Human impact on the nitrogen cycle
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Human impact on the nitrogen cycle is diverse. Agricultural and industrial nitrogen (N) inputs to the environment currently exceed inputs from natural N fixation.[1] As a consequence of anthropogenic inputs, the global nitrogen cycle (Fig. 1) has been significantly altered over the past century. Global atmospheric nitrous oxide (N2O) mole fractions have increased from a pre-industrial value of ~270 nmol/mol to ~319 nmol/mol in 2005.[2] Human activities account for over one-third of N2O emissions, most of which are due to the agricultural sector.[2] This article is intended to give a brief review of the history of anthropogenic N inputs, and reported impacts of nitrogen inputs on selected terrestrial and aquatic ecosystems.
History of anthropogenic nitrogen inputs
Food Types | Acidifying Emissions (g SO2eq per 100g protein) |
---|---|
Beef | 343.6
|
Cheese | 165.5
|
Pork | 142.7
|
Lamb and Mutton | 139.0
|
Farmed Crustaceans | 133.1
|
Poultry | 102.4
|
Farmed Fish | 65.9
|
Eggs
|
53.7
|
Groundnuts | 22.6
|
Peas
|
8.5
|
Tofu | 6.7
|
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
Until 1850, natural BNF, cultivation-induced BNF (e.g., planting of
Since the
Impacts of anthropogenic inputs on the nitrogen cycle
Between 1600 and 1990, global reactive nitrogen (Nr) creation had increased nearly 50%.[6] During this period, atmospheric emissions of Nr species reportedly increased 250% and deposition to marine and terrestrial ecosystems increased over 200%.[6] Additionally, there was a reported fourfold increase in riverine dissolved inorganic N fluxes to coasts.[6] 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.[7][8]
Atmosphere
Food Types | Greenhouse Gas Emissions (g CO2-Ceq per g protein) |
---|---|
Ruminant Meat | 62
|
Recirculating Aquaculture | 30
|
Trawling Fishery | 26
|
Non-recirculating Aquaculture | 12
|
Pork | 10
|
Poultry | 10
|
Dairy | 9.1
|
Non-trawling Fishery | 8.6
|
Eggs
|
6.8
|
Starchy Roots
|
1.7
|
Wheat | 1.2
|
Maize | 1.2
|
Legumes
|
0.25
|
Atmospheric N inputs mainly include oxides of N (NOx), ammonia (NH3), and nitrous oxide (N2O) from aquatic and terrestrial ecosystems,[4] and NOx from fossil fuel and biomass combustion.[1]
In
Biosphere
Part of a series on |
Biogeochemical cycles |
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Terrestrial and aquatic ecosystems receive Nr inputs from the atmosphere through wet and dry deposition.[1] 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]).[1] Aquatic ecosystems receive additional nitrogen from surface runoff and riverine inputs.[8]
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.
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
Anthropogenic sources of N generally reach upland forests through
A 15-year study of chronic N additions at the Harvard Forest Long Term Ecological Research (
Impacts on plant species diversity
Many plant communities have evolved under low nutrient conditions; therefore, increased N inputs can alter biotic and abiotic interactions, leading to changes in community composition. Several nutrient addition studies have shown that increased N inputs lead to dominance of fast-growing plant species, with associated declines in species richness.
In a more recent experimental study of N fertilization and disturbance (i.e., tillage) in old field succession, it was found that species richness decreased with increasing N, regardless of disturbance level. Competition experiments showed that competitive dominants excluded competitively inferior species between disturbance events. With increased N inputs, competition shifted from belowground to aboveground (i.e., to competition for light), and patch colonization rates significantly decreased. These internal changes can dramatically affect the community by shifting the balance of competition-colonization tradeoffs between species.[21] In patch-based systems, regional coexistence can occur through tradeoffs in competitive and colonizing abilities given sufficiently high disturbance rates.[27] That is, with inverse ranking of competitive and colonizing abilities, plants can coexist in space and time as disturbance removes superior competitors from patches, allowing for establishment of superior colonizers. However, as demonstrated by Wilson and Tilman, increased nutrient inputs can negate tradeoffs, resulting in competitive exclusion of these superior colonizers/poor competitors.[21]
Aquatic ecosystems
Aquatic ecosystems also exhibit varied responses to nitrogen enrichment. NO3− loading from N saturated, terrestrial ecosystems can lead to acidification of downstream freshwater systems and eutrophication of downstream marine systems. Freshwater acidification can cause aluminium toxicity and mortality of pH-sensitive fish species. Because marine systems are generally nitrogen-limited, excessive N inputs can result in water quality degradation due to toxic algal blooms, oxygen deficiency, habitat loss, decreases in biodiversity, and fishery losses.[8]
Acidification of freshwaters
Atmospheric N deposition in terrestrial landscapes can be transformed through soil microbial processes to biologically available nitrogen, which can result in surface-water acidification, and loss of biodiversity. NO3− and NH4+ inputs from terrestrial systems and the atmosphere can acidify freshwater systems when there is little buffering capacity due to soil acidification.[8] N pollution in Europe, the Northeastern United States, and Asia is a current concern for freshwater acidification.[28] Lake acidification studies in the Experimental Lake Area (ELA) in northwestern Ontario clearly demonstrated the negative effects of increased acidity on a native fish species: lake trout (Salvelinus namaycush) recruitment and growth dramatically decreased due to extirpation of its key prey species during acidification.[29] Reactive nitrogen from agriculture, animal-raising, fertilizer, septic systems, and other sources have raised nitrate concentrations in waterways of most industrialized nations. Nitrate concentrations in 1,000 Norwegian lakes had doubled in less than a decade. Rivers in the northeastern United States and the majority of Europe have increased ten to fifteen fold over the last century. Reactive nitrogen can contaminate drinking water through runoff into streams, lakes, rivers, and groundwater. In the United States alone, as much as 20% of groundwater sources exceed the World Health Organization's limit of nitrate concentration in potable water. These high concentrations can cause "blue baby disease" where nitrate ions weaken the blood's capacity to carry oxygen. Studies have also linked high concentrations of nitrates to reproductive issues and proclivity for some cancers, such as bladder and ovarian cancer.[30]
Eutrophication of marine systems
Urbanization, deforestation, and agricultural activities largely contribute sediment and nutrient inputs to coastal waters via rivers.
Integration
The above system responses to reactive nitrogen (Nr) inputs are almost all exclusively studied separately; however, research increasingly indicates that nitrogen loading problems are linked by multiple pathways transporting nutrients across system boundaries.[1] This sequential transfer between ecosystems is termed the nitrogen cascade.[6] (see illustration from United Nations Environment Programme). During the cascade, some systems accumulate Nr, which results in a time lag in the cascade and enhanced effects of Nr on the environment in which it accumulates. Ultimately, anthropogenic inputs of Nr are either accumulated or denitrified; however, little progress has been made in determining the relative importance of Nr accumulation and denitrification, which has been mainly due to a lack of integration among scientific disciplines.[1][34]
Most Nr applied to global agroecosystems cascades through the atmosphere and aquatic and terrestrial ecosystems until it is converted to N2, primarily through denitrification.[1] Although terrestrial denitrification produces gaseous intermediates (nitric oxide [NO] and nitrous oxide [N2O]), the last step—microbial production of N2— is critical because atmospheric N2 is a sink for Nr.[34] Many studies have clearly demonstrated that managed buffer strips and wetlands can remove significant amounts of nitrate (NO3−) from agricultural systems through denitrification.[35] Such management may help attenuate the undesirable cascading effects and eliminate environmental Nr accumulation.[1]
Human activities dominate the global and most regional N cycles.[36] N inputs have shown negative consequences for both nutrient cycling and native species diversity in terrestrial and aquatic systems. In fact, due to long-term impacts on food webs, Nr inputs are widely considered the most critical pollution problem in marine systems.[8] In both terrestrial and aquatic ecosystems, responses to N enrichment vary; however, a general re-occurring theme is the importance of thresholds (e.g., nitrogen saturation) in system nutrient retention capacity. In order to control the N cascade, there must be integration of scientific disciplines and further work on Nr storage and denitrification rates.[34]
See also
References
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- ^ a b c d e f Schlesinger, W. H. 1997. Biogeochemistry: An analysis of global change, San Diego, CA.
- ^ a b c d Smil, V. 2001. Enriching the earth: Fritz Haber, Carl Bosch, and the transformation of world food production. MIT Press, Cambridge, MA.
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Further reading
- Good, A. G.; Beatty, P. H. (2011). "Fertilizing Nature: A Tragedy of Excess in the Commons". PLOS Biology. 9 (8): e1001124. PMID 21857803..
- Scarsbrook M.; Barquin J.; Gray D. (2007). New Zealand coldwater springs and their biodiversity (PDF). )
- Olde Venterink, H.; Wassen, M. J.; Verkroost, A. W. M.; De Ruiter, P. C. (2003). "Species Richness–Productivity Patterns Differ Between N-, P-, and K-Limited Wetlands" (PDF). Ecology. 84 (8): 2191–2199. JSTOR 3450042. Archived from the original(PDF) on 2016-03-03. Retrieved 2009-09-03.