Biogeochemical Cycles Lecture prepared by Aimee C. Wyrick Chapter 22 Biogeochemical Cycles Lecture prepared by Aimee C. Wyrick

Chapter 22 Biogeochemical Cycles Many chemical reactions take place in abiotic components of the ecosystem Atmosphere Water Soil Parent material The biogeochemical cycle is the cyclic flow of nutrients from the nonliving to the living and back to the nonliving components of the ecosystem

22.1 There Are Two Major Types of Biogeochemical Cycles Two types of biogeochemical cycles In gaseous biogeochemical cycles, the main pools of nutrients are the atmosphere and the oceans Global Nitrogen, carbon dioxide, oxygen

22.1 There Are Two Major Types of Biogeochemical Cycles In sedimentary biogeochemical cycles, the main pool of nutrients is the soil, rocks, and minerals Inorganic sources of minerals are released to living animals through weathering and erosion Phosphorus

22.1 There Are Two Major Types of Biogeochemical Cycles Hybrid of gaseous and sedimentary cycles occur Sulfur

22.1 There Are Two Major Types of Biogeochemical Cycles Both gaseous and sedimentary cycles Involve biological and nonbiological processes Are driven by the flow of energy through the ecosystem Are tied to the water cycle Biogeochemical cycles could not exist without the water cycle

22.1 There Are Two Major Types of Biogeochemical Cycles All biogeochemical cycles have a common structure Inputs Internal cycling Outputs

Figure 22.1

22.2 Nutrients Enter the Ecosystem via Inputs The input of nutrients depends on the cycle Nutrients with a gaseous cycle enter the ecosystem via the atmosphere Nutrients with a sedimentary cycle enter the ecosystem via weathering of rocks and minerals

22.2 Nutrients Enter the Ecosystem via Inputs Supplementing soil nutrients (of terrestrial habitats) are carried by rain, snow, air currents, and animals Wet fall are those nutrients supplied by precipitation Dry fall are the nutrients brought in by airborne particles and aerosols The sources of nutrients for aquatic ecosystems From the surrounding land in the form of drainage water, detritus, sediment Form the atmosphere in precipitation

22.3 Outputs Represent a Loss of Nutrients from the Ecosystem The output (export) of nutrients depends on the cycle Release of CO2 from expiration of heterotrophic organisms Organic matter can be carried out of an ecosystem Through surface flow of water or underground flow of water By herbivores Nutrients are released slowly from organic matter as it is decomposed

22.3 Outputs Represent a Loss of Nutrients from the Ecosystem Human harvesting (farming and logging) Nutrient loss must be replaced by fertilizers Fire converts a portion of the standing biomass and soil organic matter to ash Leaching and erosion of soil

22.4 Biogeochemical Cycles Can Be Viewed from a Global Perspective Often, the output from one ecosystem represents an input to another The exchange of nutrients among ecosystems requires us to view the biogeochemical processes on a broad spatial scale This is particularly true of nutrients that go through a gaseous cycle

22.5 The Carbon Cycle Is Closely Tied to Energy Flow Carbon is so closely tied to energy flow that the two are inseparable Ecosystem productivity = grams C fixed/m2/year Inorganic carbon dioxide is the source of all carbon The inorganic carbon is fixed into the living component through photosynthesis Carbon dioxide is again released following respiration

22.5 The Carbon Cycle Is Closely Tied to Energy Flow Terrestrial cycling of carbon Input: photosynthesis Output: respiration, decomposition, combustion Net primary productivity = carbon uptake (photosynthesis) – carbon loss (respiration) Net ecosystem productivity = difference in rates

22.5 The Carbon Cycle Is Closely Tied to Energy Flow The rate of carbon cycling is determined by the rates of primary productivity and decomposition The rates of primary productivity and decomposition are directly affected by temperature and precipitation In warm, wet ecosystems (e.g., tropical rain forest), production and decomposition rates are high and carbon cycles through the ecosystem quickly When dead material has not completely decomposed in the past (e.g., in swamps) the matter has formed fossil fuels

22.5 The Carbon Cycle Is Closely Tied to Energy Flow Aquatic cycling of carbon Input: photosynthesis, diffusion, transport Output: respiration, decomposition, diffusion Significant amounts of carbon can be bound as carbonates incorporated into exoskeletons (e.g., shells) of many aquatic organisms

Figure 22.2

22.6 Carbon Cycling Varies Daily and Seasonally Carbon dioxide concentration fluctuates throughout the day This is a function of the difference in photosynthetic activity in response to sunlight and temperature

Figure 22.3

22.6 Carbon Cycling Varies Daily and Seasonally The production and use of carbon dioxide fluctuates with the seasons This is a function of temperature and timing of the growing and dormant seasons With the onset of the growing season, the atmospheric concentration begins to drop as plants withdraw carbon dioxide through photosynthesis The fluctuations are greater in terrestrial environments as compared to aquatic ecosystems Fluctuations are much greater in the Northern Hemisphere due to the larger land area

Figure 22.4

22.7 The Global Carbon Cycle Involves Exchanges among the Atmosphere, Oceans, and Land Earth’s carbon budget is linked to the atmosphere, land, and oceans and to the mass movements of air currents The Earth contains 1023 grams (or 100 million gigatons) of carbon! All but a small fraction of this carbon is buried in sedimentary rock and is not actively involved in the global carbon cycle

22.7 The Global Carbon Cycle Involves Exchanges among the Atmosphere, Oceans, and Land Carbon pool involved in the global carbon cycle amounts to 55,000 gigatons (Gt) Fossil fuels: 10,000 Gt Oceans: 38,000 Gt (mostly as bicarbonate and carbonate ions) Dead organic matter: 1650 Gt Living matter (mostly phytoplankton): 3 Gt Terrestrial Dead organic matter (in soil): 1500 Gt Living matter: 560 Gt Atmosphere: 750 Gt

Figure 22.5

22.7 The Global Carbon Cycle Involves Exchanges among the Atmosphere, Oceans, and Land The surface water acts as the site of main exchange of carbon dioxide between atmosphere and ocean Uptake of CO2 depends on its reaction with carbonate ions (CO32–) to form bicarbonates (HCO3–) Carbon circulates physically by means of currents and biologically through photosynthesis and movement through the food chain Net uptake of carbon in oceans = 1 Gt/year Net loss of carbon in oceans (due to sedimentation) = 0.5 Gt/year

22.7 The Global Carbon Cycle Involves Exchanges among the Atmosphere, Oceans, and Land Recent studies suggest that the terrestrial surface is a carbon sink, with a net uptake of CO2 from the atmosphere Uptake of CO2 from the atmosphere by terrestrial systems is determined by photosynthesis CO2 losses from terrestrial systems are a function of respiration (especially decomposition)

22.7 The Global Carbon Cycle Involves Exchanges among the Atmosphere, Oceans, and Land More carbon is stored in soils than in living matter The average carbon/volume of soil increases from the tropical regions poleward to the boreal forest and tundra The greatest accumulation of organic matter occurs in areas where decomposition is inhibited (e.g., frozen or waterlogged soils)

22.8 The Nitrogen Cycle Begins with Fixing Atmospheric Nitrogen Nitrogen is an essential constituent of protein, a building block of all living tissue Nitrogen is available to plants in two forms Ammonium (NH4+) Nitrate (NO3–) The Earth’s atmosphere is 80 percent nitrogen in the form of N2 This form is unavailable to plants for assimilation

Figure 22.6

22.8 The Nitrogen Cycle Begins with Fixing Atmospheric Nitrogen Nitrogen enters the ecosystem via two pathways Atmospheric deposition via wetfall and dryfall provides nitrogen in a form already available for plant uptake High-energy fixation occurs when gaseous nitrogen (N2) is converted to ammonia and nitrate by energy from cosmic radiation, meteorite trails, or lightning — this accounts for only 0.4 kg N/ha annually

22.8 The Nitrogen Cycle Begins with Fixing Atmospheric Nitrogen Atmospheric nitrogen can be converted into a usable form biologically — this accounts for 10 kg N/ha annually This fixation is carried out by: Symbiotic bacteria living in mutualistic associations with plants Free-living aerobic bacteria Cyanobacteria (blue-green algae) Nitrogen fixation requires considerable energy To fix 1 g of nitrogen, nitrogen-fixing bacteria must expend about 10 g of glucose!

22.8 The Nitrogen Cycle Begins with Fixing Atmospheric Nitrogen Rhizobium bacteria are symbiotic organisms and form nodules in the roots of host plants Associated with leguminous plants Free-living soil bacteria (Azotobacter, Clostridium) are prominent in converting nitrogen into a usable form Cyanobacteria (Nostoc, Calothrix) fix nitrogen in terrestrial and aquatic ecosystems Certain lichens may also fix nitrogen

22.8 The Nitrogen Cycle Begins with Fixing Atmospheric Nitrogen Ammonification occurs when ammonium (NH4+) is converted to NH3 as a waste product of microbial activity Loss of gaseous NH3 from the soil to the atmosphere is influenced by soil pH

22.8 The Nitrogen Cycle Begins with Fixing Atmospheric Nitrogen Nitrification is the stepwise conversion of NH4+ to NO2– (by Nitrosomonas) and then conversion of NO2– to NO3– (by Nitrobacter) The nitrate may be taken up by plant roots or returned to the atmosphere

22.8 The Nitrogen Cycle Begins with Fixing Atmospheric Nitrogen Denitrification is the chemical reduction of NO3– to N2O and N2 (by Pseudomonas) which are then returned to the atmosphere This reduction requires anaerobic conditions This process is common in wetland ecosystems and bottom sediments of aquatic ecosystems

Figure 22.7

22.8 The Nitrogen Cycle Begins with Fixing Atmospheric Nitrogen Nitrate is the most common form of nitrogen exported from terrestrial ecosystems in stream water The amount of nitrogen recycled is usually much greater than inputs or outputs of nitrogen

22.8 The Nitrogen Cycle Begins with Fixing Atmospheric Nitrogen Nitrogen fixation and nitrification are influenced by environmental conditions Bacterial activity is affected by temperature, moisture, and soil pH In highly acidic soils, bacterial action is inhibited

22.8 The Nitrogen Cycle Begins with Fixing Atmospheric Nitrogen The internal cycling of nitrogen is fairly similar from ecosystem to ecosystem Assimilation of NH4+ and NO3– by plants Return of nitrogen to the soil, sediments, and water via decomposition

22.8 The Nitrogen Cycle Begins with Fixing Atmospheric Nitrogen The nitrogen pool Atmosphere: 3.9 × 1021 g Terrestrial Biomass: 3.5 × 1015 g Soils: 95 × 1015 to 140 × 1015 g

22.8 The Nitrogen Cycle Begins with Fixing Atmospheric Nitrogen Nitrogen loss Terrestrial and aquatic denitrification: 200 × 1012 g/yr Sedimentation Nitrogen input Freshwater drainage: 36 × 1012 g/yr Precipitation: 30 × 1012 g/yr Biological fixation: 15 × 1012 g/yr

Figure 22.8

22.8 The Nitrogen Cycle Begins with Fixing Atmospheric Nitrogen Human activity has significantly influenced the global nitrogen cycle Conversion of native forests and grasslands to agricultural fields Application of chemical fertilizers to agricultural fields Auto exhaust and combustion add N2O, NO, and NO2 to the atmosphere, which leads to an increase in ozone concentration of the stratosphere

22.8 The Nitrogen Cycle Begins with Fixing Atmospheric Nitrogen Animation: Energy Flow and Nutrient Cycling – Part 1

22.8 The Nitrogen Cycle Begins with Fixing Atmospheric Nitrogen Animation: Energy Flow and Nutrient Cycling – Part 2

22.9 The Phosphorus Cycle Has No Atmospheric Pool Phosphorus (P) can only be cycled from land to sea and is not returned via the biogeochemical cycle The main reservoirs of P are rock and natural phosphate deposits Phosphorus is released by weathering, leaching, erosion, and mining In most soils, only a small fraction of total phosphorus is available to plants

Figure 22.9

22.9 The Phosphorus Cycle Has No Atmospheric Pool In freshwater and marine ecosystems, the phosphorus cycle moves through three states Particulate organic phosphorus (PP) Dissolved organic phosphates (PO) Inorganic phosphates (Pi)

22.9 The Phosphorus Cycle Has No Atmospheric Pool Organic phosphates are taken in by all forms of phytoplankton, which are eaten by zooplankton Zooplankton may excrete as much phosphorus daily as it stores in its biomass Some phosphorus is deposited in sediments  surface waters may become depleted while the deep waters become saturated This phosphorus can be returned to the surface waters and accessed by organisms when upwelling occurs

22.9 The Phosphorus Cycle Has No Atmospheric Pool Little atmospheric component although airborne transport of ~1 × 1012 g P/yr River transport = 21 × 1012 g P/yr (only 10 percent is available for NPP) Ocean waters are a significant global pool of P simply due to large volume Organic phosphorus in the surface waters is recycled very rapidly The phosphorous deposited in sediments or deep waters is unavailable to phytoplankton until upwelling

Figure 22.10

Ecological Issues Nitrogen Saturation NPP in most terrestrial forest ecosystems is limited by soil nitrogen availability. Anthropogenic activity and high-intensity agriculture have increased inputs of nitrogen oxides in the atmosphere far above natural inputs Generally, nitrogen is deposited in the region where it originated and thus will vary with geography and human population density

Figure 1

Ecological Issues Nitrogen Saturation Soil nitrogen concentration influences the rate of N uptake and plant tissue concentration Up to a point, as nitrogen concentration increases, net primary productivity increases

Ecological Issues Nitrogen Saturation As the ecosystem approaches “nitrogen saturation,” the soil and plant community suffer negative impacts Release of other important soil cations (e.g., Mg) as ammonium concentration increases Soil acidification that may contribute to toxic levels of aluminum ions

Figure 2

22.10 The Sulfur Cycle Is Both Sedimentary and Gaseous The sulfur cycle has both sedimentary and gaseous phases In the long-term sedimentary phase, sulfur is tied up in organic and inorganic deposits and is released by weathering and decomposition The gaseous phase permits sulfur to circulate on a global scale

Figure 22.11

22.10 The Sulfur Cycle Is Both Sedimentary and Gaseous Atmospheric sulfur sources (as H2S) Combustion of fossil fuels Volcanic eruptions Ocean surface exchange Decomposition Atmospheric sulfur dioxide (SO2) is carried back to the surface in rainwater as weak sulfuric acid (H2SO4)

22.10 The Sulfur Cycle Is Both Sedimentary and Gaseous Sulfur is incorporated into plants via photosynthesis and building of sulfur-bearing amino acids Excretion and death return sulfur from living material back to the soil and sediments Bacteria release it as hydrogen sulfite or sulfate Colorless, green, and purple bacteria each have a unique interaction with sulfur

22.10 The Sulfur Cycle Is Both Sedimentary and Gaseous Pyritic rocks (those that contain FeS) can be a source of sulfur if weathered or uncovered by humans (during coal mining) These products (e.g., sulfuric acid) can be extremely detrimental to aquatic ecosystems

22.11 The Global Sulfur Cycle Is Poorly Understood The annual flux of sulfur compounds (SO2, H2S, sulfate particles) through the atmosphere ~300 × 1012 g Wetfall and dryfall of sulfate particles

22.11 The Global Sulfur Cycle Is Poorly Understood Oceans are a large source sulfate aerosols, though most are redeposited in precipitation and dryfall Dimethylsulfide [(CH3)2S] is the major sulfur gas emitted (16 × 1012 g S/yr) from the oceans and is generated by biological processes H2S is the dominant sulfur form emitted from freshwater wetlands and anoxic soils

22.11 The Global Sulfur Cycle Is Poorly Understood Forest fires emit 3 × 1012 g S annually Volcanic activity contributes to the global cycle of sulfur

Figure 22.12

22.12 The Oxygen Cycle Is Largely Under Biological Control The major source of oxygen (O2) that supports life is the atmosphere and may originate from two processes Breakup of water vapor = 2 H2O  O2 + 4 H+ Photosynthetic production The input of oxygen must have exceeded its loss (due to respiration) for an overall abundance of oxygen

22.12 The Oxygen Cycle Is Largely Under Biological Control Water and carbon dioxide are other sources of oxygen Oxygen is also biologically exchangeable in various molecules that are transformed by living organisms (e.g., hydrogen sulfide to sulfates)

Figure 22.13

22.12 The Oxygen Cycle Is Largely Under Biological Control Due to oxygen’s reactivity, its cycling in the ecosystem is complex Carbon dioxide + calcium  carbonates Nitrogen compounds  nitrates Iron compounds  ferric oxides

22.12 The Oxygen Cycle Is Largely Under Biological Control Ozone (O3) is an atmospheric gas In the stratosphere (10 to 40 km above Earth) it acts as a UV shield Close to the ground, it is a pollutant

22.12 The Oxygen Cycle Is Largely Under Biological Control In the stratosphere, O2 is freed by solar radiation and freed oxygen atoms rapidly combine with O2 to form O3 (this reaction is reversible) Under natural conditions, a balance exists between ozone formation and destruction Human activity has interrupted this balance, and various molecules (e.g., CFCs) reduce the production of O3

22.13 The Various Biogeochemical Cycles Are Linked The biogeochemical cycles are linked through their common membership in compounds that form an important component of their cycles Nitrate and oxygen in nitrate Autotrophs and heterotrophs require nutrients in different proportions for different processes Stoichiometry is the branch of chemistry that deals with the quantitative relationships of elements in combination

22.13 The Various Biogeochemical Cycles Are Linked The limitation of one nutrient can affect the cycling of all the others (e.g., macro and micro plant nutrients) Nitrogen availability will influence a plant’s rubisco concentration Rubisco concentration affects photosynthetic rate and carbon assimilation The carbon cycle is directly affected by nitrogen availability