WATER QUALITY MANAGEMENT IN CONNECTED CATCHMENT- SHALLOW LAKE SYSTEMS - Integrating GIS, Remote Sensing, and Mathematical Modelling for Surface Water Quality Management - page 26

compacted soil. Another pathway for nitrogen loss from plant availability is the loss of the nitrogen
through the process of soil erosion by water.
Nitrogen Cycle-Hydrologic Cycle: Interactions
Since nitrogen and water are so vital for all organisms, it is inevitable that components of the nitrogen
and hydrologic cycles are closely related. These relations have particular importance for agriculture,
and lawn and turf management. By understanding these interactions, we can better understand the
effects of human activities on water resource quality. Nitrogen, mostly in the form of ammonium and
nitrate, reaches the Earth's surface as a result of atmospheric lightning, precipitation and industrial
pollution, this is accounted for as atmospheric production.
Denitrification
Nitrifying organisms can only function when free oxygen (O 2 ) is present. In saturated soils, free
oxygen is very low, suppressing the growth of the nitrifying organisms, often causing nitrogen
deficiencies in excessively wet soils. This condition is enhanced by denitrifying bacteria since they
thrive in an oxygen-free environment, like a saturated soil, and therefore consume nitrate at a rapid
rate. Excessive rainfall promotes nitrogen loss not only by promoting nitrate leaching from the plant
root zone, but also by creating wet soil conditions that favor denitrification. Evaporation works in the
opposite way to remove water from the upper soil layers. Space then becomes available for oxygen,
thereby making the environment suitable for the growth of nitrifying bacteria.
Surface Volatilization
In agricultural situations, surface volatilization (vaporization of urea to ammonia gas) may occur when
urea is applied on crop residues, and is not in good contact with soil particles. To limit volatilization of
the urea, producers usually incorporate it into the soil by tillage to bring the urea into contact with the
soil. Limited rainfall also helps with proper incorporation of the urea in the upper portion of the soil
profile. When water and urea combine, the result is the ammonium ion (NH 4 + ), which has a positive
charge and attaches to negatively charged soil particles. Both tillage and rainfall can help make
nitrogen available for plant use. Unfortunately, the interaction between tillage and excessive rainfall
increases the potential for soil erosion. After tillage, the soil is more susceptible to being carried away
by water during heavy rainfall.
Nitrogen Movement through the Soil
The nitrate ion (NO 3 - ) is the most water-soluble form of nitrogen as well as the form least attracted to
soil particles. Therefore, its interaction with the hydrologic cycle is very important since it moves
where water moves. Precipitation, evaporation and transpiration may affect the movement of nitrate in
the near-surface soil profile. Rainfall that infiltrates the soil surface may cause nitrate ions to move
down through the soil profile by percolation. The more rain that infiltrates into the soil, the further
down in the profile nitrate ions move. Nitrate movement below the plant root zone is called nitrate
leaching. Soil texture, structure and permeability, along with other soil properties, affect nitrate
leaching. Deep percolation of water through the soil profile potentially allows the movement of nitrate
out of the root zone and downward, where it may pollute the underlying aquifer. In contrast to the
nitrate ion, the ammonium ion has a strong attraction for soil, and therefore is considered to be
immobile in most soils. However, in soils with very high sand and low organic matter contents, the
ammonium ion will move in the direction of water movement.
Surface evaporation and transpiration may help nitrate move toward the soil surface within the root
zone as a result of capillary movement as the plant withdraws water from the soil profile. Upward
movement of nitrate occurs mainly in the summer when evaporation and transpiration exceed rainfall.
Once nitrates get into the surface water or the groundwater, the greatest concerns are for infants less
than one year old and for young or pregnant animals. High levels of nitrates can be toxic to newborns,
causing anoxia, or internal suffocation.