The Nitrogen Cycle Aquatic Ecology Animation - Classzone

Nitrogen Cycle Link

Back to Chemistry What are the forms of N? Where do we naturally find N (forms of N)?

Forms of Nitrogen Urea  CO(NH2)2 Ammonia  NH3 (gaseous) Ammonium  NH4+ Nitrate  NO3- Nitrite  NO2- Atmospheric Dinitrogen N2 Organic N

N Cycle in Tanks

Reservoirs of Nitrogen Rocks - weathering Atmosphere – 78% of atmosphere Oceans – Soluble in water Freshwater – Headwater Streams = sinks Primary Producers – Use ammonium & nitrates to make proteins Consumers – digest proteins into AA

Nitrogen History • More money and effort are spent on the management of N and S than any other mineral nutrient: – Deficiencies are world wide, in cultivated and natural environments – Excesses cause a degradation of the quality of life, N pollution

Nitrogen History • Adding N to soils is one of the most costly parts of agriculture

Significance of N • Nitrogen (N) is an essential component of DNA, RNA and proteins, the building blocks of life • All organisms require nitrogen to live and grow • Although the majority of the air we breathe is N2, most of the nitrogen in the atmosphere is unavailable for use by organisms • This is because the strong triple bond between the N atoms in N2 molecules makes it relatively inert

Significance of N Nitrogen is an incredibly versatile element, existing in both inorganic and organic forms as well as many different oxidation states • The movement of nitrogen between the atmosphere, biosphere and geosphere in different forms is described by the nitrogen cycle

Significance of N N2 gas must first be converted to more a chemically available form such as ammonium (NH4+), nitrate (NO3-), or organic nitrogen (e.g. urea - (NH3)2CO) • The inert nature of N2 means that biologically available nitrogen is often in short supply in natural ecosystems, limiting plant growth and biomass accumulation

Chemistry of N

Chemistry of N • The valence range which N undergoes in its biogeochemical cycling is full – Going from loss of all five of its outer shell electrons (+5) to other elements – To the gain of three electrons from other elements (-3) to complete all of the orbitals of its outer electron shell

Chemistry of N On the right-hand side of the depicted N cycle, the N atom can eventually lose all five of its outer shell electrons to O • With this, N can eventually become fully oxidized as nitrate (NO3-)

Chemistry of N • On the left-hand of the depicted N cycle, N can eventually add three electrons to fill all of its outer shell electron orbitals from elements such as hydrogen (H) and carbon (C) • With such gain of electrons, N can be fully reduced to ammonia (NH3) − which most commonly exists in its ionic form, ammonium (NH4+) • Or N can be fully-to-partially reduced in organic compounds

Nitrogen Cycle - Animation

6 Important Processes Nitrogen fixation Nitrogen uptake/Assimilation Nitrogen mineralization Nitrification Denitrification Volatilization

Nitrogen Fixation N2 N2O NH4 NO2 R-NH2 NO NO2 NO3

Nitrogen fixation N2 → NH4+ • N2 is converted to ammonium • Essential because it is the only way that organisms can attain nitrogen directly from the atmosphere Energy intensive process: N2 + 8H+ + 8e- + 16 ATP = 2NH3 + H2 + 16ADP + 16 Pi

Nitrogen fixation Certain bacteria, Rhizobium, are the only organisms that fix nitrogen through metabolic processes • N fixing bacteria often form symbiotic relationships with host plants (e.g. beans, peas, and clover) • N fixing bacteria inhabit legume root nodules and receive carbohydrates and a favorable environment from their host plant in exchange for some of the nitrogen they fix

N fixation w/ Blue-Green Algae In aquatic environments, blue-green algae (really a bacteria called cyanobacteria) is an important free-living nitrogen fixer Plates 19 & 20 Anacystis bloom; Ford Lake August 2002.

Measuring water transparency with a Secchi disk on Ford Lake during a bloom of Aphanizomenon. August 2004.

N Fixation Cont’d ½ can be contributed by N-fixing org. The rest comes from atmospheric deposition (lightning) or runoff. Salmon Alders

Nitrification N2 N2O NH4 NO2 R-NH2 NO NO2 NO3

Nitrification • NH4+ → NO3- or NO2 • Some of the ammonium produced by decomposition is converted to nitrate via a process called nitrification– Nitrosomonas and Nitrobacter • The bacteria that carry out this reaction gain energy • Requires the presence of oxygen – circulating or flowing waters and the very surface layers of soils and sediments

Drinking Water The U.S. Environmental Protection Agency has established a standard for nitrogen in drinking water of 10 mg per liter nitrate-N Unfortunately, many systems (particularly in agricultural areas) already exceed this level By comparison, nitrate levels in waters that have not been altered by human activity are rarely greater than 1 mg/L Where would there be higher levels of N in drinking water?

Methemoglobinemia Nitrate is one of the most common groundwater contaminants in rural areas. It is regulated in drinking water primarily because excess levels can cause methemoglobinemia, or "blue baby" disease. Affects nursing infants b/c gut is too acidic (pH 2) for denitrifying bacteria to reduce nitrate to nitrite. Nitrite combines w/ hemoglobin to produce methemoglobin, does not break down easily or carry Oxygen. What is Methemoblobenemia

Methemoglobinemia Nitrate in groundwater originates primarily from fertilizers, septic systems, & manure storage or spreading operations. Fertilizer nitrogen that is not taken up by plants, volatilized, or carried away by surface runoff leaches to the groundwater in the form of nitrate (nitrification NH4 → NO3). This not only makes the nitrogen unavailable to crops, but also can elevate the concentration in groundwater above the levels acceptable for drinking water quality. Nitrogen from manure similarly can be lost from fields, barnyards, or storage locations. Septic systems also can elevate groundwater nitrate concentrations because they remove only half of the nitrogen in wastewater, leaving the remaining half to percolate to groundwater.

Denitrification N2 N2O NH4 NO2 R-NH2 NO NO2 NO3

Denitrification NO3- → N2+ → N2O • Oxidized forms of nitrogen such as nitrate and nitrite (NO2-) are converted to dinitrogen (N2) and, to a lesser extent, nitrous oxide gas • An anaerobic process that is carried out by denitrifying bacteria, which convert nitrate to dinitrogen in the following sequence: • NO3- → NO2- → NO → N2O → N2.

Denitrification Effluent of sewage treatment plants. Denitrification by bacteria converts nitrogen-oxygen compounds into nitrogen gas. N leaves treatment plant as a gas to reduce the amount of DIN in effluent.

N Uptake/Assimilation NH4 + → Organic N • The ammonia produced by nitrogen fixing bacteria is usually quickly incorporated into protein and other organic nitrogen compounds, either by a host plant, the bacteria itself, or another soil organism • When organisms nearer the top of the food chain eat, they are using nitrogen that has been fixed initially by nitrogen fixing bacteria

Ammonification or Mineralization NH4 NO2 R-NH2 NO NO2 NO3

Ammonification/Mineralization Organic N → NH4+ • After nitrogen is incorporated into organic matter, it is often converted back into inorganic nitrogen • During this process, usually called decay, a significant amount of the nitrogen contained within the dead organisms is converted to ammonium • Once in the form of ammonium, nitrogen is available for use by plants

Ammonia Volatilization • The process of nitrogen loss as ammonia gas from urea forms under alkaline conditions • During this process, ammonium is converted into NH3 gas which is then lost to the air • In cooler conditions the enzyme breaks down urea much slower • Thus, little ammonia gas is lost when urea is applied to cool soils Urea may originate from animal manure, urea fertilizers and, to a lesser degree, the decay of plant materials.

Excretion N compounds are metabolized by animals for energy & NH3 is a waster product. If O2 is present = oxidized to NO3 or NO4 If O2 is absent = NH3 will accumulate. Aquatic Snails – N is excreted by diffusion of (highly toxic) ammonia NH3 into the water. Terrestrial Snails – excrete N as cyclic C-N compounds (uric acid) b/c NH3 cannot be easily washed away. NH3 would poison their lungs.

Human Influence Early in the 20th century, a German scientist named Fritz Haber figured out how to fix nitrogen chemically at high temperatures and pressures, creating fertilizers that could be added directly to soil This technology has spread rapidly over the past century, and, along with the advent of new crop varieties, the use of synthetic nitrogen fertilizers has led to an enormous boom in agricultural productivity

Surface Contamination Added nitrogen can lead to nutrient overenrichment, particularly in coastal waters receiving the inflow from polluted rivers This nutrient over-enrichment, also called eutrophication, has been blamed for Increased frequencies of coastal fish-kill events, increased frequencies of harmful algal blooms, and species shifts within coastal ecosystems

Acid Rain Reactive nitrogen (like NO3- and NH4+) present in surface waters and soils, can also enter the atmosphere as the smog component nitric oxide (NO) an nitrous oxide (N2O) Eventually, this atmospheric nitrogen can be blown into nitrogen-sensitive terrestrial environments, causing long-term changes Acid rain from nitrogen oxides has been blamed for forest death and decline in parts of Europe and the Northeast United States

Acid Rain and Species Shifts Increases in atmospheric nitrogen deposition have also been blamed for more subtle shifts in dominant species and ecosystems On nitrogen-poor serpentine soils of northern Californian grasslands, plant assemblages have historically been limited to native species that can survive without a lot of nitrogen There is now some evidence that elevated levels of atmospheric N input from nearby industrial and agricultural development have paved the way for invasion by non-native plants