If N2 were depleted from the atmosphere, thus shutting off nitrogen fixation, life on Earth outside of the oceans would be greatly restricted. A nitrogen atom cycles on average 10 times between atmospheric N2 and the oceans before it is transferred to the lithosphere. Atmospheric oxygen is produced by photosynthesis, which we represent stoichiometrically as. This source of O2 is balanced by loss of O2 from the oxidation of biomass, including respiration by living biomass and microbial decay of dead biomass:.
The cycling of oxygen by reactions R1 and R2 is coupled to that of carbon, as illustrated in Figure Figure Cycling of O2 with the biosphere; orgC is organic carbon. A petagram Pg is 1x g. To assess the potential for this cycle to regulate atmospheric O2 levels, we need to determine inventories for the different reservoirs of Figure : O2, CO2, and orgC. Applying equation 6. Simple comparison of these inventories tells us that cycling with the biosphere cannot control the abundance of O2 in the atmosphere, because the inventory of O2 is considerably larger than that of either CO2 or organic carbon.
Conversely, if respiration and decay were to stop, conversion of all atmospheric CO2 to O2 by photosynthesis would increase O2 levels by only 0.
What, then, controls atmospheric oxygen? The next place to look is the lithosphere. Rock material brought to the surface in a reduced state is weathered oxidized by atmospheric O2. The total amounts of organic carbon and pyrite in sedimentary rocks are estimated to be 1. These amounts are sufficiently large that weathering of rocks would eventually deplete atmospheric O2 if not compensated by an oxygen source.
The turnover time of sedimentary rock, that is the time required for sedimentary rock formed at the bottom of the ocean to be brought up to the surface, is of the order of million years. The corresponding weathering rates are 0. The resulting loss of O2 is 0.
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On a time scale of several million years, changes in the rate of sediment uplift could conceivably alter the levels of O2 in the atmosphere. This cycling of O2 with the lithosphere is illustrated in Figure Atmospheric O2 is produced during the formation of reduced sedimentary material, and the consumption of O2 by weathering when this sediment is eventually brought up to the surface and oxidized balances the O2 source from sediment formation.
Fossil records show that atmospheric O2 levels have in fact not changed significantly over the past million years, a time scale much longer than the lifetime of O2 against loss by weathering. The constancy of atmospheric O2 suggests that there must be stabilizing factors in the O2-lithosphere cycle but these are still poorly understood. One stabilizing factor is the relative rate of oxidation vs. If sediment weathering were to increase for some reason, drawing down atmospheric O2, then more of the marine organic carbon would be buried because of slower oxidation , which would increase the source of O2 and act as a negative feedback.
Figure Cycling of O2 with the lithosphere. One molecule of O2 is produced for each organic C atom incorporated in the sediments. That is 30 times more than the 1. Where did the rest of the oxygen go? Indeed, global inventories show that these reservoirs can account for the missing oxygen.
Part V of "Matter cycles": The carbon cycle
Ice core measurements show that atmospheric concentrations of CO2 have increased from ppmv in pre-industrial times to ppmv today. Continuous atmospheric measurements made since at Mauna Loa Observatory in Hawaii demonstrate the secular increase of CO2 Figure Superimposed on the secular trend is a s easonal oscillation winter maximum, summer minimum that reflects the uptake of CO2 by vegetation during the growing season, balanced by the net release of CO2 from the biosphere in fall due to microbial decay.
The current global rate of increase of atmospheric CO2 is 1. This increase is due mostly to fossil fuel combustion. When fuel is burned, almost all of the carbon in the fuel is oxidized to CO2 and emitted to the atmosphere. We can use worldwide fuel use statistics to estimate the corresponding CO2 emission, presently 6.
Another significant source of CO2 is deforestation in the tropics; based on rates of agricultural encroachment documented by satellite observations, it is estimated that this source amounts to 1. Substituting the above numbers in a global mass balance equation for atmospheric CO2,. Only half of the CO2 emitted by fossil fuel combustion and deforestation actually accumulates in the atmosphere. The other half is transferred to other geochemical reservoirs oceans, biosphere, and soils.
We need to understand the factors controlling these sinks in order to predict future trends in atmospheric CO2 and assess their implications for climate change. A sink to the biosphere would mean that fossil fuel CO2 has a fertilizing effect, with possibly important ecological consequences.
The Carbon And Nitrogen Cycle Essay
This process is described by the chemical equilibria:. The values given here for the constants are typical of seawater and take into account ionic strength corrections, complex formation, and the effects of temperature and pressure. The average pH of the ocean is 8.
The alkalinity of the ocean is mainained by weathering of basic rocks Al2O3, SiO2, CaCO3 at the surface of the continents, followed by river runoff of the dissolved ions to the ocean. Figure Speciation of total carbonate CO2 aq in seawater vs. Let F represent the atmospheric fraction of CO2 in the atmosphere-ocean system:. The concentrations of CO2 in the atmosphere and in the ocean are related by equilibria R3 - R5 :.
Assuming the whole ocean to be in equilibrium with the atmosphere, we relate NCO2 aq to [CO2 aq ] in 6. Substituting into equation 6. For an ocean pH of 8.
Human Impacts on the Carbon, Nitrogen and Phosphorus Cycles Essay examples
The value of F is extremely sensitive to pH, as illustrated by Figure In the absence of oceanic alkalinity, most of the CO2 would partition into the atmosphere. Figure pH dependence of the atmospheric fraction F of CO2 at equilibrium in the atmosphere-ocean system equation 6. However, such an inference is flawed because it ignores the acidification of the ocean resulting from added CO2.
This buffer effect is represented by the overall equilibrium. Uptake of CO2 g is thus facilitated by the available pool of CO ions. Note that the buffer effect does not mean that the pH of the ocean remains constant when CO2 increases; it means only that changes in pH are dampened. To better pose our problem, let us consider a situation where we add dN moles of CO2 to the atmosphere. We wish to know, when equilibrium is finally reached with the ocean, what fraction f of the added CO2 remains in the atmosphere:.
Replacing into equation 6. Replacing into 6. Nitrogen is an element that can combine with itself or with other elements to make different compounds. For instance nitrogen gas, N 2 , is a compound made when two nitrogen atoms form a chemical bond. So nitrogen gas is very common and plentiful. However, only a specialized group of bacteria, and industrial fertilizer manufacture, can "fix" this largely inert compound into biologically useful nitrogen compounds.
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Fertilizer production now exceeds natural nitrogen fixation in making N 2 available to the biosphere. Nitrogen is a component of amino acids and urea. Amino acids are the building blocks of all proteins. Proteins comprise not only structural components such as muscle, tissue and organs, but also enzymes and hormones essential for the functioning of all living things. Urea is a byproduct of protein digestion.