Chemical Transformations
Transformations: Chemical transformations from one compound to another can be important in controlling the quantity of an applied chemical leaching to groundwater. The section on degradation discussed one type of transformation that occurs in pesticides. Other biological and chemical transformations occur as well.
Transformations may result in compounds that are less toxic. They may result in compounds which are more strongly adsorbed and hence move more slowly through the soil profile. They may form compounds which are less soluble. In some cases gaseous products are formed which can be lost to the atmosphere from shallow depths.
With regard to the nitrogen cycle, transformations include mineralization, nitrification, immobilization, denitrification, and volatilization. Large quantities of nitrogen exist as organic N is soils. This form of nitrogen is quite immobile. The conversion of organic N to the ammonium ion is known as mineralization. Ammonium can be converted to nitrate by a group of bacteria. This is known as nitrification. So the mineralization and nitrification processes increase the pool of nitrate available to leach. They also increase the amount of N available for plant uptake. The reverse of this process is called immobilization. It takes mineral nitrogen and converts it to organic form. If the decomposing organic residue has more than 25 g of carbon per gram of nitrogen, immobilization occurs so mineral N is transformed to organic N. If the residue has less than 20 g of carbon per gram of nitrogen, mineralization occurs and nitrate is produced.
Nitrate can be removed from the soil system by the process of denitrification. In this process, nitrate is converted to various ions and ultimately to nitrous oxide and nitrogen gases. This requires conditions of high moisture, low oxygen, readily available carbon as well as a supply of nitrogen and the presence of denitrifying bacteria. Loss of these gases from the soil is most likely if the process occurs near the soil surface, but the gases can diffuse or move with convective currents of soil air from depths of several meters.
Volitilization can also occur when ammonia gas is created near the soil surface. This can occur when nitrogen fertilizer is applied in the form of urea or ammonia. A substantial amount of nitrogen in animal wastes can also be lost in gaseous phase.
