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Managing agricultural landscapes for clean water
Nitrate, a form of nitrogen, occurs naturally in ecosystems and promotes plant growth. However, excessive nitrate concentrations in ground and surface waters can contaminate drinking water supplies, creating health risks for humans. In addition, when nitrate-rich ground and surface waters enter lakes, estuaries, or coastal ocean waters, conditions for undesirably high algae growth are created. This process, called eutrophication, can create a deadly chain reaction: as large algae blooms die, the dead organic matter becomes food for bacteria. Because there is more food available, bacterial populations increase rapidly and use up the dissolved oxygen in the water. These oxygen-depleted areas—toxic to many fish and aquatic insects—are referred to as hypoxic or dead zones in large lakes and oceans. Large algae blooms also diminish water quality for lake shore home owners and those who use lakes for fishing, swimming, and boating.
Agricultural landscapes commonly have excessive nitrate concentrations in ground and surface waters. Contamination occurs when nitrogen from fertilizer or organic applications is not used by plants and therefore leaches through the soil profile. Once deep in the soil profile, nitrate can enter into groundwater aquifers or be transported in water drained by tiles to surrounding surface waters.
KBS LTER scientists are investigating how various types of farm and land management affect nitrate losses to groundwater aquifers and to groundwater-fed streams and lakes. From 1995 through 2006, the scientists measured nitrate levels in soil water from fields under different management treatments. The treatments included four types of annual row-crop management and alfalfa and hybrid poplar tree systems. These cropped systems were compared with unmanaged fields and forests that were abandoned from agriculture years ago and are undergoing natural succession (change) toward becoming the native deciduous forest of the region.
The scientists discovered dramatic differences in nitrate leaching under these various types of land management. In the annual crops, biologically-based management (no fertilizer; cover crops supply nitrogen) reduced nitrate leaching by as much as two-thirds compared to conventional farm management. The unmanaged ecosystems leached the least nitrate. This is because they were not fertilized and because as they undergo succession, they continue to accumulate nitrogen in their biomass. Some proposed cellulosic biofuel crops, such as switchgrass, could resemble these unmanaged systems and leach very low amounts of nitrate to the groundwater.
The scientists are also investigating what happens to nitrate once groundwater enters streams and wetlands. About 75% of the nitrogen humans add to landscapes disappears in transit before rivers reach the sea. This is most likely due to the conversion of nitrate to nitrogen gas by certain bacteria in a process known as denitrification. Headwater streams and wetlands are known to be “hotspots” of denitrification because they are shallow and high in biological activity. To learn more about these hotspots, KBS LTER scientists collaborated with researchers from around the country. They sampled numerous streams in agricultural, urban, and forested watersheds. They found that stream ecosystems can be managed to preserve and enhance nitrate removal, a valuable ecosystem service that improves downstream water quality.
These results should help farmers, land managers, and policymakers develop and maintain sustainable agricultural landscapes that provide society with both food and fuel and clean water.