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Introduction

Riparian zones are areas of vegetation flanking either side of river and stream systems and are often planted with various hydrophilic grasses, shrubs, and trees. They play an important role in the function of freshwater ecosystems by preventing nutrients from entering a waterway. Riparian areas are essential in the preservation of water quality .

Riparian zones and the environment

The delicate balance of the water way is ruled by a multitude of key players. Once the equilibrium is skewed, the process to return back to original conditions is markedly tedious and laborious . Nutrients enter waterways through groundwater seepage, pollution, or agricultural run-off . The two nutrients which are notorious for instigating most problems in agriculture are nitrogen and phosphorus. The concentrations of these nutrients build up in water bodies and result in eutrophication which is an insidious water-quality problem because its effects are often not visible at the source (e.g., agricultural fields). Excessive levels of nutrients in water systems harbor conditions that are ideal for micro and macrophytic blooms and dispersal. These blooms cause sunlight to be blocked from submerged plants. As algae die they are consumed by microbial decomposers which remove dissolved oxygen from the water bodies, creating hypoxic regions. These regions that are sometimes referred to as pseudo-dead zone and are fatal for a number of aquatic organisms (e.g., fish) These conditions that degrade lotic systems worsen if the over applied of nutrients is not halted or the problem is habitually ignored .

Nitrogen transformations in riparian zones

Nitrogen often enters riparian areas in the form of nitrate (NO3-) through either surface runoff or by subsurface ground water. Then, the nitrogen can either 1) move into surface water bodies contributing to eutrophication, 2) be taken up by plants, 3) and stored as organic matter or incorporated in other mineral phases, 4) or undergo transformations. Preferably, nitrogen is removed from the system by undergoing transformations rather than by moving into surface water bodies .

Denitrification is the conversion of nitrate to nitrogen gasses (i.e., Nitric Oxide (NO), Nitrous Oxide (N2O), or elemental nitrogen gasses (N2). Preferably all or nearly all of the nitrogen is converted to N2 gas. Less desirable is when nitric or nitrous oxides are the end products. Nitric oxide reacts with ozone and can deplete the ozone layer. Nitrous oxide is much more effective of a greenhouse gas than carbon dioxide .

Nitrogen is one of the more important fertilizers placed on agriculture lands. One of nitrogen’s intrinsic characteristics is its readiness to be influenced by subsurface water flow after application, either for human practice or natural ways, and is adsorbed by soil particles . In most cases it is transformed into usable forms by bacteria found in plant nodules beneath the surface (Chang et al., 2007). Once the nitrogen has reached groundwater, most of the seepage and recharge for certain waterways is via these underground channels. In areas of karst topography, this rate of transportation into the groundwater is quicker. Riparian zones are necessary to capture the remaining nitrogen not utilized by crops and other plants harvested. For the uptake of nitrogen, the most effective vegetation is those deep-rooted trees which maintain a thick root mat . Any other woody vegetation is equally effective . Grasses and smaller shrubs are less effective in retaining the excess nutrient but still actively participate as a function in the retention of approximately ninety-percent of the limiting nitrogen .

Phosphorous retention in riparian zones

When phosphorus enters a riparian area, it is usually from surface runoff because it is relatively insoluble and has a retarded speed of movement in slow water. Even though less phosphorus is taken up by vegetation than other nitrogen , buffers can still be effective at retaining phosphorus. For example, Wang et al. (2008) found that there was less total phosphorus in water when a reed buffer strip were used. However, unless the phosphorus is harvested, it remains in the riparian zone. Therefor, it can be lost at later times. For example, during dormancy (typically in the winter time) there is a large out flux of phosphorous from the plants into the soil .