In addition, research in Ohio and Minnesota shows that subsurface drainage promotes mellower soil conditions and reduces compaction potential. Where soil infiltration is promoted by practices such as subsurface drainage and conservation tillage, significant reductions have been found in total and dissolved phosphorus and sediment concentrations of surface waters. Improved agricultural stewardship of land and water resources has led to reductions in phosphorus and sediment loadings from agricultural land, however nitrate-N losses to surface waters have continued and are increasing in some areas, for reasons that are not completely understood. Elevated concentrations of nitrate-N have long been considered a source of drinking water pollution. Recently, N fertilizer has received attention for its contribution to hypoxia in the Gulf of Mexico.
Hypoxia is a condition of low dissolved oxygen occurring in vital Gulf of Mexico fisheries.
Oxygen depletion occurs when nutrients exported from upstream Mississippi Basin watersheds cause increased algal growth in the nutrient-limited marine ecosystem. The decomposition of dead algal matter by oxygen-using organisms results in depleted oxygen levels that kill animals in the sediment and force shrimp and fish out of preferred habitats. Certainly, subsurface drainage acts as a conduit for the movement of nitrate-N to surface waters. Research in Illinois showed that nitrate-N concentrations in drainage water from fertilized crop fields was 10 times greater than concentrations measured in drainage from a meadow. In addition, there is a great potential for nitrate-N loss when available N exceeds plant requirement.