Human impact on the nitrogen cycle is diverse. Agricultural and industrial nitrogen (N) inputs to the environment currently exceed inputs from natural N fixation. This is largely attributed to the widespread use of synthetic fertilizers produced via the Haber-Bosch process, and the expansion of fossil fuel combustion following the Industrial Revolution. As a consequence of anthropogenic inputs, the global nitrogen cycle has been significantly altered over the past century, leading to increased nitrogen deposition and disruption of natural biogeochemical processes. Global atmospheric nitrous oxide (N2O) mole fractions have increased from a pre-industrial value of ~270 nmol/mol to ~319 nmol/mol in 2005, a concerning trend given that N2O is both a greenhouse gas and an ozone-depleting substance (ODS).Alley et al. 2007. IPCC Climate Change 2007: The Physical Science Basis. Contribution of Working Group I in the Third Assessment Report of Intergovernmental Panel on Climate Change. Report Summary for Policy Makers (SPM) . Human activities account for over one-third of N2O emissions (35% of total global N2O emissions), with about 20% originating from agricultural sources such as fertilizer use, manure management, and aquaculture. An additional 12% of emissions arise from other anthropogenic activities including fossil fuel combustion, industrial processes, waste and wastewater treatment, and biomass burning. This article is intended to give a brief review of the history of anthropogenic N inputs, reported impacts of nitrogen inputs on selected terrestrial and aquatic ecosystems, and global players in N2O emissions.
History of anthropogenic nitrogen inputs Approximately 78% of Earth's atmosphere is N gas (N2), which is an inert compound and biologically unavailable to most organisms. In order to be utilized in most biological processes, N2 must be converted to reactive nitrogen (Nr), which includes inorganic reduced forms (NH3 and NH4+), inorganic oxidized forms (NO, NO2, HNO3, N2O, and NO3−), and organic compounds (urea, amines, and proteins). N2 has a strong triple bond, and so a significant amount of energy (226 kcal mol−1) is required to convert N2 to Nr. Prior to industrial processes, the only sources of such energy were solar radiation and electrical discharges. Utilizing a large amount of metabolic energy and the enzyme nitrogenase, some bacteria and cyanobacteria convert atmospheric N2 to NH3, a process known as biological nitrogen fixation (BNF).Schlesinger, W. H. 1997. Biogeochemistry: An analysis of global change, San Diego, CA. The anthropogenic analogue to BNF is the Haber-Bosch process, in which H2 is reacted with atmospheric N2 at high temperatures and pressures to produce NH3.Smil, V. 2001. Enriching the earth: Fritz Haber, Carl Bosch, and the transformation of world food production. MIT Press, Cambridge, MA. Lastly, N2 is converted to NO by energy from lightning, which is negligible in current temperate ecosystems, or by fossil fuel combustion.
Until 1850, natural BNF, cultivation-induced BNF (e.g., planting of leguminous crops), and incorporated organic matter were the only sources of N for agricultural production. Near the turn of the century, Nr from guano and sodium nitrate deposits was harvested and exported from the arid Pacific islands and South American deserts. By the late 1920s, early industrial processes, albeit inefficient, were commonly used to produce NH3. Due to the efforts of Fritz Haber and Carl Bosch, the Haber-Bosch process became the largest source of nitrogenous fertilizer after the 1950s, and replaced BNF as the dominant source of NH3 production. From 1890 to 1990, anthropogenically created Nr increased almost ninefold. During this time, the human population more than tripled, partly due to increased food production.
Since the Industrial Revolution, an additional source of anthropogenic N input has been fossil fuel combustion, which is used to release energy (e.g., to power automobiles). As fossil fuels are burned, high temperatures and pressures provide energy to produce NO from N2 oxidation. Additionally, when fossil fuel is extracted and burned, fossil N may become reactive (i.e., NOx emissions). During the 1970s scientists began to recognize that N inputs were accumulating in the environment and affecting ecosystems.
Impacts of anthropogenic inputs on the nitrogen cycle Between 1600 and 1990, global reactive nitrogen (Nr) creation had increased nearly 50%. During this period, atmospheric emissions of Nr species reportedly increased 250% and deposition to marine and terrestrial ecosystems increased over 200%. Additionally, there was a reported fourfold increase of inorganic nitrate carried by rivers from inland areas into coastal waters. Nitrogen is a critical limiting nutrient in many systems, including forests, wetlands, and coastal and marine ecosystems; therefore, this change in emissions and distribution of Nr has resulted in substantial consequences for aquatic and terrestrial ecosystems.
Atmosphere Atmospheric N inputs mainly include oxides of N (NOx), ammonia (NH3), and nitrous oxide (N2O) from aquatic and terrestrial ecosystems, and NOx from fossil fuel and biomass combustion.
In agroecosystems, fertilizer application has increased microbial nitrification (aerobic process in which microorganisms oxidize ammonium [NH4+] to nitrate [NO3−]) and denitrification (anaerobic process in which microorganisms reduce NO3− to atmospheric nitrogen gas [N2]). Both processes naturally leak nitric oxide (NO) and nitrous oxide (N2O) to the atmosphere. Of particular concern is N2O, which has an average atmospheric lifetime of 114–120 years,John T. Houghton, Y. Ding, D. J. Griggs, M. Noguer, P. J. van der Linden, X. Dai, K. Maskell, and C. A. Johnson. 2001. IPCC Climate Change 2001: The Scientific Basis. Contribution of Working Group I in the Third Assessment Report of Intergovernmental Panel on Climate Change. Cambridge University Press] and is 300 times more effective than CO2 as a greenhouse gas. NOx produced by industrial processes, automobiles and agricultural fertilization and NH3 emitted from soils (i.e., as an additional byproduct of nitrification) and livestock operations are transported to downwind ecosystems, influencing N cycling and nutrient losses. Six major effects of NOx and NH3 emissions have been cited: 1) decreased atmospheric visibility due to ammonium aerosols (fine particulate matter [PM]); 2) elevated ozone concentrations; 3) ozone and PM affects human health (e.g. respiratory diseases, cancer); 4) increases in radiative forcing and global climate change; 5) decreased agricultural productivity due to ozone deposition; and 6) ecosystem acidification and eutrophication.
Biosphere
Terrestrial and aquatic ecosystems receive Nr inputs from the atmosphere through wet and dry deposition. Atmospheric Nr species can be deposited to ecosystems in precipitation (e.g., NO3−, NH4+, organic N compounds), as gases (e.g., NH3 and gaseous nitric acid [HNO3]), or as aerosols (e.g., ammonium nitrate [NH4NO3]). Aquatic ecosystems receive additional nitrogen from surface runoff and riverine inputs.
Increased N deposition can acidify soils, streams, and lakes and alter forest and grassland productivity. In grassland ecosystems, N inputs have produced initial increases in productivity followed by declines as critical thresholds are exceeded. Nitrogen effects on biodiversity, carbon cycling, and changes in species composition have also been demonstrated. In highly developed areas of near shore coastal ocean and estuarine systems, rivers deliver direct (e.g., surface runoff) and indirect (e.g., groundwater contamination) N inputs from agroecosystems. Increased N inputs can result in freshwater acidification and eutrophication of marine waters.
Terrestrial ecosystems
Impacts on productivity and nutrient cycling
Much of terrestrial growth in temperate systems is limited by N; therefore, N inputs (i.e., through deposition and fertilization) can increase N availability, which temporarily increases N uptake, plant and microbial growth, and N accumulation in plant biomass and soil organic matter.Aber, J. D., K. J. Nadelhoffer, P. Steudler, and J. M. Melillo. 1989. "Nitrogen saturation in northern forest ecosystems". Bioscience 39:378–386 Incorporation of greater amounts of N in organic matter decreases C:N ratios, increasing mineral N release (NH4+) during organic matter decomposition by heterotrophic microbes (i.e., ammonification). As ammonification increases, so does nitrification of the mineralized N. Because microbial nitrification and denitrification are "leaky", N deposition is expected to increase trace gas emissions. Additionally, with increasing NH4+ accumulation in the soil, nitrification processes release hydrogen ions, which acidify the soil. NO3−, the product of nitrification, is highly mobile and can be leached from the soil, along with positively charged alkaline minerals such as calcium and magnesium. In acid soils, mobilized aluminium ions can reach toxic concentrations, negatively affecting both terrestrial and adjacent aquatic ecosystems.
Anthropogenic sources of N generally reach upland forests through deposition. A potential concern of increased N deposition due to human activities is altered nutrient cycling in forest ecosystems. Numerous studies have demonstrated both positive and negative impacts of atmospheric N deposition on forest productivity and carbon storage. Added N is often rapidly immobilized by microbes, and the effect of the remaining available N depends on the plant community's capacity for N uptake. In systems with high uptake, N is assimilated into the plant biomass, leading to enhanced net primary productivity (NPP) and possibly increased carbon sequestration through greater photosynthetic capacity. However, ecosystem responses to N additions are contingent upon many site-specific factors including climate, land-use history, and amount of N additions. For example, in the Northeastern United States, hardwood stands receiving chronic N inputs have demonstrated greater capacity to retain N and increase annual net primary productivity (ANPP) than conifer stands. Once N input exceeds system demand, N may be lost via leaching and gas fluxes. When available N exceeds the ecosystem's (i.e., vegetation, soil, and microbes, etc.) uptake capacity, N saturation occurs and excess N is lost to surface waters, groundwater, and the atmosphere. N saturation can result in nutrient imbalances (e.g., loss of calcium due to nitrate leaching) and possible forest decline.
A 15-year study of chronic N additions at the Harvard Forest Long Term Ecological Research (LTER) program has elucidated many impacts of increased nitrogen deposition on nutrient cycling in temperate forests. It found that chronic N additions resulted in greater leaching losses, increased pine mortality, and cessation of biomass accumulation. Another study reported that chronic N additions resulted in accumulation of non-photosynthetic N and subsequently reduced photosynthetic capacity, supposedly leading to severe carbon stress and mortality. These findings negate previous hypotheses that increased N inputs would increase NPP and carbon sequestration.
Impacts on plant species diversity