking up N.
Additions of N to the soil system
There are several ways N is added naturally to soil systems. These include N fixation by lightning, and biological N fixation by both free living organisms and legumes. In our native prairie ecosystems this can account for 20 to 40 pounds of N per acre per year. In modern managed systems we also gain N through acid deposition from burning fossil fuels, adding N fertilizers, growing leguminous crops such as alfalfa and soybeans, and applying waste materials, especially animal manure on farm lands. The total additions of N to intensively managed farmland can exceed 250 pounds N per acre per year when manure is involved.
Losses of N from the soil system
N is also lost from cropland soils each year, even in Kansas. Key loss mechanisms include:
* Leaching of nitrate through the soil, a key issue on sandy soils, especially with irrigation.
* Denitrification, the biological conversion of nitrate under anaerobic conditions to various forms of gaseous nitrogen oxide products. This process can ultimately result in nitrous oxide and nitrogen gas. This is the primary nitrogen loss mechanism on poorly drained, heavy textured or claypan soils.
* Ammonia volatilization from surface applied urea or manure, a concern at high soil pH or in high residue no-till systems.
* Soil erosion.
We also can’t forget crop removal, which is generally the largest N loss annually from an agricultural system, especially when we harvest forage crops or corn grain.