Nutrient Cycles


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Nutrient Cycles

  1. 1.
  2. 2. NUTRIENT CYCLES: ECOSYSTEM TO ECOSPHERE <ul><li>Nutrient cycling occurs at the local level through the action of the biota. </li></ul><ul><li>Nutrient cycling occurs at the global level through geological processes, such as, atmospheric circulation, erosion and weathering. </li></ul>
  3. 3. NUTRIENT CYCLES <ul><li>The atoms of earth and life are the same; they just find themselves in different places at different times. </li></ul><ul><li>Most of the calcium in your bones came from cows, who got it from corn, which took it from rocks that were once formed in the sea. </li></ul><ul><li>The path atoms take from the living (biotic) to the non-living (abiotic) world and back again is called a biogeochemical cycle . </li></ul>
  4. 4. Nutrients: The Elements of Life <ul><li>Of the 50 to 70 atoms (elements) that are found in living things, only 15 or so account for the major portion of living biomass. </li></ul><ul><li>Only around half of these 15 have been studied extensively as they travel through ecosystems or circulate on a global scale. </li></ul>
  5. 5. A GENERALIZED MODEL OF NUTRIENT CYCLING IN AN ECOSYSTEM <ul><li>The cycling of nutrients in an ecosystem are interlinked by an a number of processes that move atoms from and through organisms and to and from the atmosphere, soil and/or rocks, and water. </li></ul><ul><li>Nutrients can flow between these compartments along a variety of pathways. </li></ul>
  6. 6. Nutrient Compartments in a Terrestrial Ecosystem <ul><li>The organic compartment consists of the living organisms and their detritus. </li></ul><ul><li>The available-nutrient compartment consists of nutrients held to surface of soil particles or in solution. </li></ul><ul><li>The third compartment consists of nutrients held in soils or rocks that are unavailable to living organisms. </li></ul><ul><li>The fourth compartment is the air which can be found in the atmosphere or in the ground. </li></ul>
  7. 7. Uptake of Inorganic Nutrients from the Soil <ul><li>With the exception of CO 2 and O 2 which enter though leaves, the main path of all other nutrients is from the soil through the roots of producers. </li></ul><ul><li>Even consumers which find Ca, P, S and other elements in the water they drink, obtain the majority of these nutrients either directly or indirectly from producers. </li></ul>
  8. 8. The Atmosphere Is a Source of Inorganic Nutrients <ul><li>The atmosphere acts as a reservoir for carbon dioxide (CO 2 ), oxygen (O 2 ) and water (H 2 O). </li></ul><ul><li>These inorganic compounds can be exchanged directly with the biota through the processes of photosynthesis and respiration. </li></ul><ul><li>The most abundant gas in the atmosphere is nitrogen (N 2 );about 80% by volume. Its entry into and exit from the biota is through bacteria. </li></ul>
  9. 9. Some Processes By Which Nutrients Are Recycled <ul><li>Cycling within an ecosystem involves a number of processes. </li></ul><ul><li>These are best considered by focusing attention on specific nutrients. </li></ul>
  10. 10. CARBON, HYDROGEN AND OXYGEN CYCLES IN ECOSYSTEMS <ul><li>C, H & O basic elements of life; making up from about 98% of plant biomass. </li></ul><ul><li>CO 2 and O 2 enter biota from the atmosphere. </li></ul><ul><li>Producers convert CO 2 and H 2 O into carbohydrates (C H 2 O compounds) and release O 2 from water. </li></ul><ul><li>Producers, consumers and decomposers convert C H 2 O compounds, using O 2 , back into CO 2 and H 2 O . </li></ul>
  11. 11. CARBON, HYDROGEN AND OXYGEN CYCLES IN ECOSYSTEMS <ul><li>Carbon and oxygen cycle come out of the air as carbon dioxide during photosynthesis and are returned during respiration. </li></ul><ul><li>Oxygen is produced from water during photosynthesis and combines with the hydrogen to form water during respiration. </li></ul>
  12. 12. PHOSPHOROUS CYCLE IN ECOSYSTEMS <ul><li>Phosphorus, as phosphate (PO 4 -3 ), is an essential element of life. </li></ul><ul><li>It does not cycle through atmosphere, thus enters producers through the soil and is cycled locally through producers, consumers and decomposers. </li></ul><ul><li>Generally, small local losses by leaching are balanced by gains from the weathering of rocks. </li></ul><ul><li>Over very long time periods (geological time) phosphorus follows a sedimentary cycle. </li></ul>
  13. 13. NITROGEN CYCLE IN ECOSYSTEMS <ul><li>Nitrogen (N 2 ) makes up 78% of the atmosphere. </li></ul><ul><li>Most living things, however, can not use atmospheric nitrogen to make amino-acids and other nitrogen containing compounds. </li></ul><ul><li>They are dependent on nitrogen fixing bacteria to convert N 2 into NH 3 (NH 4 + ). </li></ul>
  14. 14. Sources of Nitrogen to the Soil <ul><li>Natural ecosystems receive their soil nitrogen through biological fixation and atmospheric deposition. </li></ul><ul><li>Agricultural ecosystems receive additional nitrogen through fertilizer addition. </li></ul>
  15. 15. Biological Sources of Soil Nitrogen <ul><li>Only a few species of bacteria and cyanobacteria are capable of nitrogen fixation. </li></ul><ul><li>Some are fee-living and others form mutualistic associations with plants. </li></ul><ul><li>A few are lichens. </li></ul>
  16. 16. Atmospheric Sources of Soil Nitrogen <ul><li>Lightning was the major source of soil nitrogen until recent times when the burning of fossil fuels became a major source of atmospheric deposition. </li></ul><ul><li>Nitrogen oxides come from a variety of combustion sources that use fossil fuels. In urban areas, at least half of these pollutants come cars and other vehicles. </li></ul>
  17. 17. Agricultural Supplements to Soil Nitrogen <ul><li>Various forms of commercial fertilizer are added to agricultural fields to supplement the nitrogen lost through plant harvest. </li></ul><ul><li>Crop rotation with legumes such as soybeans or alfalfa is also practiced to supplement soil nitrogen. </li></ul>
  18. 18. Biological Nitrogen Fixation <ul><li>Nitrogen fixation is the largest source of soil nitrogen in natural ecosystems. </li></ul><ul><li>Free-living soil bacteria and cyanobacteria (blue-green “algae”) are capable of converting N 2 into ammonia (NH 3 ) and ammonium (NH 4 + ). </li></ul><ul><li>Symbiotic bacteria (Rhizobium ) in the nodules of legumes and certain other plants can also fix nitrogen. </li></ul>
  19. 19. Nitrification <ul><li>Several species of bacteria can convert ammonium (NH 4 + ) into nitrites (NO 2 - ). </li></ul><ul><li>Other bacterial species convert nitrites (NO 2 - ) to nitrates (NO 3 - ). </li></ul>
  20. 20. Uptake of Nitrogen by Plants <ul><li>Plants can take in either ammonium (NH 4 + ) or nitrates (NO 3 - ) and make amino acids or nucleic acids. </li></ul><ul><li>These molecules are the building blocks of proteins and DNA, RNA, ATP, NADP, respectively. </li></ul><ul><li>These building blocks of life are passed on to other trophic levels through consumption and decomposition. </li></ul>
  21. 21. Ammonification <ul><li>Decomposers convert organic nitrogen (CHON) into ammonia (NH 3 ) and ammonium (NH 4 + ). </li></ul><ul><li>A large number of species of bacteria and fungi are capable of converting organic molecules into ammonia. </li></ul>
  22. 22. Denitrification <ul><li>A broad range of bacterial species can convert nitrites, nitrates and nitrous oxides into molecular nitrogen (N 2 ). </li></ul><ul><li>They do this under anaerobic conditions as a means of obtaining oxygen (O 2 ). </li></ul><ul><li>Thus, the recycling of N is complete. </li></ul>
  23. 23. NITROGEN CYCLE IN ECOSYSTEMS <ul><li>Molecular nitrogen in the air can be fixed into ammonia by a few species of prokaryotes. </li></ul><ul><li>Other bacterial species convert NH 4 - into NO 2 - and others to N0 3 - . </li></ul><ul><li>Producers can take up NH 4 - and to N0 3 - use it to make CHON. </li></ul><ul><li>Decomposers use CHON and produce NH 4 - . </li></ul><ul><li>Recycling is complete when still other species convert N0 3 - and NO 2 - into N 2 . </li></ul>
  24. 24. NUTRIENT LOSS IN ECOSYSTEMS I <ul><li>The role of vegetation in nutrient cycles is clearly seen in clear cut experiments at Hubbard Brook. </li></ul><ul><li>When all vegetation was cut from a 38-acre watershed, the output of water and loss of nutrients increased; 60 fold for nitrates, and at least 10 fold for other nutrients. </li></ul><ul><li>Freeman describes the experiments on page 1254 and in Figure 54.15. </li></ul>
  27. 27. GLOBAL NUTRIENT CYCLES <ul><li>The loss of nutrients from one ecosystem means a gain for another. (Remember the law of conservation of matter.) </li></ul><ul><li>When ecosystems become linked in this manor, attention shifts to a global scale. One is now considering the ECOSPHERE or the whole of planet earth. </li></ul>
  28. 28. GLOBAL WATER CYCLE I <ul><li>Water is the solvent in which all the chemistry of life takes place and the source of its hydrogen. </li></ul><ul><li>The earth’s oceans, ice caps, glaciers, lakes, rivers, soils and atmosphere contains about 1.5 billion cubic kilometers of H 2 O. </li></ul><ul><li>It has been estimated that all the earth’s water is split by plant cells and reconstituted by the biota about every 2,000,000 years . </li></ul>
  29. 29. GLOBAL WATER CYCLE II <ul><li>Oceans contain a little less than 98% of the earth’s water. </li></ul><ul><li>Around 1.8% is ice; found in the two polar ice caps and mountain glaciers. </li></ul><ul><li>Only 0.5% is found in the water table and ground water. </li></ul><ul><li>The atmosphere contains only 0.001% of the earth’s water, but is the major driver of weather. </li></ul>
  30. 30. GLOBAL WATER CYCLE III <ul><li>The rate at which water cycles is shown in Figure 54.16 (Freeman, 2005). </li></ul><ul><li>Evaporation exceeds precipitation over the oceans; thus there is a net movement of water to the land. </li></ul><ul><li>Nearly 60% of the precipitation that falls on land is either evaporated or transpired by plants; the remainder is runoff and ground water. </li></ul>
  32. 32. GLOBAL CARBON CYCLE I <ul><li>All but a small portion of the earth’s carbon (C) is tied up in sedimentary rocks; but the portion that circulates is what sustains life. </li></ul><ul><li>The active pool of carbon is estimated to be around 40,000 gigatons. </li></ul><ul><li>93.2 % found in the ocean; 3.7% in soils; 1.7% in atmosphere; 1.4% in vegetation. </li></ul>
  33. 33. GLOBAL CARBON CYCLE II <ul><li>The rate at which the biota exchanges CO 2 with atmosphere has been estimated to be every 300 years. </li></ul><ul><li>The rate at which carbon cycles through various components of the ecosphere is summarized in Figure 54.17 in Freeman (2005). </li></ul><ul><li>Since the industrial revolution, a new source of stored sedimentary carbon has been added to the atmosphere from the burning of fossil fuels causing a concern with respect to climate change. </li></ul>
  35. 35. GLOBAL NITROGEN CYCLE I <ul><li>99.4% of exchangeable N is found in the atmosphere; 0.5% is dissolved in the ocean; 0.04% in detritus ; 0.006% as inorganic N sources; 0.0004% in living biota. </li></ul><ul><li>Figure 54.19 in Freeman (2005) gives major pathways and rates of exchange. </li></ul>
  36. 36. GLOBAL NITROGEN CYCLE II <ul><li>Humans are adding large amounts of N to ecosystems. Some estimates of are given in Figure 54.20 in Freeman (2005). </li></ul><ul><li>Among the fossil fuel sources, power plants and automobiles are important sources of atmospheric nitrogen deposition in the US. </li></ul><ul><li>Investigations of native plant and natural ecosystem responses to nitrogen deposition and global warming will be a focus of study. </li></ul>