TKA 3104 ENVIRONMENTAL BIOLOGY

Nutrient Cycles

• The basic elements of which all organisms are composed are carbon, nitrogen, phosphorus, sulfur, oxygen, and hydrogen.

• The first four of these elements are much more limited in mass and easier to trace than are oxygen and hydrogen.

• Because these elements are conserved, they can be recycled indefinitely.

• Because the pathways used to describe the movement of these elements in the environment are cyclic, they are referred to as the carbon, nitrogen, phosphorus, and sulfur cycles.

CARBON CYCLE• What are the major reservoirs of carbon on earth, how

large are these? • What are the major fluxes of carbon on earth, which

ones predominate? At what timescales? • What are the main ways that carbon is cycled? • What are the preindustrial, modern, and projected future

concentrations of CO2 in the atmosphere? • What is the "missing sink" and why is it important? • What are the important controls on fluxes of carbon

between reservoirs?

Introduction• Understanding the carbon cycle is the first step toward understanding

possible impacts of the human-induced rise in greenhouse gasses. • Carbon is the elemental building block of all life and as such, it is

stored and exchanged between different reservoirs, each of which has its own characteristic size and response time.

• The main carbon reservoirs are carbonate rocks (limestone), soils, land plants, the oceans, the atmosphere.

• The aim of this lecture is to give you an appreciation for how large the carbon reservoirs are, what the important fluxes of carbon are between reservoirs, how we conduct mass balances between the reservoirs, and finally to underscore the magnitude of anthropogenic CO2 increases relative to these reservoirs and fluxes.

• Through fossil fuel burning and land use changes, humans have started a grand, uncontrolled experiment with carbon on earth.

• We now recognize that this experiment will change our climate, and the potential effects on people's lives have stimulated some of the largest public and policy debates of any scientific topic today.

• There have been times on earth in the past when the CO2concentration of the atmosphere has been both much less and much greater than it is today.

• If it has been greater, then one might ask "why are we so worried, the CO2 concentrations were greater than they are today and we still survived?"

• The answer to that question lies in the fact that the rate of change of CO2 in the atmosphere is faster today than at anytime in earth's history.

• It is this rapid increase in CO2, not necessary the final CO2 concentration that we may achieve, which is driving much of our concern.

• For example, because organisms (and certainly all of the human cultures on earth) have never been exposed to such rapid rates of CO2 increase, we don't know how they will respond and whether they will be able to adapt quickly enough to survive.

• These are the questions that science and society are struggling with today.

• While we know that CO2 concentrations are increasing, there have been several plans or ideas on how to control them "naturally", such as plant more trees to take up the excess CO2.

• Later in the lecture we will examine two of these ideas and determine if they could be real solutions to global warming.

The ins and outs of carbon -reservoirs and fluxes.• The basic principles of accounting as applied to understand how

carbon cycles through all the various reservoirs. • One of the key things we learn from this exercise is that there are

"fast" and "slow" carbon cycles. • Rapid carbon cycling occurs on timescales of days to seasons to

yearly timescales and is accomplished by the cycling of land and ocean life on the planet: Photosynthesis and respiration and remineralization.

• Changes in ocean carbon uptake and storage occur over timescales on many decades to many millennia.

• Even slower carbon cycling occurs over much longer timescales (millions of years) and involves the long-term processes of rock formation and weathering, as well as tectonic activity at the Earth's surface.

• There are several different forms of carbon that we have to keep track of in learning about the carbon cycle.

• The main forms are: (a) Inorganic-C in rocks (such as limestone, CaCO3); (b) organic-C (such as found in organic plant material and

soils); carbon gases such as CO2, (carbon dioxide), CH4, (methane), and CO (carbon monoxide).

• "Carbon cycling" is really the movement of C from one of these forms to another form.

• An example would the plant growth, which takes carbon from the atmosphere reservoir and "fixes" it as plant-based carbon (leaves).

• The fundamental unit of measurement of Carbon at planetary cycling scales is the Gigaton.

• One gigaton is equal to one billion tons of carbon (or 10^15 g).

• How much is one gigaton? • It is about 2750 Empire State Buildings, or about

142 million African elephants. • For reference, The US carbon emission rate

alone is about 1.4 gigtons per year (or about 5.4 tons/person annually).

• Table 1: Sizes (in gigatons, or 1015 g) of the main reservoirs of carbon on earth.

LOCATION Amount (x1015 g C)Rocks 65,000,000Oceans 39,000Soils 1,580Atmosphere* 750Land plants 610

Figure 1. The Global Carbon Cycle.

Carbon Cycling - transferring carbon between reservoirs.• There are several pathways in the carbon cycle

that are of particular importance (Figure 1 above).

• The main pathways to and from the atmosphere are diffusion into and out of the ocean, photosynthesis which consumes CO2 from the atmosphere (an output from the atmosphere), respiration which produces CO2, and the burning of fossil fuels and biomass which produces CO2.

• The magnitude of these fluxes are as follows, all in 1015 g of C per year ("-" means taken from the atmosphere, "+" means given off to the atmosphere: · Ocean uptake = -2.0 (x 1015 g C / yr)· Photosynthesis = -100· Respiration = +100· Fossil fuels = +5.4· Biomass burning = +1.6

Timescales of carbon fluxes.• Not all of the carbon reservoirs have the same response

time. • Specifically, some of the reservoirs are not really active

participants in the global carbon cycle at human timescales, but are important over geological time scales (millions of years).

• For example, rocks (limestone) are by far the largest reservoir of carbon on earth, but changes in the flux of carbon to and from this reservoirs are extremely slow so that they have no real impact on changes in the global carbon cycle at human timescales (10's to 1000's of years).

• On the other hand, changes in land and ocean productivity vary at seasonal and annual timescales and can impact global carbon cycles dramatically.

• The most pressing example is the dramatic rise in CO2 from fossil fuel burning (5.4 x 1015 g C / yr) and land-use changes (1.6 x 1015 g C / yr) which is increasing the atmospheric carbon reservoir at an unprecedented rate.

• (A) Anthropogenic rise in CO2: CO2 concentrations have been rising for over 300 years due to human activities.

• The rate of increase and the magnitude of increase are believed to be unprecedented in earth history.

• Why was there any increase in CO2 prior to the industrial revolution in the mid-1800's?

• The gradual increase in atmospheric CO2concentrations was due to forest clearing and conversion of forests to pasture lands.

• During this time trees were cut down, thus removing a efficient sink (remover) of carbon from the atmosphere.

• In addition, the trees decay over time and return the carbon to the atmosphere.

• Modern CO2 levels are close to 370 ppm, much higher (12%) than the preindustrial value of ~290 ppm.

• This increase is due to the combination of fossil fuel burning emissions and land-use changes.

• The average rate of CO2 increase is currently near 0.5% (~1.5 ppm/year) (how does this compare to the average population increase?).

• (B). Rock weathering and volcanism: Over much longer time scales (tens of million of years) atmospheric CO2 concentrations have varied tremendously due to changes in the balance between the supply of CO2 from volcanism and the consumption of CO2 by rock weathering.

• CO2 concentrations have, at times in earth history, been much higher than present or even projected future values.

• Over geological timescales, large (but very gradual) changes in atmospheric CO2 result from changes in this balance between rock weathering and volcanism.

• CO2 in the atmosphere is consumed in the weathering of rocks (CO2 combines with H2O to make carbonic acid, which slowly dissolves rocks).

• This weathering produces bicarbonate (HCO3-), a form of inorganic carbon, and calcium (Ca2+) that is then transported in river water to the oceans.

• Once in the oceans the calcium and bicarbonate are combined by organisms to form calcium carbonate, the mineral that is found in shells.

• This calcium carbonate mineral is buried in the sediments, where eventually it comes under great temperature and pressure and is melted during the process of "subduction".

• The melted rock rises to the surface in the form of magma and is released back to the surface of the earth at volcanoes.

• This high-temperature process also converts some of the calcium carbonate back to CO2, which is released during volcanic eruptions to the atmosphere to begin the cycle over again.

Figure 2. The global carbon cycle from the perspective of its control by weathering.

Past relationships between atmospheric CO2 and surface temperature.• We can learn about earth climate sensitivity to

past variations in atmospheric CO2 by drilling into ice sheets.

• Ice sheets record past concentrations of atmospheric CO2 by trapping bubbles of ancient air as the ice sheet forms.

• The figure below shows the relationship between CO2 in the atmosphere and surface temperatures over Antarctica spanning the last 150,000 years.

• As you can see, there is a very close relationship between surface temperatures and atmospheric CO2 levels.

• Note, however, the present mismatch between the current high levels of CO2 (around 365 ppm) and the relatively unchanged surface temperatures.

• If past history is a guide to the future, the data in this plot suggest we are due for very significant global warming.

Figure 3. Relationship between CO2 in the atmosphere and surface temperature changes based on ice core drilling results from Antarctica (Vostok).

• The figure below shows one projection of the magnitude of future CO2 levels due to fossil fuel emissions and land-use changes.

• Note that global CO2 levels will be doubled from the pre-Industrial value (~290 ppm) in about 70 years or by ~2075.

Figure 4. Projected CO2 increases in the atmosphere and estimated surface temperature changes for the next 100 years.

NITROGEN CYCLE• The nitrogen cycle is the biogeochemical cycle

that describes the transformations of nitrogen and nitrogen-containing compounds in nature.

• It is a cycle which includes gaseous components.

• Earth's atmosphere is 80% nitrogen, making it the largest pool of nitrogen.

• Nitrogen is essential for many biological processes; it is crucial for any life here on Earth.

• It is in all amino acids, is incorporated into proteins, and is present in the bases that make up nucleic acids, such as DNA and RNA.

• In plants, much of the nitrogen is used in chlorophyll molecules which are essential for photosynthesis and further growth.

• Processing, or fixation, is necessary to convert gaseous nitrogen into forms usable by living organisms.

• Some fixation occurs in lightning strikes, but most fixation is done by free-living or symbiotic bacteria.

• These bacteria have the nitrogenase enzyme that combines gaseous nitrogen with hydrogen to produce ammonia, which is then further converted by the bacteria to make its own organic compounds.

• Some nitrogen fixing bacteria, such as Rhizobium, live in the root nodules of legumes (such as peas or beans).

• Here they form a mutualistic relationship with the plant, producing ammonia in exchange for carbohydrates.

• Nutrient-poor soils can be planted with legumes to enrich them with nitrogen. A few other plants can form such symbioses.

• Nowadays, a very considerable portion of nitrogen is fixated in ammoniachemical plants.

• Other plants get nitrogen from the soil, and by absorption of their roots in the form of either nitrate ions or ammonium ions.

• All nitrogen obtained by animals can be traced back to the eating of plants at some stage of the food chain.

• Due to their very high solubility, nitrates can enter groundwater. • Elevated nitrate in groundwater is a concern for drinking water use because

nitrate can interfere with blood-oxygen levels in infants and cause methemoglobinemia or blue-baby syndrome.

• Where groundwater recharges stream flow, nitrate-enriched groundwater can contribute to eutrophication, a process leading to high algal, especially blue-green algal populations and the death of aquatic life due to excessive demand for oxygen.

• While not directly toxic to fish life like ammonia, nitrate can have indirect effects on fish if it contributes to this eutrophication.

• Nitrogen has contributed to severe eutrophication problems in some water bodies.

• As of 2006, the application of nitrogen fertilizer is being increasingly controlled in Britain and the United States.

• This is occurring along the same lines as control of phosphorus fertilizer, restriction of which is normally considered essential to the recovery of eutrophied waterbodies.

• Ammonia is highly toxic to fish and the water discharge level of ammonia from wastewater treatment plants must often be closely monitored.

• To prevent loss of fish, nitrification prior to discharge is often desirable.

• Land application can be an attractive alternative to the mechanical aeration needed for nitrification.

• During anaerobic (low oxygen) conditions, denitrification by bacteria occurs.

• This results in nitrates being converted to nitrogen gases (NO, N2O, N2) and returned to the atmosphere.

• Nitrate can also be reduced to nitrite and subsequently combine with ammonium in the anammox process, which also results in the production of dinitrogen gas.

Schematic representation of the flow of Nitrogen through the environment. The importance of bacteria in the cycle is immediately recognized as being a key element in the cycle, providing different forms of nitrogen compounds assimilable by higher organisms.

PHOSPHORUS CYCLE• Phosphorus is an important element for all forms of life. • As phosphate (PO4), it makes up an important part of the

structural framework that holds DNA and RNA together. • Phosphates are also a critical component of ATP – the

cellular energy carrier – as they serve as an energy ‘release' for organisms to use in building proteins or contacting muscles.

• Like calcium, phosphorus is important to vertebrates; in the human body, 80% of phosphorous is found in teeth and bones.

• The phosphorus cycle differs from the other major biogeochemical cycles in that it does not include a gas phase; although small amounts of phosphoric acid (H3PO4) may make their way into the atmosphere, contributing – in some cases – to acid rain.

• The water, carbon, nitrogen and sulfur cycles all include at least one phase in which the element is in its gaseous state.

• Very little phosphorus circulates in the atmosphere because at Earth's normal temperatures and pressures, phosphorus and its various compounds are not gases.

• The largest reservoir of phosphorus is in sedimentary rock.

• It is in these rocks where the phosphorus cycle begins. • When it rains, phosphates are removed from the rocks

(via weathering) and are distributed throughout both soils and water.

• Plants take up the phosphate ions from the soil. • The phosphates then moves from plants to animals when

herbivores eat plants and carnivores eat plants or herbivores.

• The phosphates absorbed by animal tissue through consumption eventually returns to the soil through the excretion of urine and feces, as well as from the final decomposition of plants and animals after death.

• The same process occurs within the aquatic ecosystem. • Phosphorus is not highly soluble, binding tightly to molecules in soil,

therefore it mostly reaches waters by traveling with runoff soil particles.

• Phosphates also enter waterways through fertilizer runoff, sewage seepage, natural mineral deposits, and wastes from other industrial processes.

• These phosphates tend to settle on ocean floors and lake bottoms. • As sediments are stirred up, phosphates may reenter the phosphorus

cycle, but they are more commonly made available to aquatic organisms by being exposed through erosion.

• Water plants take up the waterborne phosphate which then travels up through successive stages of the aquatic food chain.

• While obviously beneficial for many biological processes, in surface waters an excessive concentration of phosphorus is considered a pollutant.

• Phosphate stimulates the growth of plankton and plants, favoring weedy species over others.

• Excess growth of these plants tend to consume large amounts of dissolved oxygen, potentially suffocating fish and other marine animals, while also blocking available sunlight to bottom dwelling species.

• This is known as eutrophication.

• Humans can alter the phosphorus cycle in many ways, including in the cutting of tropical rain forests and through the use of agricultural fertilizers.

• Rainforest ecosystems are supported primarily through the recycling of nutrients, with little or no nutrient reserves in their soils.

• As the forest is cut and/or burned, nutrients originally stored in plants and rocks are quickly washed away by heavy rains, causing the land to become unproductive.

• Agricultural runoff provides much of the phosphate found in waterways.

• Crops often cannot absorb all of the fertilizer in the soils, causing excess fertilizer runoff and increasing phosphate levels in rivers and other bodies of water.

• At one time the use of laundry detergents contributed to significant concentrations of phosphates in rivers, lakes, and streams, but most detergents no longer include phosphorus as an ingredient.

SULFUR CYCLE• Sulfur (S), the tenth most abundant element in

the universe, is a brittle, yellow, tasteless, and odorless non-metallic element.

• It comprises many vitamins, proteins, and hormones that play critical roles in both climate and in the health of various ecosystems.

• The majority of the Earth's sulfur is stored underground in rocks and minerals, including as sulfate salts buried deep within ocean sediments.

• The sulfur cycle contains both atmospheric and terrestrial processes. • Within the terrestrial portion, the cycle begins with the weathering of

rocks, releasing the stored sulfur. • The sulfur then comes into contact with air where it is converted into

sulfate (SO4). • The sulfate is taken up by plants and microorganisms and is

converted into organic forms; animals then consume these organic forms through foods they eat, thereby moving the sulfur through the food chain.

• As organisms die and decompose, some of the sulfur is again released as a sulfate and some enters the tissues of microorganisms.

• There are also a variety of natural sources that emit sulfur directly into the atmosphere, including volcanic eruptions, the breakdown of organic matter in swamps and tidal flats, and the evaporation of water.

• Sulfur eventually settles back into the Earth or comes down within rainfall.

• A continuous loss of sulfur from terrestrial ecosystem runoff occurs through drainage into lakes and streams, and eventually oceans.

• Sulfur also enters the ocean through fallout from the Earth's atmosphere.

• Within the ocean, some sulfur cycles through marine communities, moving through the food chain.

• A portion of this sulfur is emitted back into the atmosphere from sea spray.

• The remaining sulfur is lost to the ocean depths, combining with iron to form ferrous sulfide which is responsible for the black color of most marine sediments.

• Since the Industrial Revolution, human activities have contributed to the amount of sulfur that enters the atmosphere, primarily through the burning of fossil fuels and the processing of metals.

• One-third of all sulfur that reaches the atmosphere – including 90% of sulfur dioxide – stems from human activities.

• Emissions from these activities, along with nitrogen emissions, react with other chemicals in the atmosphere to produce tiny particles of sulfate salts which fall as acid rain, causing a variety of damage to both the natural environment as well as to man-made environments, such as the chemical weathering of buildings.

• However, as particles and tiny airborne droplets, sulfur also acts as a regulator of global climate.

• Sulfur dioxide and sulfate aerosols absorb ultraviolet radiation, creating cloud cover that cools cities and may offset global warming caused by the greenhouse effect.

• The actual amount of this offset is a question that researchers are attempting to answer.