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The Nitrogen Cycle I

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The Nitrogen Cycle I A. Global Pools: Nutrient Cycling 1: The nitrogen cycle I. Intro to the Nitrogen Cycle - most in the atmosphere, but not biologically available I. Introduction - reactive N in atmosphere: trace gases A. Changes to the global N cycle (Ch. 15) Productivity of many ecosystems (managed & unmanaged) is - lots in sediments and rocks, but not available 1. Global pools and fluxes - inorganic N in ocean is next largest 2. Changes limited by nitrogen availability: - organic pools in plants and soils follow that 3. Consequences terrestrial – temperate, boreal, arctic B. Overview of the ecosystem N cycle (Ch. 9) 1. Major pools and fluxes aquatic – open oceans 2. Main points II. Controls on N cycle fluxes in soils (Ch. 9) A. Inputs 1. N fixation 2. N deposition B. Internal cycling 1. Mineralization/immobilization 2. Nitrification C. Outputs 15.4 1. Gaseous losses (esp. denitrification) 2. Leaching 12 III. Plant uptake and loss (Ch. 8) Pools in Tg = 10 g Fluxes in Tg yr-1 Powerpoint modified from Harte & Hungate (http://www2.for.nau.edu/courses/hart/for479/notes.htm) and Chapin (http://www.faculty.uaf.edu/fffsc/) Fluxes: several important biosphere-atmosphere N exchanges Biological cycling within systems greatly outweighs B. Human-mediated fluxes in the global N cycle now exceed - biological: fixation, denitrification, nitrification inputs/outputs (i.e., N cycle is much more “closed” than the C ‘natural’ (pre-industrial) fluxes - abiotic: industrial fixation, lightning fixation, cycle) fossil fuel and biomass burning, deposition 15.4 15.4 Pools in Tg Pools in Tg Fluxes in Tg yr-1 Fluxes in Tg yr-1 15.5 1 C. Consequences Consequences •Eutrophication •Eutrophication • Species changes/losses • Species changes/losses How much N is added in agriculture? • Atmospherically active trace gases • Atmospherically active trace gases Cotton 56-78 Kg/ha • Iowa corn 170-225 Kg/ha N fert Æ increasing prod. • Taiwan rice: 270 Kg/ha N fert Æ increasing dominance, decreasing diversity Tilman 1987 Consequences •Eutrophication Consequences Consequences •Eutrophication • Species changes/losses • Species changes/losses •N deposition Æ increased growth (C sequestration)…to a • Atmospherically active trace gases • Atmospherically active trace gases –NO + NO2 (NOx): fossil fuel combustion point. –NH3: domestic animals, ag fields (fert), biomass burning • NO (highly reactive) Æ smog, tropospheric O3 formation - • N saturation: availability exceeds demand •Acid rain (NO2 + OH Æ HNO3) • Atmospherically active Æ aerosols, air pollution –N2O: increased fertilizer application Æ denitrification • Deposition, N availability downwind – Associated with decreases in forest productivity, potentially due • Potent greenhouse gas (200x more effective than CO2, 6% of total forcing) to indirect effects such as acidification, altered plant cold • Chemically inert in troposphere, but catalyzes destruction of O3 in stratosphere tolerance –NH3 • N saturation Æ N losses Æ “opening” of the N cycle 15.3 15.4 2 B. Overview of Ecosystem N cycle (Ch. 9) 1. Major pools & fluxes II. Controls on N cycle fluxes in soil 2. Main Points 1. Biological N Fixation 1. Inputs~outputs A. N Inputs 2. Open (C) vs. closed (N) 3. Plant needs met by internal recycling 1. Biological N fixation a. What is it? 4. Available soil pools are + 2. Atmospheric N deposition • Conversion of atmospheric N2 to NH4 small relative to organic pools. 3. Mineral weathering? (actually, amino acids) 5. Microbes rule bg • Under natural conditions, nitrogen fixation is the main pathway by which new, available nitrogen enters terrestrial ecosystems 9.2 Nitrogen fixation Types of N-fixers Types of N fixers b. Who does it? • There’s no such thing as a N-fixing • Carried out by bacteria plant – Symbiotic N fixation (e.g., legumes, alder) • Associative N fixers – Heterotrophic N fixation (rhizosphere and other carbon- • Symbiotic N-fixers rich environments) – High rates of fixation (5-20+ g-N m-2 y-1) – Occur in rhizosphere of plants (non-nodulated); – Phototrophs (bluegreen algae) with plants supplying the C (and the plant moderate rates with C supply from plant root • The characteristics of nitrogenase, the enzyme that receiving N) -2 -1 + turnover and exudates (1-5 g-N m y ) catalyzes the reduction of N2 to NH4 , dictate much of the –Protection from O2 via leghemoglobin biology of nitrogen fixation (legumes) –Reduced [O2] by rapid respiration from plant – High-energy requirement (N triple bond) – Microbial symbiont resides in root roots • Requires abundant energy and P for ATP nodules – Inhibited by O2 • Bacteria (Rhizobia) – Legumes (Lupinus, – Azotobacter, Bacillus – Requires cofactors (e.g., Mo, Fe, S) Robinia) • Actinomycetes (Frankia) - Alnus, Ceanothus (woody non-legumes) – N-fixation rates reduced in presence of high N availability in the soil 3 Types of N fixers Red alder in secondary succession following • Free-living N fixers clearcutting near Lake Whatcom – Heterotrophic bacteria that get organic C from environment and where N is limiting (e.g., decaying logs) C. When/where does it happen? -2 – Rates low due to low C supply and lack of O2 protection (0.1-0.5 g-N m y-1) N-fixing species are common in •Also, cyanobacteria (free-living photo-autotrophs); symbiotic early succession lichens (cyanobacteria with fungi offering physical protection) - Lichens early in primary succession following deglaciation in Alaska. - Alder at later stages. Photo: D. Hooper Photo: D. Hooper Alder and the other woody hosts of Frankia are typical Environmental limitations to pioneer species that d. Paradox of N limitation invade nutrient-poor soils. These plants N fixation probably benefit from the nitrogen- • Nitrogen is the element that most • Energy availability in closed-canopy fixing association, while supplying the frequently limits terrestrial NPP ecosystems bacterial symbiont with photosynthetic – N-fixers seldom light-limited in well- products. •N2 is the most abundant component of the atmosphere mixed aquatic ecosystems (e.g., lakes) • Why doesn’t nitrogen fixation occur • Nutrient limitation (e.g., P, Mo, Fe, S) almost everywhere? – These elements may be the ultimate controls over N supply and NPP • Why don’t N fixers have competitive advantage until N becomes non- •Grazing limiting? – N fixers often preferred forage 4 Deposition depends on upwind sources A. Inputs 2. Nitrogen Deposition Wet deposition typically scales • Wet deposition: dissolved in precipitation Dry deposition a with greater proportion of • Dry deposition: dust or aerosols by precipitation. total deposition in more arid climates sedimentation (vertical) or impaction (Pawnee, CO) (horizontal) • Cloud water: water droplets to plant Dry deposition surfaces immersed in fog; only important in can be significant even coastal and mountainous areas in humid climates. Adirondacks Appalachians N species in deposition depends on type B. Internal Cycling of Nitrogen of source 3. Rock weathering as a source of N? • In natural ecosystems, most N taken up by • Some sedimentary rocks contain plants becomes available through substantial amounts of N with high rates decomposition of organic matter of N release (up to 2 g-N m-2 y-1); however, – Over 90% of soil nitrogen is organically bound in detritus in a form unavailable to organisms most rocks contain little N. – 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 http://pah.cert.ucr.edu/aqm/ndep/results.shtml 5 Internal Cycling of Nitrogen • The pools Net Ain’t Gross –Plant biomass Net Ain’t Gross – SOM (solid; including litter) • Net rates of N transformations – Microbial biomass (mineralization and nitrification) • Similarly… – DON (a variable portion “plant - available”) Net nitrification = Δ NO3 pool + - –NH+ (plant available) Net N mineralization = Δ (NH4 +NO3 pools) - 4 = gross nitrification – gross NO3 - –NO3 (plant available) = gross N mineralization-gross N immobilization immobilization • The processes: – (Gross) N mineralization – (Gross) N immobilization – (Gross) autotrophic nitrification –Nuptake (and assimilation) by plants 9.2 1. Mineralization/immobilization -Mineralization is closely linked to decomposition. Critical litter C:N for net N min. (box 9.1) -Plant functional types: effects via litter quality influence on both 2. Nitrification breakdown of plant material and immob by microbes. a. Why is Nitrification Important? -Climate affects mineralization via decomposition (microbial activity). • Microbial C:N ~10:1 -Species effects can be much greater than differences in climate. • Microbial growth efficiency ~40% • Nitrate is more mobile than ammonium, so more • So, for 100 units C, 40 units Æ mic readily leached from soil biomass, 60 units respired. • Substrate for denitrification (N loss as a gas) • For mic C:N of 10:1, need 4 units of N per • Generates acidity if nitrate is lost from soil 40 units C. • Loss of nitrate results in loss of base cations • So substrate needs C:N of 100:4 (i.e., 25:1) for net N mineralization. 9.3 6 - Substrate limitation is common. Nearly all nitrogen that is - Nitrifiers are obligate aerobes. 2.b. Controls on Nitrification mineralized in these systems is nitrified on a net basis. + - + •NH4 + 2O2 Æ NO3 +2H + H2O -In contrast, net nitrification is frequently – Two-step process conducted by less than 25% of net chemoautotrophic bacteria: mineralization in • First step conducted by Nitrosomonas (other temperate
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