Altering the Nitrogen Cycle: Massive Human Intervention in Nature
Introduction
Nitrogen is a key nutrient for plant growth, but to be useful to plants, it must occur in a form called “fixed nitrogen.” Throughout most of history, fixed nitrogen has been in relatively scarce supply – the quantity limited, for the most part, by the rate at which certain types of algae and bacteria could fix it. However, because of human activities and technological developments, the pool of fixed nitrogen on the landscape has been rising rapidly over the last century. This trend has had major consequences for ecosystems, many of which are becoming more and more apparent.
Historically, fixed nitrogen has been scarce in nature despite the fact that the atmosphere is predominantly nitrogen gas. Nitrogen gas is N2, which plants cannot use directly. Nitrogen bonded with oxygen to form nitrate (NO3-), or with hydrogen to form ammonium (NH4+), as well as smaller organic nitrogen molecules, are forms that plants can use and are called fixed nitrogen. The mechanism by which nitrogen moves between the atmosphere and the living world is termed the nitrogen cycle (see The Nitrogen Cycle as it Was, below).
Industry causes increases in nitrogen in the natural environment
In the first half of the 20th century, industrial methods of simulating what certain micro- organisms do – in this case, turning atmospheric nitrogen gas into fixed nitrogen – were developed. This led to the ability to make nitrogen-based fertilizers for plants, which had a dramatic effect on global food production. This agricultural practice was one of several modern developments that have led to an increase in the availability of nitrate and ammonium in the natural environment. When a synthetic fertilizer containing ammonium and nitrate is applied to crops, much of it may be taken up by the crops. But some of these nitrogen compounds may percolate into groundwater or run off into streams – mostly in the form of nitrate. Fixed nitrogen in fertilizers may also be converted by bacteria into nitrogen gas and released back to the atmosphere. Nitrogen also escapes from farm fields in the form of nitrous oxide (N20), a greenhouse gas and ozone- depleting substance.
Livestock operations
Livestock operations may also raise nitrogen levels in an area. The manure of farm animals like cows, pigs and sheep is rich in nitrogen compounds. When farm animals are relatively small in number and spread over a large pasture area, the nitrogen load tends to be relatively light and more or less balanced by bacterial processes and plant uptake. Synthetic fertilizers used to grow feed may allow more livestock to be raised in a given area, sometimes leading to higher nitrogen loads in that area. The traditional agricultural practice of planting leguminous crops, e.g., peas or beans, which support nitrogen-fixing bacteria has also greatly enriched the amount of nitrogen stored in the soils of agricultural areas.
Industrial and combustion processes
Industrial and combustion processes fix nitrogen in a number of forms. When naturally occurring nitrogen gas from the atmosphere enters a high temperature combustion process, nitric oxide, NO, and nitrogen dioxide, NO2, can be formed (taken together, NO and NO2 are often referred to as nitrogen oxides or NOx). In the atmosphere, NO2 can be converted to NO under the influence of ultraviolet radiation leading to the production of ozone (O3). Ozone can build up, but it can also convert NO back into NO2. These reactions are a major element of photochemical smog. Nitrogen dioxide can also mix with water in the atmosphere to form nitric acid (HNO3) and precipitate out as acid rain – often at great distances from the emission source. Major sources of NOx include vehicle engines and fossil fuel burning for electricity or industrial applications.
Many of these activities have become widespread throughout the world, and some are so massive in scale that they are causing a noticeable increase in fixed nitrogen in the natural environment. Over the last 50 years, the increase has been substantial enough to lead researchers to believe that humans may be fundamentally altering the nitrogen cycle. Evidence that human activities have radically altered the nitrogen cycle over the last century includes the fact that the amount of nitrogen available for uptake at any given time has more than doubled. While natural processes contribute about 140 million tonnes annually, human activities generate about 210 million tonnes of fixed nitrogen per year. This means that human activities now contribute more to the global supply of fixed nitrogen each year than do natural processes.
Consequences of releases of fixed nitrogen
There are many consequences of releases of fixed nitrogen to the natural environment. Nitrogen-based emissions to air can lead to acid precipitation, smog, climate change and ozone depletion. On many land-based ecosystems, certain problems appear when vegetation can no longer respond to further inputs of nitrogen, a situation called nitrogen saturation. Consequences of excess nitrogen loadings can include habitat and species loss, shifts in species composition, and releases of fixed nitrogen to water, which can lead to the death of aquatic life. The scientific understanding of the negative consequences of nitrogen pollution has advanced to the point that scientists believe it is advancing the rate of extinction for certain plant species. A study of species diversity in grasslands in Britain found that species richness was significantly lower in areas of high nitrogen pollution. Furthermore, native plant species, and those adapted to low nitrogen conditions, were often found to fare less well than introduced species.
| The Nitrogen Cycle as it Was… |
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| Nitrogen is part of every living organism. In fact, after oxygen, carbon and hydrogen, it is the most abundant element in living organisms. But prior to the industrial age, nitrogen in a form useable by plants (fixed nitrogen) was in scarce supply in the natural environment. In fact, nitrogen was scarce enough to be considered a limiting plant nutrient – a nutrient that can limit the growth of plants by its lack of availability. Nitrogen was fixed only by natural processes, which supplied an amount less than could be taken up by all the plants
in most environments, that is, there was no excess. Sources of fixed nitrogen for plant use arose primarily from two natural phenomena – nitrogen-fixing micro-organisms and lightning. Nitrogen-fixing bacteria, such as Rhizobium, are found on the root nodules of leguminous plants such as clover, alfalfa, and acacia trees. These organisms take nitrogen from the atmosphere and convert it to fixed forms of nitrogen. An arc of lightning can unite nitrogen gas (N2) with oxygen to form nitrogen dioxide (NO2), which reacts with rain water to form nitric acid (HNO3). When this rain water reaches soil, nitrate (NO3-) may be liberated for plant uptake. Additions of fixed nitrogen were small in comparison to the total amount already on the landscape that had been taken up by plants and animals. Living organisms are a major nitrogen reservoir in nature, as nitrogen is needed to make protein – an important component of cells in plants and animals. Because of this need, the size and number of higher animal life forms, e.g., mammals, were also limited historically by the availability of fixed nitrogen (since higher animals ultimately rely on the supply of nitrogen available from plant life). While in the cells of living organisms, nitrogen is effectively unavailable to support new plant life until an organism dies. When plants and animals die, bacteria and other organisms decompose this biological material. Some of these bacteria transform the nitrogen in the dead plant or animal material back into the ammonia form, (NH3 and NH4+), of nitrogen which may then be converted by bacteria to nitrite (NO2-), then to nitrate(NO3-), which plants may use. Other bacteria transform some of this nitrate to nitrogen gas (N2) and return it to the atmosphere, which makes it once again inaccessible to plants. At any given time, the total amount of biologically active nitrogen in the biosphere was controlled by a series of processes that added nitrogen to the biosphere in a form useable by plants (e.g., NO3-, NH4+, amino acids, proteins), as well as other processes that returned it to the atmosphere as nitrogen gas (N2). Through these nitrogen-fixing and denitrifying processes and because of plant uptake, fixed nitrogen availability in the environment was scarce. This is a key characteristic of the nitrogen cycle – that only so much of the element in a biologically useable form is available at any given time and that the available fixed nitrogen is readily used by plants. Little excess ever exists. This was the case until the industrial revolution. |
In Ontario, the impacts of nitrogen on the province’s forests and waters are substantial and well documented. These impacts have been the focus of a number of programs and standards, recently established or revised, which attempt to limit the damage of nitrogen emissions to the natural environment.
Nitrogen’s impact on aquatic ecosystems
In southwestern Ontario, surface water quality has become problematic because of run-off from farm fields, septic system discharge, effluent from sewage treatment plants and other problems that have arisen in the past few decades. In our 2001/2002 annual report, the ECO noted that nitrate concentrations appeared to be trending upward in surface waters in many of the river systems in agricultural areas of Ontario where sandy soils predominate. For example, the nitrate concentrations in the Middle Maitland River rose from below 1.0 mg/L in the 1970s to about 4.5 mg/L in 1994. Many forms of aquatic life are adversely affected by elevated nitrate levels. Population declines of frog and salamander species have been linked to rising nitrate levels in water, according to Environment Canada.
Nitrogen’s impact on forests
The impacts of heavy nitrogen loadings on the forest landscape are becoming better understood. Nitrogen can have at least two major effects on forests. It can act as a nutrient that stimulates forest growth in those forest ecosystems where nitrogen used to be scarce. Some researchers speculate that this is having a major effect on the earth’s carbon cycle by accelerating the withdrawal of carbon from the atmosphere. In regions where there is not enough carbonate rock to buffer the acidity from NOx emissions, these emissions can lead to acidification of forest soils. In these areas, heavy nitrogen emissions can lead to forest dieback. Consequently, loadings of fixed nitrogen in Ontario can be a point of concern for certain forested areas if the forests have evolved in an environment of low fixed nitrogen availability or in soils with very little buffering capacity. The impacts of nitrogen loadings on forest ecosystems warrant continued monitoring and investigation.
Standards to manage nitrogen in water
Ontario has two sets of water standards, the Provincial Water Quality Objectives which apply to surface water, and the Ontario Drinking Water Standards, which apply to drinking water. The former are not enforceable, and even if they were, they have no objective for nitrogen in surface water. The latter has a standard of 10.0 mg/L nitrate-N; drinking water suppliers must comply with this standard and report non-compliance to the Ministry of the Environment. Nationally, a guideline of 3.0 mg/L nitrate-N has recently been established as a Canadian Water Quality Guideline (CWQG) for surface water. In 2003, Environment Canada added ammonia to the List of Toxic Substances regulated under the Canadian Environmental Protection Act. This could require many waste water treatment systems to reassess their ammonia controls. The new CWQG for nitrate draws attention to the impact of nitrate on aquatic ecosystems, but at present there is no mechanism by which this guideline would be enforced. The Nutrient Management Act and its requirement for plans for nutrient application (see The Nutrient Management Regulation) may begin to address the issue of nitrogen release to water from some non-point sources. But its initial impact will be modest.
Standards for airborne nitrogen emissions
Both federally and provincially, there have been initiatives ongoing over the past decade to control airborne emissions of nitrogen oxides (NOx). Programs such as Ontario’s Anti- Smog Action Plan and the bi-national Canada-US Ozone Annex have set broad targets to control NOx emissions and have also worked out some of the details for emission reductions, generally established through negotiations with industrial sectors or individual companies. Emission limits based upon these agreements may then be incorporated into the certificates of approval or memoranda of understanding. As with emissions to water, reduction targets generally need to be incorporated into legal documents to allow enforceability. For an overview of some of these programs and initiatives, see Emissions Reduction Trading and NOx and SO2 Emission Limits for the Electricity Sector in the 2001/2002 ECO annual report and Ontario's Anti-Smog Action Plan in our 2002/2003 annual report.
Conclusion
Despite existing programs and standard-setting processes, nitrogen’s impacts on Ontario’s ecosystems and landscapes appear to be growing, particularly those related to water quality. For this reason, MOE should adopt a provincial water quality objective following from the federal CWQG initiative on nitrate. Second, all agencies involved in nitrogen management need to adopt a more holistic view of the impact of nitrogen emissions on the global nitrogen cycle, not unlike the way we have come to think about carbon dioxide emissions and their resulting alteration of the earth’s natural carbon cycle.
| Recommendation 14:
The ECO recommends that MOE adopt a Provincial Water Quality Objective for nitrate consistent with the Canadian Water Quality Guideline for this substance. |
| This is an article from the 2003/04 Annual Report to the Legislature from the Environmental Commissioner of Ontario. |
Citing This Article
Environmental Commissioner of Ontario. 2004. "Altering the Nitrogen Cycle: Massive Human Intervention in Nature." Choosing our Legacy, ECO Annual Report, 2003-04. Toronto, ON : Environmental Commissioner of Ontario. 186-191.
