|Ecosystem Stress From Chronic Exposure to Low Levels of Nitrate (EPA/600/R-05/087) October 2005
Throughout the eastern United States, from the Front Range of the Rocky Mountains to the Atlantic Ocean, bioavailable nitrogen has been falling in the rain since the industrial revolution. Bioavailable nitrogen is a limiting nutrient throughout this region.
Long-term research conclusively demonstrates that exposure of soil ecosystems to large doses of bioavailable nitrogen leads to deleterious environmental impacts, such as eutrophication, toxic algae blooms, hypoxia, toxicity, acid rain, and global climate change. These impacts can compromise people’s health and the economic vigor of communities.
Symptoms of compromised ecosystem function that may be attributable to chronic exposure to bioavailable nitrogen are widespread; many forests routinely leach nitrogen to surface and ground water, and nitrate-N concentration in estuaries perturbs aquatic food webs and affects fisheries. These observations, among others, support the hypothesis that ecosystem function can be (and has been) deleteriously affected by chronic exposure to low doses of bioavailable nitrogen.
In 1998, EPA initiated an integrated multidisciplinary study to investigate these effects on sixteen 40x40-meter plots in south-central Oklahoma. Complementary short-term field and laboratory studies were also conducted.
Four study plots received fertilizer only (16.3 kilograms N ha-1 yr-1), herbivory manipulation only (fenced), a combination of fertilizer and herbivory manipulation, or were left as controls.
Herbivory population was manipulated by constructing a 2-meter-tall chain link fence of 2.5-centimeter wire mesh. In this nitrogen-limited system, the ability of the soil system to adapt to new nitrogen inputs was compromised after one year of exposure. Concentrations of nitrate-N in the soil peaked at 1,169 percent more than expected and averaged 254 percent more than expected.
Plant growth was affected by nitrogen application; biomass increased on fertilized plots and diversity was related to distribution of Festuca arundinacea. Microbial activity was naturally limited in this system by carbon availability, but this tendency was exacerbated by additional inputs of nitrogen. Furthermore, microbial population response was not qualitatively different in soils that received small nitrogen additions vs. soils that received larger nitrogen additions.
The presence of large numbers of small mammals coincided with high concentrations of soil nitrate-N. Herbivores may be able to recirculate up to an estimated 67 percent of the bioavailable nitrogen deposited back into the plant and microbial pathways, thereby producing a self-reinforcing positive feedback loop leading to ever greater concentrations of soil nitrate-N. This could lead to increased nitrate-N leaching to surface and ground water.
The ability of detritus pathways to process nitrogen inputs was compromised after six months, and this tendency was increased when macroinvertebrate communities were restricted.
These experiments demonstrate that even the relatively small amounts of bioavailable nitrogen that are deposited during precipitation have the capacity to change many aspects of ecosystem nitrogen retention, sequestration, and processing. The changes observed are always deleterious; they lead to greater concentrations of nitrate-N, thereby making more available for leaching to surface and ground water.
As outputs of nitrogen to the atmosphere can reasonably be expected to increase in the foreseeable decades, it is prudent to identify and develop management options now to both restore ecosystems that are already compromised and to buffer effects to ecosystems that are at risk from new nitrogen inputs.
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