Natural and Anthropogenic Cycles of Carbon, Phosphorus, and Nitrogen

This figure is a simplified schematic of the carbon cycle, showing the processes of photosynthesis and respiration (discussed in more detail here), as well as the cycling of carbon between CO₂ and carbonate rocks (mostly limestone, CaCO₃). The two primary anthropogenic perturbations to this cycle bothinvolve the accelerated conversion of organic carbon to CO₂, which is a greenhouse gas and the primary driver of climate change:

• Combustion of fossil fuels (coal, oil, natural gas) takes organic carbon deposits that have accumulated over millions of years and converts them to CO₂. Combustion is similar to respiration in that it involves the exothermic oxidation of organic matter using O₂.
• Land-use change—primarily deforestation—takes organic carbon in the form of vegetation (and soil organic matter) and converts it to CO₂, either by burning it or by cutting it down and letting it decompose.

A third, smaller change to the carbon cycle is the conversion of CaCO₃ to CaO and CO₂ during the production of cement (not shown in this figure).

This figure is a simplified schematic of the P cycle. “Uptake” refers to the creation of cellular organic P compounds (such as nucleic acids) during photosynthesis. The pool of P that is available for uptake consists mostly of the simplest dissolved inorganic form of P (ortho-phosphate, PO₄^3-) and was traditionally called Dissolved Inorganic Phosphorus (DIP). However, this pool is now referred to as SRP (Soluble Reactive Phosphorus) to acknowledge that some small organic P-containing molecules can also serve as P sources for algae, while some large inorganic forms, such as polyphosphates, can’t be used. “Decomposition” (also called mineralization) is the breakdown of large organic P compounds back to PO₄^3- during respiration. 

P can also be found in minerals such as hydroxyapatite, Ca₅(PO₄)3(OH); rock weathering releases P from these forms into biologically available forms, while formation of new rocks through geological processes returns P back into rock-bound unavailable forms. The release of P from rocks is much slower than its biological uptake, so the rate of P mineral weathering sets the amount of P available for biological productivity. The primary anthropogenic perturbation to the P cycle has been the mining of P minerals and their conversion to phosphate, mostly for use as agricultural fertilizer. 

The N cycle is the most complex of the three. N has two primary dissolved inorganic forms—NH4⁺ (ammonium) and NO₃^- (nitrate)—referred to collectively as Dissolved Inorganic Nitrogen (DIN). Both forms of DIN can be used as nitrogen sources for the biological production of organic nitrogen compounds, such as proteins. Decomposition of organic nitrogen releases NH₄⁺, which, in the presence of O₂, can be converted to NO₃^- by microbes. Depending on pH, NH₄⁺ can also be converted to NH₃ (ammonia), which can volatilize as a gas. Denitrification—which occurs only in reducing environments—converts NO₃^- to N₂, which is the most abundant form of N, but is very stable and not generally available for biological cycling. Both nitrification and denitrification lead to formation of the gas N₂O as a byproduct.

We will use the term reactive N to refer to all forms of N except for N₂, in order to distinguish between the actively-cycling, biologically-relevant forms, on the one hand, and the inert gas on the other. There are several ways that N₂ can be converted to reactive N. Some specialized types of bacteria can use N₂ as a nitrogen source, a process referred to as biological nitrogen fixation (BNF). N2 can also undergo fixation by reaction with O₂ to yield NO and NO₂ (referred to collectively as NO_x) during combustion or through lighting strikes. Humanity has accelerated the creation of reactive N through three processes:

• Legumes have symbiotic relationships with N-fixing bacteria, so planting large areas of legumes increases reactive N in soils.
• Combustion of fossil fuels increases the rate of NO_x production. 
• An energy-intensive industrial process known as the Haber-Bosch process converts N2 to NH₃, for use as an agricultural fertilizer and in industrial applications. 

Thanks to the Haber-Bosch process, there is no risk of humanity running out of N (unlike P), but the environmental impacts of the acceleration of the N cycle are far-reaching.

This table shows the magnitude of natural and anthropogenic fluxes of these three elements. Before anthropogenic disturbance, the natural carbon cycle was approximately balanced, with about 170 Pg of C undergoing photosynthesis each year, and a similar amount undergoing respiration. Anthropogenic impacts (mostly fossil fuel combustion and land-use change) are currently adding an additional 11 PgC/yr of CO₂ to the atmosphere, of which about half is being removed from the atmosphere though natural processes on land and in the oceans; these land and ocean carbon sinks reflect the fact that higher atmospheric CO₂ drives higher photosynthesis and air-water transfer. The portion remaining in the atmosphere is the main driver of climate change, while the portion in the ocean causes ocean acidification. 

The mining of P rocks has led to a 65% increase in the rate at which P is made available for biological uptake, fueling growing concerns over both the environmental impacts of excess P (primarily eutrophication) and the potential depletion of the world’s supply of accessible P-containing rocks.

The N cycle has experienced a larger anthropogenic impact than either C or P, with humanity’s annual production of reactive N now about equal in magnitude to natural production. The extent to which natural N sinks—particularly denitrification—have increased in response to higher N availability is poorly understood, but we do know that our acceleration of the N cycle is driving a variety of impacts, sometimes referred to as the “N cascade”:

• DIN drives eutrophication of coastal and marine water bodies. 
• NO₃^- (nitrate) is toxic to people when present at high concentrations in drinking water. 
• N₂O (nitrous oxide) is a greenhouse gas and a contributor to the depletion of stratospheric ozone.
• NO_x is an air pollutant that contributes to the formation of photochemical smog. It can also be oxidized in the atmosphere to form nitrate aerosols (a component of unhealthy particulate matter) and nitric acid (a component of acid rain).
• NH₃ (ammonia) in the atmosphere is a harmful air pollutant and can contribute to the formation of nitrate and sulfate aerosols. 
• Atmospheric deposition of N (originating as either NO_x or NH₃) can bring DIN—and thus eutrophication problems—to remote areas.