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NUTRIENT CYCLES Prof. Murray, Univ of Illinois at Chicago
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 + ).
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.
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.
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.
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.
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.
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 - ).
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.
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.
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.
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.
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
NUTRIENT LOSS IN ECOSYSTEMS II
NUTRIENT LOSS IN ECOSYSTEMS III
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.
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.
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.
GLOBAL WATER CYCLE
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.
GLOBAL CARBON CYCLE
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.
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