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Nutrient loadings which cause eutrophication of coastal marine systems are transported into the watershed via atmospheric, surface flow, and groundwater pathways (Keeny and DeLuca 1993, David et al. 1997). Intensive agricultural practices that cause high outputs of sediment and nutrients into the water system also cause some forms of nitrogen (ammonia) to become air-borne. Along with emissions from fossil-fuel burning electrical stations and internal combustion engines, the air-borne nitrogen then returns to earth through precipitation and dry fallout.
Nitrate-nitrogen, the form of nitrogen that is readily obtainable by plants, is mobile and can therefore be lost from the soil profile by leaching. Nitrogen present in the basin is about 61% nitrate, which is unnaturally high for the area. Subsequent transport of nitrate-nitrogen to surface waters occurs primarily through subsurface drainage. Nitrogen is the primary nutrient involved in the eutrophication of the gulf; thus, subsurface runoff through the drain tiles used underneath much of the cropland of the basin provides an efficient way for nitrogen to enter waterways. Drain tiles are a common management practice in highly productive agricultural areas of the Mississippi River Basin where poorly drained soils have seasonally dynamic water tables or shallow groundwater. (Matson et. al., 1997) (Jackson et. al., 1973; Logan et. al., 1994 and Mitsch, 1999).
Very little nitrate-nitrogen is lost from the agricultural landscape via surface runoff; however, the majority of phosphorus lost from agricultural systems is transported over the surface via runoff and erosion. Phosphorus is an important ingredient to nutrient enrichment with 31% of the phosphorus that reaches the Gulf coming from fertilizer and 41% coming from unsourced sediment erosion that can be caused by tillage and ground disturbance associated with agriculture. Heavy metals that are toxic to many living things are often bound to soil particles, and thus are also transported into waterways through increased erosion due to agricultural practices.