Before human civilization, reactive nitrogen was in very short supply. Natural reactive nitrogen is created only through lightning-induced atmospheric combustion and by biological nitrogen fixation. Humans have added to the global pool of reactive nitrogen by cultivating legumes and rice, burning fossil fuels, and transforming atmospheric nitrogen to produce synthetic nitrogen fertilizers. The rate of conversion of nitrogen from non-reactive to reactive form has accelerated exponentially over the past 50 years. More reactive nitrogen is now produced by human activities than by all natural sources combined. This makes alteration of the global nitrogen cycle one of the five most important elements of global change, along with increases in greenhouse gases, land use change, appropriation of freshwater for human uses, and loss of biodiversity.
This abundance of reactive nitrogen has had both positive and negative consequences. According to the International Nitrogen Initiative, 40% of the world’s population consumes crops supported by human-altered nitrogen. Nitrogen fertilizer increases the yield and nutritional quality of agricultural products; however, nitrogen escaping from agricultural use or produced as a byproduct of industry and transportation can have far-reaching effects on natural ecosystems and human health (Galloway et al. 2003). A surplus of nitrogen in rivers, lakes, groundwaters, and coastal zones leads to excessive plant growth, or eutrophication, and plant decay, which consumes oxygen. Anoxia – the lack of oxygen in waters – reduces water quality and renders aquatic environments such as coastal waters unable to support fish and other aquatic species. What’s more, very high nitrate concentrations in water pose serious risks to human health. Nitrogen availability enables some plants to outcompete others, and the dominance of certain invasive weeds has been tied to nitrogen fertilization. In the atmosphere, nitrogen oxides are precursors to ground-level ozone pollution, which causes human respiratory ailments and reduces crop and forest productivity. Nitrous oxide, which is present in the atmosphere in increasing concentrations, is a potent greenhouse gas that contributes to global warming and loss of stratospheric ozone.
Increased emissions of nitrogen have led to commensurate increases in the amount of the element deposited to the Earth’s surface in rain, snow, or particles in some regions. The long-term ecological consequences include reduced soil fertility and shifts in vegetation composition. Nitrogen saturation, a condition that occurs when terrestrial plants and soils are unable to use or store all the nitrogen they receive, leads to nitrogen leaching, lake and stream eutrophication, and eventual acidification (see Nitrogen pathways diagram). Most studies of ecological responses come from regions receiving chronically high amounts of nitrogen, with deposition rates exceeding 10 kg per hectare per year. But because reactive, readily available nitrogen has been so rare through evolutionary history, any increase above background has the potential to initiate ecological response. This has been scientifically understudied due to three prevailing assumptions: (1) phosphorus, not nitrogen, was held to be the major limitation to primary productivity in freshwaters; (2) forest growth response was not expected at nitrogen fertilization amounts of less than 100 kg per hectare per year; and (3) the environmental response to atmospheric deposition was acidification.
Subtle effects
So what are the consequences of low, chronic inputs of nitrogen? We explored this question in otherwise undisturbed Colorado Rocky Mountain ecosystems, looking for subtle evidence of changes due to nitrogen deposition. With every human-mediated ecological change, there must be a point at which ecosystem structure and function begin to respond, but alterations are often not detected until far down the trajectory of change. In protected natural reserves such as national parks, any ecological response to atmospheric pollution is unwanted, and the National Park Service, while not a regulatory agency, has a legal responsibility to protect resources in some parks potentially adversely affected by air pollution. A communication conundrum occurs when subtle air pollution effects are reported: whereas management or policy intervention can be applied to reverse trends most effectively during the onset of environmental response, these early stages of change are often not dramatic enough to capture the attention of public and policy makers.
Case Study: Front Range of the Colorado Rocky Mountains
In the western United States estimated rates of pre-industrial, or background, inorganic-nitrogen deposition range from 0.4 to 0.7 kg per hectare per year. Current rates of deposition, however, are at least an order of magnitude greater than background rates, and they are spatially heterogeneous – ranging widely from 1.0 to 4.0 kg per hectare per year over much of the region to as high as 30.0 to 90.0 kg per hectare per year in heavily populated southern California (Fenn et al. 2003). Inorganic nitrogen deposition is greater near large sources of nitrogen emissions, such as highly urbanized or agricultural regions, and at high elevations, where orographic (mountainous) conditions promote more precipitation. Along the eastern flanks of Colorado’s Front Range, rates of gaseous and precipitation nitrogen deposition are approximately 3 to 5 kg per hectare per year.
My colleagues and I addressed the question of whether we could find discernible responses to low levels of nitrogen deposition in the high mountain alpine tundra, forests and waters of Colorado’s Front Range. Over many years we have employed a variety of methods, including long-term monitoring, experiments, regional surveys, ecological models, and paleo-historical reconstructions of past changes recorded in lake sediments. Our surveys and monitoring revealed evidence of nitrogen enrichment in plant tissues, soils, soil microbial activity, lakes and streams (Baron et al. 2000). Observations of nitrogen-driven changes in alpine flora and in assemblages of lake algae were confirmed through experiments (Burns 2005). Records of lake sediments, including algal diatoms, organic compounds, and stable isotopes, indicate that the onset of change due to nitrogen fertilization from atmospheric deposition in the mountain environment occurred between 1950 and 1960 (Enders et al. in press), and our reconstruction revealed that just 1.5 kg per hectare of wet nitrogen deposition was enough to initiate the change.
So we defined 1.5 kg per hectare per year of nitrogen per hectare as the critical load, or threshold, at which eutrophication occurs and initiates ecological changes in this system. Other studies of Rocky Mountain ecosystems have indicated that annual critical loads are 4 kg of nitrogen per hectare for acidification to cause changes in some alpine species, and 10 kg of nitrogen per hectare for fertilization-driven changes to alpine vegetation communities (Bowman et al. 2006).
Messages for policy
Taken individually, the observations and experiments from forests, soil microbial activity, and alpine vegetation and lakes, provide interesting but unremarkable scientific findings; however, the cumulative body of evidence pointing to atmospheric nitrogen deposition as the cause for ecological changes was compelling. What’s more, since the observed changes indicated early stages of eutrophication along a trajectory that would eventually lead to ecosystem acidification, early action to reverse the trend of increasing emissions and deposition of nitrogen was warranted before more changes could occur. The evidence was sufficient to convince State and Federal agencies to adopt a nitrogen deposition-reduction plan with the ultimate management goal of limiting the annual load of wet nitrogen deposition to 1.5 kg per hectare, the threshold below which ecosystem changes are unlikely to occur.
An interim target load – 2.7 kg per hectare per year of wet nitrogen deposition – is to be achieved by 2012, and by 2009 a contingency plan will be developed to prepare for the possibility that reduction programmes and voluntary efforts won’t be enough to achieve the milestones. Rapid environmental response to decreased input of nitrogen should be detectable in stream nitrate concentration and possibly in numbers and species of lake algae. Terrestrial vegetation and soils will take much longer to respond, since nitrogen is tightly cycled and retained in terrestrial environments. The legacy of past deposition inputs will influence the response rates according to ecosystem type – terrestrial or aquatic – for years.
At first glance, this appears to be a textbook case of how the application of science to policy is supposed to work. It began with good scientific evidence from many different sectors that pointed toward a common conclusion, matured as collegial public servants and interested civic groups came to collective agreement on a solution, and ended with implementation by farsighted and wise policy makers. Problem solved.
Confounding factors
Or is it? There are still several outstanding issues regarding the source of emissions and whether in-State emissions reductions will be sufficient to reduce wet nitrogen deposition. In addition, there are recent observations of what appear to be climate- rather than pollution-related increases in nitrate concentrations in, and nitrogen losses from, several Colorado Front Range streams.
Correlations of wet deposition chemistry with wind directions along the Front Range have long suggested that nitrogen deposition was greatest when winds blew from east to west. The upslope winds were thought to entrain pollutants from the metropolitan and agricultural areas east of the Colorado Rocky Mountains. A recent intensive sampling effort that captured and analysed airborne particles and gases from many locations in Colorado confirmed that the highest concentrations of nitrogen originate from sources east of the mountains and Rocky Mountain National Park. How much nitrogen originates from Colorado and how much comes from industrial or agricultural sources outside the State is still under study. Large outside sources may hinder Colorado’s ability to control its own nitrogen deposition through either voluntary or regulatory means.
More troubling is the observation since 2000 of a rapid rise in stream nitrogen concentrations and nitrogen flux that is unrelated to atmospheric deposition – an unexpected nitrogen surprise. Wet nitrogen deposition has not increased since 1994, and precipitation since 2000 has been at or below the long-term average. A marked increase in summertime temperatures and an increase in runoff coincided with the increase in nitrogen flux. A narrowing in the precipitation : runoff ratio since 2000 strongly points toward the melting of permanent ice features in alpine basins. Measurable nitrate concentrations associated with bacterial populations in glaciers and glacial melt have been reported from other alpine basins in the Rocky Mountains, the Alps, and in the high Arctic. So the recent change in stream nitrogen dynamics may be a climate-driven phenomenon that confounds scientists’ ability to discern whether reductions in nitrogen emissions and wet deposition are having the desired effect on protected national park ecosystems.
Interactions among ecosystem drivers, in this case climate change and atmospheric deposition, are common, but remain difficult to manage. Ours is a good example of how climate change complicates matters and must be included when considering resource management issues now and in the future. The interactions between climate change and resource management issues challenge policy makers, managers, and scientists alike to develop flexible and sophisticated strategies that protect natural resources from multiple disturbances.