sign in sign up
<<
Home , Nitrification, Nitrogen cycle

A. Global Pools: Nutrient Cycling 1: The cycle I. Intro to the - most in the , but not biologically available I. Introduction - reactive N in atmosphere: trace gases A. Changes to the global N cycle (Ch. 15) Productivity of many (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 and follow that 3. Consequences terrestrial – temperate, boreal, arctic B. Overview of the 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. C. Outputs 15.4 1. Gaseous losses (esp. ) 2. 12 III. 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 -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, fixation, cycle) and 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 • 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 point. –NH3: domestic animals, ag fields (fert), biomass burning • NO (highly reactive) Æ , tropospheric O3 formation - • N saturation: availability exceeds demand • (NO2 + OH Æ HNO3) • Atmospherically active Æ , air pollution –N2O: increased application Æ denitrification • Deposition, N availability downwind – Associated with decreases in forest productivity, potentially due • Potent (200x more effective than CO2, 6% of total forcing) to indirect effects such as acidification, altered plant cold • Chemically inert in , but catalyzes destruction of O3 in 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 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, 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 plant – Symbiotic N fixation (e.g., , alder) • Associative N fixers – Heterotrophic N fixation (rhizosphere and other - • 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 , the 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 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, – , 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, (free-living photo-); 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 of 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 (exoenzymes) such as proteases, ribonucleases, and chitinases to break down large polymers into water-soluble units such as amino acids and 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)nitrificationautotrophic

–Nuptake (and ) 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% • is more mobile than , 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 (other temperate coniferous forests. + - Nitroso-), NH4 Æ NO2 , mono-oxygenase, need O2 - Semi-arid forests tend • Second step conducted by , NO - Æ NO - to show more net 2 3 nitrification relative to – Controls: net N mineralization + •NH4

•O2 • Slow growth of nitrifiers 9.6 - The relationship between net nitrogen mineralization and net nitrification (μg nitrogen g-1 of dry soil for a 10-day incubation) across a range of tropical forest ecosystems (Vitousek and Matson 1984). 9.5

- Nitrification and denitrification occur under different conditions. -High nitrate concentration, much labile C, and lack of C. N outputs - Gaseous losses for both follow the “hole-in-the-pipe” model. - H-in-the-P depends on rate of flux and percent of losses. together lead to high denit. rates.

1. Gaseous losses + + – Ammonia gas (pK = 8.2, NH4 Æ NH3 + H ) –Fire

– Oxides of N (NO, N2O, N2) • NO and N2O from autotrophic nitrification

•NO, N2O, N2 from denitrification – Most denitrification conducted by heterotrophic bacteria (many are facultative - - anaerobes that use NO3 as a terminal e acceptor in the absence of O2) - • Controls: NO3 , C availability, O2, 9.4 9.7

7 Denitrification – where? • Very important in , riparian areas. C. N outputs - Leaching losses of • Spatially very patchy in well-drained soils. 2. Leaching nitrate and cations decrease with forest regrowth at Hubbard Brook. • Erosional losses -Plant and microbial • Solution losses demand - + –NO3 >> DON >NH4 – Greatest when water flux is high and biological demand for N is low (e.g., after snowmelt!)

http://en.wikipedia.org/wiki/Image:Riparian_zone_florida_everglades 9.8 http://www.wldelft.nl/cons/area/mse/ecom/im/-1.jpg

-Leaching increases when Consequences of Mississippi River N runoff: Summary: small Æ big plant and microbial The “” • Controls on mineralization (C quality, demand are exceeded AET) are similar to those for decomposition, and this is the major (e.g., N saturation). source of plant nutrients for natural ecosystems. • Humans are influencing N inputs to ecosystems: N fixation, N deposition. • Higher N availability Æ 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 pollution in (toxicity)

– Chemically active N species (NOx) in atmosphere

– Radiatively active N species (N2O) in atmosphere – Increased output to aquatic ecosystems (eutrophication). Fig. 9.9 9.2

8