110/4/07 Announcements• Some Homeworks back today• Graduates: 1-page statement of proposal topic should have been turned in. If you haven’t, talk to Julie or Scott ASAPGlobal Change: Terrestrial ecosystems & biogeochemistry #2Global Change Lecture #11, S. Saleska, 4-Oct-2007I. Review main points of Ecosystem developmentII. Carbon storage and C:N stoichiometryIII. The Nitrogen Cycle & Global ChangeI. Review: things to take away from Tuesday lecture:• Definitions: Biogeochemistry, Ecosystem Ecology, and weathering• The basics of the long vs. short term carbon cycle• The ecosystem development paradigm and concept of resource limitation of NPP.• Alterations of the rate of nutrient supply can themselves be global change factors• Stoichiometry is a basic concept in ecosystem ecologyII. Stoichiometry (relations between elements, e.g. C:N:P ratios) in plant components:• Widely variable from one plant or soil component to the next • Tightly constrained within components• Fundamental indicators of nutrient economy of plants2Is N-limitation important to the excess carbon-sink debate?Leaves: 10-50Wood: 500Soils: 10-15C:N ratiospotential C-storage = (C:N) * (available N)Or, inverse problem:N-demand = C-stored / (C:N) Whole-tree: 200Carbon:Nitrogen StoichiometryLeaves: 10-50Wood: 500Soils: 10-15C:N ratiospotential C-storage = (C:N) * (available N)Or, inverse problem:N-demand = C-stored / (C:N) Question:if a model said that ecosystems could store 500 Pg C, how much more N would be needed ?Assume: ½ in soil (with C:N=15), ½ in trees (with C:N=200)Whole-tree: 200Carbon:Nitrogen StoichiometryLeaves: 10-50Wood: 500Soils: 10-15C:N ratiospotential C-storage = (C:N) * (available N)Or, inverse problem:N-demand = C-stored / (C:N) Question:if a model said that ecosystems could store 500 Pg C, how much more N would be needed ?Assume: ½ in soil (with C:N=15), ½ in trees (with C:N=200)Whole-tree: 200Answer: N-demand (soil) = 250 Pg / 15 = 16.67N-demand (trees) = 250 Pg / 200 = 1.25TOTAL 17.9 Pg NCarbon:Nitrogen Stoichiometry3Six models featured in IPCC (2001)Simulated C storage by 2100 due to:elevated CO2 aloneelevated CO2 +climate changeHungate et al (2003)Estimated N requirement to achieve modeled carbon storageMaximum estimated available Nitrogen50018Why N-limitation is important to the excess carbon-sink debateIII. NitrogenAtmosphere: N2(78%) + trace N2O, NH3, NOxSoil solution: NO3-(nitrate)(and waters) NH4+ (ammonium)Plants & soil organic matter: organic N (inorganic N)preferred form for plant uptakeIII. NitrogenWhy N is essential nutrient for plants:Atmosphere: N2(78%) + trace N2O, NH3, NOxSoil solution: NO3-(nitrate)(and waters) NH4+ (ammonium)Plants & soil organic matter: organic N Also: amino acids, proteins, etc. (inorganic N)Leaf nitrogen determines photosynthetic capacity4The Nitrogen Cycle (local)DON =dissolved org NMicrobes key to soil N transform-ationsN2, N2OfixationProcesses in the Nitrogen cycleNitrogen glossary(for reference, not memorization!)• Processes:–N fixation: N2==> NH4–Nitrification: NH4+==> NO3-(NO, N2O, NO2can be byproducts)–Denitrification: NO3==> N2; N2O byproduct dep. on conditions– Ammonification: Heterotrophic respiration ==> NH4byproduct– Mineralization: process that releases CO2 and nutrients in inorganic form– Immobilization: sequestering of nutrients in microbial biomass•Compounds:–NO2- nitrogen dioxide– NO - nitric oxide–NO3--nitrate– NOx - sum of above three –N2- nitrogen gas–N2O - nitrous oxide –NH4+- ammonium– NH3 - ammonia gas–NH2 -aminesNitrogen fixation• Conversion of atmospheric N2to NH4+• Requires abundant energy and P• Inhibited by oxygen• Carried out by bacteria– Symbiotic N fixation (e.g., Rhizobium in association with legumes, alder)– Heterotrophic N fixation (rhizosphereand other carbon-rich environments)5Paradox of nitrogen limitation(Vitousek & Howarth, 1991)• 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?Environmental limitations to N fixation• Energy availability in closed-canopy ecosystems– N fixers seldom light-limited in well-mixed aquatic ecosystems (e.g., lakes)• Nutrient limitation (e.g., P, Mo, Fe, S)– These elements may be the ultimate controls over N supply and NPP•Grazing– N fixers often preferred forageGlobal Change and the N-cycleHuman alteration of N-cycle is LARGE(> 100% in fixation relative to the background rate of 90-140 Tg N/yr):• fertilizer production (80-90 Tg/yr)• Expansion of N-fixing crops (legumes) (40 Tg/yr)• Fossil fuel combustion (20+ Tg/yr)(from N in fossil fuel + oxidation of atmospheric N2 in high-temp combustion)Vitousek et al., 1997Global Change and the N-cycleHuman alteration of N-cycle is LARGE(> 100% in fixation relative to the background rate of 90-140 Tg N/yr):Vitousek et al., 1997No place on earth unaffected by this6Units:Stocks (Tg N)orFlows (Tg N/yr)Impacts of N-cycle alterationImpacts of N-cycle alterationEutrophication of estuaries and coastal ecosystemsImpacts of N-cycle alterationEutrophication of estuaries and coastal ecosystemsBottom-dwelling plants of marine ecosystemsNatural rates of N(high diversity & spacing)Few species, leaves covered with algae7Impacts of N-cycle alterationVitousek et al., 1997Fraction of total gaseous N-emissions caused by human activities Greenhouse gas(global impact)Reactive gases = local effectNO Æ major precursor (with VOCs) to ground-level ozone (smog); acid rainNH3Æ acid-neutralizing agent in the atmosphereAcid rain (high NO-, NO3-, which also releases H+) impacts on forestImpacts of N-cycle alterationNote: pH = -log [H+] (high H+ Æ low pH Æ strong acid )Impacts of N-cycle alterationAcid rain (high NO-, NO3-, which also releases H+) impacts on forestÆ Base cation (Ca+, Mg+) depletion of soilsÆ Increased NO3-leaching and runoff (toxic in drinking water, esp. for children)Impacts of N-cycle alterationAcid rain (high NO-, NO3-, which also releases H+) impacts on forest8N-deposition and biodiversityGrassland experiments on the interactions between species composition and nitrogenMinnesota grasslandsTilman et al. (1987)N-deposition and biodiversityN-depositionTilman et al. (1987)Impacts of N-cycle alteration•