APES 1.5-1.6: Nitrogen and Phosphorus Cycles

Nitrogen cycle

The biogeochemical cycling of nitrogen among atmospheric, terrestrial, and marine reservoirs through physical, chemical, and biological transformations.

Why is atmospheric nitrogen not directly available to most organisms?

Although $N_2$ makes up about 78% of Earth's atmosphere, its strong triple bond makes it relatively unreactive. Most organisms require nitrogen in fixed forms such as $NH_4^+$, $NO_2^-$, $NO_3^-$, or organic nitrogen compounds.

Important chemical forms of environmental nitrogen

Nitrogen occurs as organic nitrogen, ammonium ($NH_4^+$), ammonia ($NH_3$), nitrite ($NO_2^-$), nitrate ($NO_3^-$), nitrogen gas ($N_2$), nitric oxide ($NO$), and nitrous oxide ($N_2O$).

Nitrogen fixation

Nitrogen fixation converts atmospheric $N_2$ into biologically usable nitrogen compounds, especially ammonia or ammonium. It occurs through biological, industrial, and atmospheric processes.

Biological nitrogen fixation

Diazotrophic bacteria and some Archaea use nitrogenase to reduce $N_2$ to ammonia, which can be incorporated into organic compounds. The enzyme commonly contains molybdenum-containing metal centers.

How do legumes obtain an advantage from symbiotic nitrogen fixation?

Bacteria such as Rhizobium live in root nodules of legumes and fix atmospheric $N_2$. The plant supplies the bacteria with carbohydrates, while receiving fixed nitrogen in return, increasing nitrogen availability in the soil.

Industrial nitrogen fixation

The Haber–Bosch process uses high temperature and pressure to synthesize ammonia from nitrogen gas and a hydrogen source: $N_2 + 3H_2 \rightleftharpoons 2NH_3$.

Major nonbiological source of fixed nitrogen

Lightning provides enough energy to convert atmospheric nitrogen and oxygen into reactive nitrogen compounds, including nitrogen oxides that can ultimately form nitrate.

Assimilation in the nitrogen cycle

Assimilation is the incorporation of inorganic nitrogen into organic molecules. Plants absorb $NO_3^-$ or $NH_4^+$ and use the nitrogen to synthesize amino acids, nucleic acids, chlorophyll, and other biomolecules.

How is nitrate processed before plants incorporate its nitrogen?

Plants reduce nitrate to nitrite and then to ammonium before incorporating the nitrogen into organic compounds: $NO_3^- \rightarrow NO_2^- \rightarrow NH_4^+$.

How do heterotrophic organisms acquire nitrogen?

Animals, fungi, and many other heterotrophs generally obtain nitrogen by consuming amino acids, nucleotides, and other organic molecules. Some microorganisms can instead use inorganic nitrogen, such as ammonium.

Ammonification (mineralization)

Ammonification is the microbial conversion of organic nitrogen in dead organisms and wastes into ammonia ($NH_3$) and often ammonium ($NH_4^+$). Bacteria and fungi use enzymes such as proteases and deaminases in this decomposition process.

Nitrification

Nitrification is the aerobic oxidation of ammonium to nitrate, usually through two microbial steps: $NH_4^+ \rightarrow NO_2^- \rightarrow NO_3^-$. It is carried out by nitrifying microorganisms.

Which microorganisms perform the two conventional steps of nitrification?

Ammonia-oxidizing organisms such as Nitrosomonas oxidize ammonia or ammonium to nitrite. Nitrite-oxidizing organisms such as Nitrobacter then oxidize nitrite to nitrate.

Why does nitrification require oxygen?

Nitrification is an aerobic oxidation process. Nitrifying microorganisms use oxygen while converting ammonium first to nitrite and then to nitrate.

Why does nitrate readily contaminate groundwater?

Nitrate is highly soluble, and soil generally retains anions poorly. Consequently, nitrate can leach through soil into groundwater.

Denitrification

Denitrification is the microbial reduction of nitrate to gaseous nitrogen, commonly under anaerobic conditions. The overall transformation returns nitrogen to the atmosphere: $NO_3^- \rightarrow N_2$.

Why can denitrifying bacteria respire without oxygen?

Under anaerobic conditions, denitrifying bacteria use nitrate as an electron acceptor in place of $O_2$. The reduced nitrogen products can include $N_2O$ and ultimately $N_2$, which is unavailable to most plants.

How have human activities altered the global nitrogen cycle?

Industrial fertilizer production, cultivation of legumes, fossil-fuel combustion, biomass burning, livestock production, and wastewater release have greatly increased the movement of reactive nitrogen into ecosystems and the atmosphere.

Why does excess nitrogen fertilizer promote eutrophication?

Nitrate and other soluble nitrogen compounds can run off into water bodies, stimulating excessive algal and cyanobacterial growth. Decomposition of this biomass consumes dissolved oxygen, producing hypoxic or anoxic conditions.

Why can eutrophication cause fish kills?

Algal blooms eventually decompose, and microbial respiration consumes dissolved oxygen. Oxygen depletion can create hypoxic or anoxic water in which fish and other aquatic organisms cannot survive.

Environmental effects of excessive nitrogen deposition

Excess nitrogen can cause eutrophication, freshwater and marine acidification, biodiversity loss, soil acidification, metal mobilization, and reduced ecosystem resilience. It can also favor nitrogen-demanding plant species over species adapted to nitrogen-poor conditions.

How does nitrogen deposition acidify soils?

Nitrification and atmospheric deposition can add acidic species or generate $H^+$ ions. Increased acidity promotes leaching of base cations and can mobilize aluminum and other metals that may be toxic to plants.

Major human-related sources and effects of atmospheric $N_2O$

Agricultural fertilization, biomass burning, livestock operations, and industry increase $N_2O$ emissions. Nitrous oxide is a potent greenhouse gas and contributes to stratospheric ozone destruction.

What is nitrogen saturation in an ecosystem?

Nitrogen saturation occurs when nitrogen inputs exceed the biological demand and processing capacity of an ecosystem. Excess nitrogen can then leach into water, acidify soils, alter species composition, and damage plants and aquatic organisms.

Phosphorus cycle

The biogeochemical movement of phosphorus among rocks, soil, water, and living organisms. Unlike nitrogen, phosphorus has no major atmospheric reservoir and cycles primarily through geological and biological processes.

What is the main reservoir of phosphorus?

The main reservoir of phosphorus is phosphate-containing rock and marine sediments. Weathering releases phosphate ions into soil and water.

How does phosphorus enter ecosystems?

Weathering and erosion of phosphate-containing rocks release phosphate into soil and freshwater. Plants absorb the phosphate, and animals obtain phosphorus by consuming plants or other organisms.

How is phosphorus returned to soil and water?

Decomposition of organisms and wastes returns organic phosphorus to soil and water, where decomposers convert it into inorganic phosphate that can be reused by plants.

Why is the phosphorus cycle slower than the nitrogen cycle?

Phosphorus is released mainly through the slow weathering and erosion of rocks and is often carried into sediments. Because it lacks a substantial gaseous atmospheric phase, its cycle is dominated by slow geological processes.

How can excess phosphorus cause eutrophication?

Phosphate runoff from fertilizers, animal waste, sewage, and erosion can stimulate excessive algal growth in aquatic ecosystems. When the algae decompose, microbial respiration lowers dissolved oxygen and may create hypoxic or anoxic conditions.

How do nitrogen and phosphorus limit primary productivity?

Nitrogen and phosphorus are essential nutrients for producers. When either is scarce, it can limit plant or algal growth; adding the limiting nutrient can increase productivity until another factor becomes limiting.