ISSUES IN ECOLOGY NUMBER FIFTEEN WINTER 2012 Box 1.CRITICAL LOADS:HOW MUCH NITROGEN IS TOO MUCHI acidificat outrient imbala of b rsity.To w h to much I loads u hey are widely used in Europe and Canada to evaluate hownitro ds to be improved,in order to reduce excess nitrogen loadings and to restore harmed areas to extinction.The herbivores that feed on these the U.S.is considered "most disturbed"with plants are also affected.For example,checker with respect to replacement of native with invasive nito IMPACTS ON CLIMATE y feed upon Th in Early global climate models focu the greer sinks of carbon dioxide.but did not include reactive nitrogen in terrestrial n up by plants or some Roughly two-thirds of U.S. astal systems have cial regulator of the carbon (C)cycle,climate recently been c to n ng pro es m ith ni increased frequency,severity,and extent of Deposition of airb me reactive nitogen onto (low oxygen)and ano ho oxygen) land affects terrestrial carbon sinks through two TCtscehnocCenceofcoasalhypowicnd e the their wood.The decade th stimulation is ot some debate zones along the U.S. e sea on is con ond in matter down processes is an area of active research d by into changes communities, and enzyme produ straints by both nitroger y one thunt ot the The most direct effect of nitrogen on climate is,the third most impor tant anthropogenic greenhouse gas,contribut- The Ecological Society of Americaesahq@esa.org esa 5
© The Ecological Society of America • esahq@esa.org esa 5 ISSUES IN ECOLOGY NUMBER FIFTEEN WINTER 2012 to extinction. The herbivores that feed on these plants are also affected. For example, checkerspot butterfly populations in serpentine grasslands of California have declined following replacement of native with invasive nitrogenloving grasses. In other cases, herbivore populations expand when the plants they feed upon become enriched with higher tissue concentrations of nitrogen, and lower concentrations of defensive chemicals. The expansion of nitrogenloving, non-native, highly flammable grasses in the western U.S. has increased fire risk. (e.g., see cover photos) Much of the reactive nitrogen in terrestrial ecosystems that is not taken up by plants or retained in soils ends up in aquatic ecosystems. Roughly two-thirds of U.S. coastal systems have recently been classified as moderately to severely impaired due to nutrient loading. Overenrichment with nitrogen is associated with increased frequency, severity, and extent of hypoxic (low oxygen) and anoxic (no oxygen) events, harmful and nuisance algae blooms, and species shifts leading to biodiversity loss. An increase in occurrence of coastal hypoxic and anoxic zones has been reported every decade since the early 1900s, with nearly 300 hypoxic zones along the U.S. coastline. In New England estuaries, phytoplankton (microscopic algae) now dominate over native sea grasses, resulting in aquatic ecosystems with much less structural complexity and lower water clarity. Enrichment of nitrogen in freshwaters often has negative impacts similar to those seen in coastal waters, and also affects drinking water quality (see human health impacts section). Although plant and algal growth in freshwater systems is strongly constrained by phosphorus (P), there is considerable evidence for co-constraints by both nitrogen and phosphorus. Recent surveys carried out by the EPA indicate that roughly one-third of the total stream length in the U.S. is considered “most disturbed” with respect to total nitrogen concentrations, and roughly one-fifth of all U.S. lakes are ranked poor with respect to total nitrogen concentrations. IMPACTS ON CLIMATE The importance of the nitrogen cycle in regulating climate is gaining increasing attention. Early global climate models focused solely on the physics of greenhouse gas effects; later models incorporated biological sources and sinks of carbon dioxide, but did not include carbon-nitrogen interactions. In recent years, a few earth system models have added some representation of the nitrogen cycle as a crucial regulator of the carbon (C) cycle, climate, and atmospheric chemistry, but the representation of nitrogen cycling processes in climate models remains far from complete. Deposition of airborne reactive nitrogen onto land affects terrestrial carbon sinks through two key processes. First, inputs of nitrogen often increase the growth of trees, which store high amounts of carbon in their wood. The magnitude of growth stimulation is of some debate, but is likely greatest in regions of moderate nitrogen deposition. Increasing atmospheric carbon dioxide concentrations also stimulate plant growth, but this stimulation is constrained by the availability of nitrogen to plants. Second, inputs of reactive nitrogen slow breakdown of dead plant material and soil organic matter in many, but not all forest soils. Why nitrogen deposition slows these breakdown processes is an area of active research into changes in soil microbial communities, microbial biomass, and enzyme production needed to break down complex organic matter. The most direct effect of nitrogen on climate is through nitrous oxide, the third most important anthropogenic greenhouse gas, contributBox 1. CRITICAL LOADS: HOW MUCH NITROGEN IS TOO MUCH? Excess nitrogen can disrupt natural ecosystems, causing acidification, nutrient imbalances, and loss of biodiversity. To manage nitrogen effectively, it is important to know how much nitrogen can be added to an ecosystem without provoking harmful effects. The term “critical load” describes how much nitrogen is too much. Critical loads usually refer to nitrogen deposited from air pollution and are expressed as loading rates of nitrogen in a given area over time, usually as kilograms of nitrogen per hectare per year. They are widely used in Europe and Canada to evaluate how nitrogen, sulfur, and other air pollutants affect streams, lakes, and forests, and are now being developed for ecosystems in the U.S. Maps of critical loads are combined with maps of air pollution to show where pollution loads exceed the estimated local critical load, putting ecosystems at risk. For example, in California, maps of critical loads combined with actual nitrogen loads highlight areas where nitrogen is likely affecting forests (Figure 3), grasslands, coastal sage, desert, and streams. This information helps air quality managers determine where and how much air quality needs to be improved, in order to reduce excess nitrogen loadings and to restore harmed areas
ISSUES IN ECOLOGY NUMBER FIFTEEN WINTER 2012 N pollution has both warming and cooling effects on the climate clouds to be 1338333333383 是症森森森泰泰森麻热 al 1101400000 乐桑森泰森泰泰森森泰森 101000444 森森森森泰森泰森泰森乘 110410001001 森泰森森秦泰桑森森寨泰 U.S.specific.Efforts are underway to create a 10410000000 桑森森森森秦森森森森森 hdco sequestration attri 1290 oughly cancel the warming effect of nitrous (Figure 4).Putting these estimates into ing 6%of total human-induced global warming. nitrogen 15 of car on di more than 10 vears).Armospheric concentra IMPACTS ON HUMAN HEALTH Drinking water and human health emis increased after World War I.The EPA sti- Nitrate concentrations in g undwater are s are raising and ide n used for domestic water The ng. l (MCL)for Reactive nitrogen also affeets methane. another important green Intergov water stic wells d small compared to those ding.S.Geological Survey (USGS)report.T king into account the irectly,nitro regional sources of nitrate and regional differ- er atmosphere). dire a a greenhouse to pla reasing ri pheric carbon dioxide by as much as 20% Both nitrogen oxides and ammonia affect hat abur lion Americans use private ormation of ing we leconcentratid nd about a half million Ame ans use wells particles influence the formation of cloud that exceed the MCL of 10 milligrams nitrogen 6 esa The Ecological Society of America.esahg@esa ora
ISSUES IN ECOLOGY NUMBER FIFTEEN WINTER 2012 6 esa © The Ecological Society of America • esahq@esa.org ing 6% of total human-induced global warming. It has about 300 times the per-molecule warming potential of carbon dioxide and is long-lived in the atmosphere (a “mean residence time” of more than 110 years). Atmospheric concentrations of nitrous oxide have increased rapidly since 1860, as livestock herds increased globally and as use of synthetic-nitrogen fertilizers increased after World War II. The EPA estimates that agricultural activities are directly or indirectly responsible for emissions of 0.48 million tons of nitrogen as nitrous oxide per year, which is about 80% of total U.S. nitrous oxide production (the remainder is from energy and industrial sources) and about 10% of the global nitrous oxide emissions from agriculture. Reactive nitrogen also affects methane, another important greenhouse gas, through chemical reactions that destroy atmospheric methane and through inhibition of methane production and consumption by microbes in soils and wetlands. However, the overall climate impacts of reactive nitrogen via methane are small compared to those of nitrous oxide and carbon sequestration. While not greenhouse gases directly, nitrogen oxides often affect the production of ozone in the troposphere (the lower atmosphere). Ozone affects climate directly as a greenhouse gas, and it is also toxic to plants, decreasing photosynthesis and plant uptake of atmospheric carbon dioxide by as much as 20%. Both nitrogen oxides and ammonia affect the formation of tiny airborne particles, also known as aerosols, and their chemical properties. The abundance and properties of these particles influence the formation of cloud droplets. In some cases this causes clouds to be brighter and longer-lived, which has important effects on precipitation and temperature patterns. Overall, aerosols have a short-term cooling effect, but the long-term effect is small because the aerosols are frequently washed out of the air by rain. The above discussion of the impacts of reactive nitrogen on climate is global in scope, not U.S.-specific. Efforts are underway to create a U.S.-specific nitrogen assessment, with preliminary findings shown in Figure 4. It compares the long-term warming potentials of nitrogen gases and particulates and carbon sequestration attributable to U.S. emissions of reactive nitrogen. The cooling effects of nitrogen deposition through carbon sequestration roughly cancel the warming effect of nitrous oxide (Figure 4). Putting these estimates into a broader perspective, these contrasting warming and cooling effects of nitrogen are equivalent to less than 10% of the warming effect of U.S. emissions of carbon dioxide from fossil fuel combustion. IMPACTS ON HUMAN HEALTH Drinking water and human health Nitrate concentrations in groundwater are increasing in many parts of the U.S., raising concerns for human health, particularly in rural agricultural areas where shallow groundwater is often used for domestic water supplies. The EPA’s maximum contaminant level (MCL) for public drinking water supplies is 10 milligrams per liter as nitrate-nitrogen (or about 45 milligrams per liter as nitrate). Nitrate concentrations above the MCL are relatively uncommon in streams and deep aquifers used for drinking water supplies. However, the MCL was exceeded in 22% of shallow (less than 100 feet below the water table) domestic wells in agricultural areas, an increase from a decade earlier, according to a 2010 U.S. Geological Survey (USGS) report. Taking into account the regional sources of nitrate and regional differences in geology that affect its movement to groundwater, the USGS study shows large areas in agricultural and urban regions with shallow groundwater nitrate exceeding 10 milligrams nitrogen per liter (Figure 5). Based on a USGS model of drinking water quality, it is estimated that about 1.2 million Americans use private drinking wells with nitrate concentrations between 5 and 10 milligrams nitrogen per liter, and about a half million Americans use wells that exceed the MCL of 10 milligrams nitrogen Figure 4. The cooling effects resulting from U.S. nitrogen deposition (which allows trees to remove carbon from the atmosphere and causes reflection of the sun by nitrogen-containing haze and particles in the air) slightly outweigh the warming effect of U.S. nitrous oxide emissions. However, uncertainties in these calculations are large, yielding the following ranges of estimates: +180 to +400 for N2O; -240 to -540 for tree growth; and -2 to -16 for haze, where positive numbers indicate warming and negative numbers cooling. Because various different greenhouse gases and aerosols are included in this analysis, all are converted to the common currency of “CO2-equivalents” on a 100-year global warming potential time frame, using the methodologies of the Intergovernmental Panel on Climate Change