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“Since 1870, land conversion in the Mississippi Basin due to agricultural use has converted over 50 million acres of land into cropland drained by tile lines, ditches, and other means to lower the water table to make farming more economical and efficient,” states Mitsch et. al., (1999) in a report about the Gulf of Mexico’s hypoxic zone. Drainage systems, such as the drain tiles present under many fields, are a more direct pathway for nutrient runoff to go into the water system. Thus, a higher concentration of nutrients goes into the water system from fertilized fields. Previous to agricultural development, much of the land in the basin was covered with wetlands and forests and grasslands in the west of the basin. Since much of this land has been converted into pasture or cropland, the native ecosystems have been destroyed in much of the basin.
This destruction, combined with tillage farming and croplands that border streams and rivers, intensifies the nutrient loading from agricultural fields. Tilling the land increases erosion, which increases the runoff of organic matter, sediment, and phosphorus into the water. When croplands border streams and rivers, there is no barrier to decrease the levels of nutrients and sediments running into the water as there would be if a wetland or riparian zone, which is an area of native habitat present between field and stream, were present. Hence, nitrate concentrations in the Mississippi River and some of its tributaries have increased two to five fold in the last century.
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