N cycle
- 1. Nutrient Cycling 1: The nitrogen cycle I. Introduction A. The global N cycle 1. Global pools and fluxes 2. Changes 3. Consequences B. The ecosystem N cycle 1. Major pools and fluxes 2. Main points II. Controls on N cycle in soils A. Inputs 1. N fixation 2. N deposition B. Internal cycling 1. Mineralization/immobilization 2. Nitrification C. Outputs 1. Denitrification 2. Leaching III. Plant uptake and loss
- 2. I. Intro to the Nitrogen Cycle Productivity of many ecosystems (managed & unmanaged) is limited by nitrogen availability: terrestrial – temperate, boreal, arctic aquatic – open oceans
- 3. Pools in Tg = 1012 g Fluxes in Tg yr-1 A. Global Pools: - most in the atmosphere, but not biologically available - lots in sediments and rocks , but not available - inorganic N in ocean is next largest - organic pools in plants and soils follow that
- 4. Pools in Tg Fluxes in Tg yr-1 Fluxes: several important biosphere-atmosphere N exchanges - biological: fixation, denitrification, nitrification - abiotic: industrial fixation, lightning fixation, fossil fuel and biomass burning, deposition
- 5. Pools in Tg Fluxes in Tg yr-1 Biological cycling within systems greatly outweighs inputs/outputs (i.e., N cycle is much more “closed” than the C cycle)
- 6. B. Human-mediated fluxes in the global N cycle now exceed ‘natural’ (pre-industrial) fluxes
- 7. C. Consequences • Eutrophication • Species changes/losses • Atmospherically active trace gases
- 8. Consequences • Eutrophication • Species changes/losses • Atmospherically active trace gases Tilman 1987 N fert increasing prod. N fert increasing dominance, decreasing diversity
- 9. Consequences• Eutrophication • Species changes/losses • Atmospherically active trace gases – NH3: domestic animals, ag fields (fert), biomass burning • Atmospherically active aerosols, air pollution • Deposition, N availability downwind 15.4
- 10. B. Overview of Ecosystem N cycle 1. Major pools & fluxes 2. Main Points 1. Inputs~outputs 2. Open (C) vs. closed (N) 3. Plant needs met by internal recycling 4. Available soil pools are small relative to organic pools. 5. B-G microbes rule! 9.2
- 11. II. Controls on N cycle fluxes in soil A. N Inputs 1. Biological N fixation 2. Atmospheric N deposition 3. Mineral weathering
- 12. 1. Biological N Fixation a. What is it? • Conversion of atmospheric N2 to NH4 + (actually, amino acids) • Under natural conditions, nitrogen fixation is the main pathway by which new, available nitrogen enters terrestrial ecosystems
- 13. Nitrogen fixation b. Who does it? • Carried out only by bacteria – Symbiotic N fixation (e.g., legumes, alder) – Heterotrophic N fixation (rhizosphere and other carbon- rich environments) – Phototrophs (bluegreen bacteria) • The characteristics of nitrogenase, the enzyme that catalyzes the reduction of N2 to NH4 +, dictate much of the biology of nitrogen fixation – High-energy requirement (N triple bond) • Requires 16 ATP per N2 molecule fixed!! – Inhibited by O2
- 14. Types of N-fixers • There’s no such thing as a N-fixing plant • Symbiotic N-fixers – High rates of fixation (5-20 gN m-2 y-1) with plants supplying the C (and the plant receiving N) – Protection from O2 via leghaemoglobin (legumes) – Microbial symbiont resides in root nodules • Bacteria (Rhizobium) – Legumes (Lupinus, Robinia)
- 15. Types of N-fixers • Part of a clover root system bearing naturally occurring nodules of Rhizobium. • Each nodule is about 2-3 mm long • Clover root nodules showing two partly crushed nodules (arrowheads) with pink- colored contents. • This color is caused by the presence of the pigment leghaemoglobin - a unique metabolite of this type of symbiosis. • Leghaemoglobin is found only in the nodules and is not produced by either the bacterium or the plant when grown alone.
- 16. Types of N fixers • Rhizobium – root nodules of leguminous plants • Azotobacter – aerated soil • Clostridium – anaerobic soils • Cyanobacteria – wet soil
- 17. Types of N fixers • Free-living N fixers – Heterotrophic bacteria that get organic C from environment and where N is limiting (e.g., decaying logs) – Rates low due to low C supply and lack of O2 protection (0.1-0.5 g-N m-2 y-1) • Also, cyanobacteria (free-living photo-autotrophs); symbiotic lichens (cyanobacteria with fungi offering physical protection)
- 18. When/where does it happen? N-fixing species are common in early succession - Lichens early in primary succession following deglaciation in Alps. - Alder at later stages.
- 19. Red alder in secondary succession following logging
- 20. Paradox of N limitation • Nitrogen is the element that most frequently limits terrestrial NPP • N2 is the most abundant component of the atmosphere • Why doesn’t nitrogen fixation occur almost everywhere??? • Why don’t N fixers have competitive advantage until N becomes non- limiting?
- 21. Environmental limitations to N fixation • Nutrient limitation (e.g., P, Mo, Fe, S) – These elements may be the ultimate controls over N supply and NPP • Grazing – N fixers are often the preferred food of herbivores
- 22. Internal Cycling of Nitrogen • In ecosystems, most N taken up by plants becomes available through decomposition of organic matter – Over 90% of soil nitrogen is organically bound in detritus in a form unavailable to organisms – The soil microflora secrete extracellular enzymes (exoenzymes) such as proteases, ribonucleases, and chitinases to break down large polymers into water-soluble units such as amino acids and nucleotides that can be absorbed
- 23. • The pools – Plant biomass – SOM (solid; including litter) – Microbial biomass – DON (a variable portion “plant available”) – NH4 + (plant available) – NO3 - (plant available) • The processes: – (Gross) N mineralization – (Gross) N immobilization – (Gross) autotrophic nitrification – N uptake (and assimilation) by plants Internal Cycling of Nitrogen
- 24. 2. Nitrification a. Why is Nitrification Important? • Nitrate is more mobile than ammonium, so more readily leached from soil • Substrate for denitrification (N loss as a gas) • Generates acidity if nitrate is lost from soil • Loss of nitrate results in loss of base cations
- 25. 2.b. Controls on Nitrification • NH4 + NO2 NO3 - – Two-step process conducted by chemoautotrophic bacteria: • First step conducted by Nitrosomonas ( NH4 + NO2 - , ammonia mono-oxygenase, need O2 ) • Second step conducted by Nitrobacter, NO2 - NO3 - – Controls: • Level of NH4 + • Presence of O2 • Naturally slow growth of nitrifiers
- 26. 9.4 - Nitrification and denitrification occur under different conditions. - Gaseous losses for both follow the “hole-in-the-pipe” model. - H-in-the-P depends on rate of flux and percent of losses.
- 27. Denitrification – where? • Very important in wetlands areas… • But very patchy in well-drained agricultural soils. http://www.wldelft.nl/cons/area/mse/ecom/im/wetland-1.jpg http://en.wikipedia.org/wiki/Image:Riparian_zone_florida_everglades.
- 28. N outputs 2. Leaching • Erosional losses • Solution losses – NO3 - >> DON >NH4 + – Greatest when water flux is high and biological demand for N is low (e.g., after snowmelt!)
- 29. Consequences of Mississippi River N runoff: The Gulf of Mexico “Dead Zone”
- 30. Summary: • Humans are influencing N inputs to ecosystems: N fixation, N deposition. • Higher N availability greater plant growth, until demand saturates. • Microbes compete with plants for available N. • Presence of substrate (NH4 +) is a major controller of nitrification; nitrate is much more susceptible to loss than ammonium. • Losses of N cause – Nitrate and nitrite pollution in groundwater (toxicity) – Increased output to aquatic ecosystems (eutrophication). 9.2
- 31. Questions • 1. Suggest why aerial deposition of nitrogen is higher near motorways. • 2. Suggest two reasons why insects spend less time chewing leaves with a high nitrogen content. • 3. Suggest two reasons why this might result in a higher population of insects.