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NUTRIENT CYCLES Prof. Murray, Univ of Illinois at Chicago.

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1 NUTRIENT CYCLES Prof. Murray, Univ of Illinois at Chicago

2 NUTRIENT CYCLES: ECOSYSTEM TO ECOSPHERE  Nutrient cycling occurs at the local level through the action of the biota.  Nutrient cycling occurs at the global level through geological processes, such as, atmospheric circulation, erosion and weathering.

3 NUTRIENT CYCLES  The atoms of earth and life are the same; they just find themselves in different places at different times.  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.  The path atoms take from the living (biotic) to the non-living (abiotic) world and back again is called a biogeochemical cycle.

4 Nutrients: The Elements of Life  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.  Only around half of these 15 have been studied extensively as they travel through ecosystems or circulate on a global scale.

5 A GENERALIZED MODEL OF NUTRIENT CYCLING IN AN ECOSYSTEM  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.  Nutrients can flow between these compartments along a variety of pathways.

6 Nutrient Compartments in a Terrestrial Ecosystem  The organic compartment consists of the living organisms and their detritus.  The available-nutrient compartment consists of nutrients held to surface of soil particles or in solution.  The third compartment consists of nutrients held in soils or rocks that are unavailable to living organisms.  The fourth compartment is the air which can be found in the atmosphere or in the ground.

7 Uptake of Inorganic Nutrients from the Soil  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.  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.  E.g. you get calcium from milk which came from the diet of the cow – producers.

8 The Atmosphere Is a Source of Inorganic Nutrients  The atmosphere acts as a reservoir for carbon dioxide (CO 2 ), oxygen (O 2 ) and water (H 2 O).  These inorganic compounds can be exchanged directly with the biota through the processes of photosynthesis and respiration.  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.

9 Some Processes By Which Nutrients Are Recycled  Cycling within an ecosystem involves a number of processes.  These are best considered by focusing attention on specific nutrients.

10 CARBON, HYDROGEN AND OXYGEN CYCLES IN ECOSYSTEMS  C, H & O basic elements of life; making up from about 98% of plant biomass.  CO 2 and O 2 enter biota from the atmosphere.  Producers convert CO 2 and H 2 O into carbohydrates (CH 2 O compounds) and release O 2 from water.  Producers, consumers and decomposers convert CH 2 O compounds, using O 2, back into CO 2 and H 2 O.

11 CARBON, HYDROGEN AND OXYGEN CYCLES IN ECOSYSTEMS  Carbon and oxygen cycle come out of the air as carbon dioxide during photosynthesis and are returned during respiration.  Oxygen is produced from water during photosynthesis and combines with the hydrogen to form water during respiration.

12 PHOSPHOROUS CYCLE IN ECOSYSTEMS  Phosphorus, as phosphate (PO 4 -3 ), is an essential element of life.  It does not cycle through atmosphere, thus enters producers through the soil and is cycled locally through producers, consumers and decomposers.  Generally, small local losses by leaching are balanced by gains from the weathering of rocks.  Over very long time periods (geological time) phosphorus follows a sedimentary cycle.

13 NITROGEN CYCLE IN ECOSYSTEMS  Nitrogen (N 2 ) makes up 78% of the atmosphere.  Most living things, however, can not use atmospheric nitrogen to make amino-acids and other nitrogen containing compounds.  They are dependent on nitrogen fixing bacteria to convert N 2 into NH 3 (NH 4 + ).

14 Sources of Nitrogen to the Soil  Natural ecosystems receive their soil nitrogen through biological fixation and atmospheric deposition.  Agricultural ecosystems receive additional nitrogen through fertilizer addition.

15 Biological Sources of Soil Nitrogen  Only a few species of bacteria and cyanobacteria are capable of nitrogen fixation.  Some are fee-living and others form mutualistic associations with plants.  A few are lichens.

16 Atmospheric Sources of Soil Nitrogen  Lightning was the major source of soil nitrogen until recent times when the burning of fossil fuels became a major source of atmospheric deposition.  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.

17 Agricultural Supplements to Soil Nitrogen  Various forms of commercial fertilizer are added to agricultural fields to supplement the nitrogen lost through plant harvest.  Crop rotation with legumes such as soybeans or alfalfa is also practiced to supplement soil nitrogen.

18 Biological Nitrogen Fixation  Nitrogen fixation is the largest source of soil nitrogen in natural ecosystems.  Free-living soil bacteria and cyanobacteria (blue-green “algae”) are capable of converting N 2 into ammonia (NH 3 ) and ammonium (NH 4 + ).  Symbiotic bacteria (Rhizobium) in the nodules of legumes and certain other plants can also fix nitrogen.

19 Nitrification  Several species of bacteria can convert ammonium (NH 4 + ) into nitrites (NO 2 - ).  Other bacterial species convert nitrites (NO 2 - ) to nitrates (NO 3 - ).

20 Uptake of Nitrogen by Plants  Plants can take in either ammonium (NH 4 + ) or nitrates (NO 3 - ) and make amino acids or nucleic acids.  These molecules are the building blocks of proteins and DNA, RNA, ATP, NADP, respectively.  These building blocks of life are passed on to other trophic levels through consumption and decomposition.

21 Ammonification  Decomposers convert organic nitrogen (CHON) into ammonia (NH 3 ) and ammonium (NH 4 + ).  A large number of species of bacteria and fungi are capable of converting organic molecules into ammonia.

22 Denitrification  A broad range of bacterial species can convert nitrites, nitrates and nitrous oxides into molecular nitrogen (N 2 ).  They do this under anaerobic conditions as a means of obtaining oxygen (O 2 ).  Thus, the recycling of N is complete.

23 NITROGEN CYCLE IN ECOSYSTEMS  Molecular nitrogen in the air can be fixed into ammonia by a few species of prokaryotes.  Other bacterial species convert NH 4 - into NO 2 - and others to N  Producers can take up NH 4 - and to N0 3 - use it to make CHON.  Decomposers use CHON and produce NH 4 -.  Recycling is complete when still other species convert N0 3 - and NO 2 - into N 2.

24 NUTRIENT LOSS IN ECOSYSTEMS I  The role of vegetation in nutrient cycles is clearly seen in clear cut experiments at Hubbard Brook.  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.  Freeman describes the experiments on page 1254 and in Figure



27 GLOBAL NUTRIENT CYCLES  The loss of nutrients from one ecosystem means a gain for another. (Remember the law of conservation of matter.)  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.

28 GLOBAL WATER CYCLE  Oceans contain a little less than 98% of the earth’s water.  Around 1.8% is ice; found in the two polar ice caps and mountain glaciers.  Only 0.5% is found in the water table and ground water.  The atmosphere contains only 0.001% of the earth’s water, but is the major driver of weather.

29 GLOBAL WATER CYCLE  The rate at which water cycles is shown in Figure (Freeman, 2005).  Evaporation exceeds precipitation over the oceans; thus there is a net movement of water to the land.  Nearly 60% of the precipitation that falls on land is either evaporated or transpired by plants; the remainder is runoff and ground water.


31 GLOBAL CARBON CYCLE  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.  The active pool of carbon is estimated to be around 40,000 gigatons.  Of active carbon, 93.2 % found in the ocean; 3.7% in soils; 1.7% in atmosphere; 1.4% in vegetation.


33 GLOBAL NITROGEN CYCLE I  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; % in living biota.  Figure in Freeman (2005) gives major pathways and rates of exchange.

34 GLOBAL NITROGEN CYCLE II  Humans are adding large amounts of N to ecosystems.  Among the fossil fuel sources, power plants and automobiles are important sources of atmospheric nitrogen deposition in the US.  Investigations of native plant and natural ecosystem responses to nitrogen deposition and global warming will be a focus of study.  E.g. invasive species tend to be more devastating to ecosystems with high soil nitrogen content


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