Nitrogen dilemma

In the past decade, nitrogen management has increasingly attracted the attention of agricultural and climate scientists.

NITROGEN (N) is Janus-faced: it has a good side and a bad side. It is necessary for all forms of life it is an essential component of nucleic acids and proteins, the building blocks of life and crucial for the increased food production needed to feed the world's population. On the other hand, the increasing nitrogen load in the atmosphere due to nitrogen leakages from diverse anthropogenic activities, in particular agriculture and energy production from fossil fuels, has several adverse effects on the environment, human health and biodiversity and contributes to global warming. Herein lies the basic dilemma, and in the past decade or so, nitrogen management has increasingly attracted the attention of agricultural and climate scientists.

Although 78 per cent of the atmosphere by volume is nitrogen, it is not in a form that is biologically available to organisms. This is because the triple bond between the nitrogen atoms in molecular nitrogen (N2) is the strongest bond in nature. Breaking it requires a significant amount of energy. For plants and animals to use nitrogen, this molecular nitrogen needs to be converted, or fixed, to nitrogen compounds that are chemically reactive as against the inert N2. Reactive nitrogen (Nr) thus includes all biologically, photochemically and radiatively active nitrogen compounds in the earth's atmosphere and biosphere essentially all forms of nitrogen excluding N2. Lack of an adequate supply of Nr can limit the growth of crops, preventing them from achieving their maximum production potential. In many ecosystems, the limiting factor for growth is nitrogen. However, excess Nr causes a range of environmental and human health problems, which have negative social and economic consequences.

Nr can be created either by natural processes or by human intervention. Under natural conditions, Nr is created either by high-temperature processes that can break the triple bond, such as lightning and forest fires, or by certain specialised microbes through a process called biological nitrogen fixation (BNF). These microbes, actually bacteria, reside symbiotically in certain legumes, including soybeans. In oceans, BNF is carried out by blue-green algae or cyanobacteria.

Before the mid-19th century, the Nr necessary to support human populations was adequately met through BNF, the use of animal manure or crop rotation. By the turn of the 19th century, however, the increasing human demand for food and the resultant expansion of agriculture, on the one hand, and the rising demand for nitrates to manufacture munitions, led Fritz Haber in Germany to invent in 1909 a high-pressure chemical process to produce reactive forms of nitrogen from its molecular form. This was later improved and scaled up to industrial production by Carl Bosch in 1913. This method of artificially fixing nitrogen led to the mass production of chemical fertilizers through the synthesis of ammonia (NH3) from the gases hydrogen and nitrogen.

Even after 100 years, the Haber-Bosch process remains the most economical process of anthropogenic fixing of nitrogen. Ammonia is the major reactive form of nitrogen used for industrial processes and for fertilizer production. This additional Nr availability has contributed to increased crop and meat production (through the increased production of fodder such as soybean) and to intensive agriculture. The increase in agricultural yields due to synthetic fertilizers sustains nearly half of the current world population. At the same time, many areas of the world do not have access to this anthropogenic Nr, resulting in the malnutrition of over one billion people.

Today, human activity has doubled the level of Nr in circulation. In the past six decades, anthropogenic production of Nr has outstripped production from all natural ecosystems. Although most of the anthropogenic Nr is associated with the use of nitrogenous fertilizers in food production, energy production from burning fossil fuels also contributes significantly to the release of Nr. Compared with the late 19th century, food and energy production has now increased the anthropogenic Nr creation rate by a factor of 10. Excess Nr in the atmosphere due to human intervention has a snowballing adverse effect on human health and ecosystems. Agricultural activities cause the emission of NH3, nitrous oxide (N2O) and nitrogen monoxide (NO) into the atmosphere and the release of nitrate (NO3-) and ammonium (NH4+) ions into groundwater and surface water. The combustion of fossil fuels by fixed sources (households and industries, particularly in the energy sector) and mobile sources (transportation) causes emissions of NO and nitrogen dioxide, or NO2, (collectively known as NOx). Fossil fuel is used in the Haber-Bosch process too.

NOx also contributes to acidification and eutrophication when it and other forms of Nr are deposited through rain or snowfall on land or in waterbodies. (Eutrophication refers to excessive accumulation of nutrients in aquatic ecosystems, which results in the domination of the plant community by one or a few species, usually harmful algal blooms). While acidification decreases the availability of important nutrients by leaching from the soil, eutrophication leads to a depletion of oxygen and results in hypoxic or anoxic conditions developing in waterbodies and a reduction in biodiversity. This deposition of Nr can affect biodiversity and the health of aquatic and terrestrial ecosystems. NOx also contributes to the formation of nitrogenous particulate matter (aerosols), which can lead to various respiratory illnesses when inhaled.

More than half of the synthetic N-fertilizer ever produced has been manufactured since 1985. From 1998 to 2010, the amount of Nr created has increased from 150 million tonnes to 200 million tonnes. It is projected to increase in a linear fashion in the years to come. The Food and Agriculture Organisation (FAO) stated in 2009 that by 2050 the world would have to produce 70 per cent more food than is being currently produced to sustain the then expected world population of around nine billion people. This, according to estimates, will require an average annual increase in crop production of 44 million tonnes, which implies a 38 per cent increase over historical production increases to be sustained for 40 years. Many parts of the world are limited by the availability of agricultural land. If encroachment on natural forests, grasslands and wetlands is to be avoided, intensification of agriculture seems to be the only option. The increasing population, the need to reduce malnutrition, increasing prosperity, increasing meat consumption and increasing production of bio-energy crops (especially biofuel crops in the context of climate change) are other factors that make the growth of N-fertilizer production and application inevitable.

The ever-increasing population and its increased generation of sewage and industrial waste have also increased the inputs of Nr into the environment. In many developing countries, the majority of wastewater is released untreated directly into waterways. According to the United Nations Environment Programme (UNEP), fewer than 35 per cent of the cities in developing countries have sewage treatment. Where it does exist, it is usually only primary treatment, which is not enough to remove nitrogen. Even in the developed world, treatment facilities often do not include tertiary treatment, which is necessary for nitrogen removal. In some localities, says the UNEP, wastewater is the largest source of release of Nr into the environment.

While carbon dioxide (CO2) is the most important greenhouse gas (GHG) contributing to climate change, nitrous oxide, or N2O, which is chiefly produced in nitrogen-rich soils and waterbodies, also has an impact on climate change. Its global warming potential (when integrated over 100 years) is 298 times that of an equal mass of CO2. As noted in the Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC), the average lifetime of N2O is about 120 years, making it one of the most persistent GHGs. The global atmospheric concentration of N2O is now 18 per cent higher than in pre-industrial times, and it has continued to increase by 0.3 per cent every year since 1980.

According to the UNEP, more than a third of all N2O emissions are caused by human activities, chiefly agriculture with its increased use of N-fertilizers and animal manure. At present, N2O is responsible for 11 per cent of the net anthropogenic radiative forcing. NOx also contributes to positive radiative forcing as it is a precursor to the formation of ozone (O3) in the troposphere (lower atmosphere), an important GHG. It is also a major pollutant affecting human health and terrestrial ecosystems. Some forms of nitrogen contribute to aerosol formation, which results in negative forcing (cooling). In addition, nitrogen also has indirect links to climate through carbon-nitrogen (C-N) interactions in ecosystems. On the other hand, Nr can stimulate plant photosynthesis and thus help absorb atmospheric CO2 (carbon sequestration).

In the context of climate change, it has thus become increasingly clear that the carbon cycle (C-cycle) and the nitrogen cycle (N-cycle) are closely linked and both are changing rapidly. There is a high correlation between the increase in Nr production and CO2 emissions. As J.W. Erisman of the Energy research Centre of the Netherlands (ECN) points out, both N-cycle changes and GHG emissions have the same drivers, food and energy production, and the sources are also similar.

The linkages

International climate negotiations, including the IPCC assessments, have hitherto failed to include these N-cycle changes in assessing the long-term effect on the climate except indirectly by including N2O as one of the GHG gases whose emissions have to be cut down. Scientists point out that fossil fuel CO2 emission targets to stabilise atmospheric CO2 may not be sufficient if one takes into account C-N interactions. Such C-N linkages have, however, not been adequately quantified, which is one of the reasons why climate negotiations have so far ignored the role of nitrogen. But an understanding of the linkages between heightened levels of Nr and the C-cycle and role of nitrogen in climate is growing rapidly.

Given the importance of the increasing Nr in the atmosphere and its diverse effects, the scientific community in different parts of the world is making efforts to quantitatively assess its relation to climate and arrive at an appropriate framework for policy guidelines for nitrogen management. This led to the formation in 2003 of the International Nitrogen Initiative (INI) under the International Geosphere-Biosphere Programme (IGBP) of the International Council for Science (ICSU). In India, the Indian Nitrogen Group (ING), too, has initiated studies to quantify the impact of anthropogenic Nr. Indeed, this initiative was in part responsible for the 5th International Nitrogen Conference (N2010) coming to India and being held in December 2010 in New Delhi.

Recent research suggests that nitrogen management will generate substantial reductions in carbon emissions and realise other benefits, including improved provision of ecosystem services such as the provision of clean water. With this perspective in mind, during the U.N. Climate Change Conference in Copenhagen (COP-15) in December 2009, a side event was organised where many organisations came together to discuss issues concerning nitrogen and climate. The meeting decided to produce a draft report on Nitrogen and Climate to identify options of nitrogen management so that its availability for food and energy production and carbon storage could be increased while its impact could be reduced.

The report was initiated by the executive body of the Convention on Long-Range Transboundary Air Pollution (CLRTAP) of the U.N. Economic Commission for Europe (UNECE) through the institution of a Task Force on Reactive Nitrogen (TFRN). An outline of this report was presented to the IPCC in October 2010. An IPCC workshop on Nitrogen and Climate will be held in 2011. Erisman presented the draft report at the Delhi nitrogen conference. The final report will be presented and launched at an upcoming conference on Nitrogen and Global Change in April 2011, which will provide inputs to the IPCC's Fifth Assessment Report (AR5) scheduled to be issued in 2014.

About half the CO2 emitted into the atmosphere is taken up by the biosphere but nitrogen has an influence on the uptake capacity. Until the pre-industrial era, there was equilibrium between the C- and N-cycles. Changes in the natural N-cycle have an effect on GHG emissions and climate, and conversely, changes in the CO2 cycle and climate affect the natural N-cycle. Natural variations before industrialisation were much smaller compared with the current perturbations caused by human activities. The effects of anthropogenic Nr are both direct and indirect.

Once an Nr molecule is created, it may remain in the environment for a considerable time. It is estimated that about half the Nr created through agriculture will end up in the food chain or in industrial products or is denitrified back to N2. The rest is, however, lost in reactive form to the atmosphere and to aquatic ecosystems, causing a host of problems including groundwater pollution, freshwater eutrophication, estuary eutrophication and hypoxia, human health impacts, and ecosystem and crop yield impacts as it cascades through various components of the environment. The end point of the cascade is ultimately the emission of denitrified N2 or N2O.

The important direct links between Nr and climate include, as mentioned earlier, N2O formation with its long residence time in the atmosphere and high global warming potential, ground-level formation of ozone, an important GHG, by the interaction of NOx with volatile organic compounds (VOCs) and aerosol formation by the action of NH3 on gaseous acids such as nitric acid to yield finely dispersed particles of ammonium nitrate and ammonium sulphate, which are transported across great distances in the atmosphere and result in the precipitation of Nr many thousands of kilometres from the source. Aerosols have a cooling effect directly through their negative radiative forcing and indirectly through cloud formation.

The indirect links are equally important in their impact on climate. Since half the CO2 emitted is taken up by the biosphere, an increased supply of Nr leads to changes in the capacities of biospheric sinks of CO2. Increased Nr affects the net CO2 uptake by terrestrial systems, rivers, estuaries and the open ocean by increasing their net productivity or reducing the rate of organic decay. However, this also has a long-term counter-effect. Increased oceanic uptake of CO2 results in acidification, which reduces CO2 uptake.

These competing processes, however, need to be quantified. Excess Nr deposition on ecosystems can either increase or decrease ecosystem productivity and hence carbon sequestration. So the levels of Nr production and deposition are important, which will, of course, differ from system to system. In the Indian context, the ING has noted that there are no data on N-deposition. On the basis of data from the European Nitrogen Assessment (ENA) project, the ING has emphasised that N-deposition in Indian conditions will be significant and that data need to be collected to remove uncertainties.

When Nr input from natural or anthropogenic sources is small, there is a net CO2 uptake up to a maximum. This is particularly significant when availability of Nr is the limiting nutrient factor for growth. When this maximum is exceeded, the net CO2 uptake may remain stable. But at some point, as the Nr input increases even more, CO2 uptake can decline as existing plant-soil systems get destabilised. Further, when Nr input reaches levels far in excess of plant requirements, more N2O is produced, which negates the benefits of higher biospheric C-sequestration. To maximise the second stage, where there is a net benefit of CO2 uptake, careful management of Nr and soil conditions are necessary.

According to the draft TFRN report on Nitrogen and Climate, migration from the last to the second stage is a no-regret environmental action as it has a net cooling effect and at the same time reduces other environmental problems. Migration from the second to the first stage, however, has the best societal benefits, and these could be large if one considers health impacts alone, says the report. The report, therefore, emphasises the need to consider the full cascade of Nr and include impacts on water and air quality, human health, biodiversity and other ecosystem services.

Currently, as mentioned before, there is only a qualitative estimate of these linkages globally. Within the ENA project, the first quantitative estimate was made recently. The table describes the current state of knowledge globally. According to the ENA's quantitative estimates, the net cooling effects (43 milliwatt/m2), arising chiefly from aerosols and additional CO2 uptake due to Nr, are larger than the warming effects (25 mW/m2) by nearly a factor of 2. However, the report points out that the net cooling effect of 18 mW/m2 is not significantly different from zero given the uncertainties in the estimates.

The converse effect of climate change on the N-cycle also needs to be quantified. Climate change leads to temperature and precipitation changes, causing shifts in growing seasons, different wind regimes, changes in sea level, and so on. Emissions of VOCs and NH3 are, for example, strongly temperature dependent, and a warmer climate will enhance their emission, atmospheric transport and deposition. Similarly, BNF, mineralisation from organic matter, nitrification and denitrification are affected by temperature, Nr availability and moisture. The extent of warming also affects concentrations of O3, CO2 and oxidised (NOx) and reduced nitrogen (NHx) compounds, N2O- and N-containing aerosol distribution, which would have feedbacks on the net GHG balance.

The biogeochemical cycle of Nr, says the report, is therefore linked to climate in profound but non-linear ways that are, at present, difficult to predict. Nevertheless, the potential for significant amplification of Nr-related impacts is substantial. There is currently not enough quantitative information about the influence of climate change and increasing CO2 concentrations on the N-cycle. Therefore, it is not possible to determine what we under- or overestimate if we do not take these relationships into account.