Why is biogeochemical cycles important to environmental science

Deeper underground are fossil fuels, the anaerobically decomposed remains of plants and algae that lived millions of years ago. Fossil fuels are considered a non-renewable resource because their use far exceeds their rate of formation.

A non-renewable resource is either regenerated very slowly or not at all. Another way for carbon to enter the atmosphere is from land including land beneath the surface of the ocean by the eruption of volcanoes and other geothermal systems.

Carbon sediments from the ocean floor are taken deep within Earth by the process of subduction : the movement of one tectonic plate beneath another. Carbon is released as carbon dioxide when a volcano erupts or from volcanic hydrothermal vents.

Getting nitrogen into living organisms is difficult. Plants and phytoplankton are not equipped to incorporate nitrogen from the atmosphere where it exists as tightly bonded, triple covalent N 2 even though this molecule comprises approximately 78 percent of the atmosphere. Nitrogen enters the living world through free-living and symbiotic bacteria, which incorporate nitrogen into their organic molecules through specialized biochemical processes.

At this point, the nitrogen-containing molecules are used by plants and other producers to make organic molecules such as DNA and proteins. This nitrogen is now available to consumers. Organic nitrogen is especially important to the study of ecosystem dynamics because many ecosystem processes, such as primary production, are limited by the available supply of nitrogen.

As shown in Figure 4 below, the nitrogen that enters living systems is eventually converted from organic nitrogen back into nitrogen gas by bacteria Figure 4.

The process of denitrification is when bacteria convert the nitrates into nitrogen gas, thus allowing it to re-enter the atmosphere. Human activity can alter the nitrogen cycle by two primary means: the combustion of fossil fuels, which releases different nitrogen oxides, and by the use of artificial fertilizers which contain nitrogen and phosphorus compounds in agriculture, which are then washed into lakes, streams, and rivers by surface runoff.

A major effect from fertilizer runoff is saltwater and freshwater eutrophication , a process whereby nutrient runoff causes the overgrowth of algae, the depletion of oxygen, and death of aquatic fauna.

In marine ecosystems, nitrogen compounds created by bacteria, or through decomposition, collects in ocean floor sediments. Although the movement of nitrogen from rock directly into living systems has been traditionally seen as insignificant compared with nitrogen fixed from the atmosphere, a recent study showed that this process may indeed be significant and should be included in any study of the global nitrogen cycle.

Phosphorus is an essential nutrient for living processes. It is a major component of nucleic acids and phospholipids, and, as calcium phosphate, it makes up the supportive components of our bones. Phosphorus is often the limiting nutrient necessary for growth in aquatic, particularly freshwater, ecosystems. Phosphorus occurs in nature as the phosphate ion PO 4 In addition to phosphate runoff as a result of human activity, natural surface runoff occurs when it is leached from phosphate-containing rock by weathering, thus sending phosphates into rivers, lakes, and the ocean.

This rock has its origins in the ocean. Phosphate-containing ocean sediments form primarily from the bodies of ocean organisms and from their excretions. However, volcanic ash, aerosols, and mineral dust may also be significant phosphate sources.

Figure below. Phosphorus is also reciprocally exchanged between phosphate dissolved in the ocean and marine organisms. The movement of phosphate from the ocean to the land and through the soil is extremely slow, with the average phosphate ion having an oceanic residence time between 20, and , years. Excess phosphorus and nitrogen that enter these ecosystems from fertilizer runoff and from sewage cause excessive growth of algae.

The subsequent death and decay of these organisms depletes dissolved oxygen, which leads to the death of aquatic organisms such as shellfish and fish.

This process is responsible for dead zones in lakes and at the mouths of many major rivers and for massive fish kills, which often occur during the summer months see Figure 6 below. A dead zone is an area in lakes and oceans near the mouths of rivers where large areas are periodically depleted of their normal flora and fauna. These zones are caused by eutrophication coupled with other factors including oil spills, dumping toxic chemicals, and other human activities. The number of dead zones has increased for several years, and more than of these zones were present as of One of the worst dead zones is off the coast of the United States in the Gulf of Mexico: fertilizer runoff from the Mississippi River basin created a dead zone of over 8, square miles.

Phosphate and nitrate runoff from fertilizers also negatively affect several lake and bay ecosystems including the Chesapeake Bay in the eastern United States. Sulfur is an essential element for the molecules of living things.

As part of the amino acid cysteine, it is involved in the formation of proteins. As shown in Figure 7 below, sulfur cycles between the oceans, land, and atmosphere. Atmospheric sulfur is found in the form of sulfur dioxide SO 2 , which enters the atmosphere in three ways: first, from the decomposition of organic molecules; second, from volcanic activity and geothermal vents; and, third, from the burning of fossil fuels by humans.

On land, sulfur is deposited in four major ways: precipitation, direct fallout from the atmosphere, rock weathering, and geothermal vents. Consequently, the atmospheric reservoir plays a paramount role in the cycling of nitrogen Figure 5. Model of the Global Nitrogen Cycle.

Nitrogen occurs in three main compartments: the atmosphere, terrestrial organic material, and oceanic organic material. Based on data from Hutzinger and Freedman These trace gases typically occur in atmospheric concentrations much less than 1 ppm, although there may be larger amounts close to sources of anthropogenic emissions. Nitrogen occurs in many additional forms in terrestrial and aquatic environments. These chemicals range in character from simple amino acids, through proteins and nucleic acids, to large and complex molecules that are components of humified organic matter.

Nitrogen in ecosystems also occurs in a small number of inorganic compounds, the most important of which are N 2 and NH 3 gases and the ions nitrate, nitrite NO 2 — , and ammonium. The nitrogen cycle involves the transformation and cycling of the various organic and inorganic forms of nitrogen within ecosystems. Because the two nitrogen atoms in dinitrogen gas are held together by a strong triple bond, N 2 is a highly unreactive compound.

For this reason N 2 can be directly used by only a few specialized organisms, even though it is extremely abundant in the environment. These nitrogen-fixing species, all of which are microorganisms, have the ability to metabolize N 2 into NH 3 gas, which can then be used for their nutrition.

More importantly, the NH 3 also becomes indirectly available to the great majority of autotrophic plants and microorganisms that cannot fix N 2 themselves. Biological nitrogen fixation is a critical process — most ecosystems depend on it to provide the nitrogen that sustains their primary productivity. In fact, because nitrogen is not an important constituent of rocks and soil minerals, N 2 fixation is ultimately responsible for almost all of the organic nitrogen in the biomass of organisms and ecosystems throughout the biosphere.

The only other significant sources of fixed nitrogen for ecosystems are the atmospheric deposition of nitrate and ammonium in precipitation and dustfall, and the uptake of NO and NO 2 gases by plants. However, these are generally minor sources in comparison with biological N 2 fixation.

The best known of the N 2 -fixing microorganisms are bacteria called Rhizobium, which live in specialized nodules on the roots of leguminous plants, such as peas and beans. Some non-legumes, such as alders, also live in a beneficial symbiosis a mutualism; see Chapter 9 with N 2 -fixing microorganisms.

So do most lichens, which are a mutualism between a fungus and an alga. Many other N 2 -fixing microbes are free-living in soil or water, such as cyanobacteria blue-green bacteria.

Non-biological nitrogen fixation also occurs, for instance during a lightning event when atmospheric N 2 combines with O 2 under conditions of great heat and pressure. Humans can also cause N 2 to be fixed. For example, nitrogen fertilizer is manufactured by combining N 2 with hydrogen gas H 2 , which is manufactured from CH 4 , a fossil fuel in the presence of iron catalysts to produce NH 3.

In addition, NO gas is formed in the internal combustion engines of vehicles, where N 2 combines with O 2 under conditions of high pressure and temperature. Large amounts of NO are emitted to the atmosphere in vehicle exhaust, contributing to air pollution Chapter This is a globally important component of the modern nitrogen cycle and is comparable in magnitude with non-human N 2 fixation about million tonnes per year.

Most species in the pea family Fabaceae , such as these soybeans, develop a mutualism with Rhizobium bacteria. The Rhizobium live in nodules on the roots and fix nitrogen gas N 2 into ammonia NH 3 , which the plant can use as a nutrient. Source: D. After an organism dies, its organically bound nitrogen must be converted to inorganic forms; otherwise, the recycling of its fixed nitrogen would not be possible Figure 5.

As such, ammonification is a component of the complex process of decay, but one that is specific to the nitrogen cycle. Ammonification is carried out by a variety of microorganisms.

The resulting ammonium is a suitable source of nutrition for many species of plants, particularly those that live in environments with acidic soil. Nitrification is the process by which nitrate is synthesized from ammonium. Once the nitrite is formed, it is rapidly oxidized to nitrate by Nitrobacter bacteria.

Because Nitrosomonas and Nitrobacter are sensitive to acidity, nitrification does not occur in acidic soil or water. This is why plants growing in acidic habitats must be able to use ammonium as their source of nitrogen. Important Transformations of Fixed Nitrogen in Ecosystems. The diagram indicates the key transformations of nitrogen among its most important inorganic forms in soil and aquatic ecosystems.

Source: Modified from Freedman In denitrification, also performed by a wide variety of microbial species, nitrate is converted to either of the gases N 2 O or N 2 , which are released to the atmosphere. Denitrification occurs under anaerobic conditions, and its rate is greatest when there is a large concentration of nitrate, for example in fertilized agricultural land that is temporarily flooded. In some respects, denitrification can be considered a counter-balancing process to nitrogen fixation.

In fact, global rates of nitrogen fixation and denitrification are in a rough balance, so the total amount of fixed nitrogen in the biosphere is not changing much over time. Phosphorus is a key constituent of many biochemicals, including fats and lipids, nucleic acids such as the genetic materials DNA and RNA, and energy-carrying molecules such as ATP.

However, phosphorus is required by organisms in much smaller quantities than nitrogen or carbon. Nevertheless, phosphorus is often in short supply and so it is a critical nutrient in many ecosystems, particularly in freshwater and agriculture. In contrast to the carbon and nitrogen cycles, that of phosphorus does not have a significant atmospheric phase.

Although phosphorus compounds do occur in the atmosphere, as trace quantities in particulates, the resulting inputs to ecosystems are small compared with the amounts available from soil minerals or from the addition of fertilizer to agricultural land. Phosphorus tends to move from the terrestrial landscape into surface waters and then eventually to the oceans, where it deposits to sediment that acts as a long-term sink. Although some phosphorus minerals in oceanic sediment are eventually recycled to the land by geological uplift associated with mountain building, this is an extremely slow process and is not meaningful in ecological time scales.

Therefore, aspects of the global phosphorus cycle represent a flow-through system. Nevertheless, certain processes do return some marine phosphorus to portions of the continental landscape. For example, some kinds of fish spend most of their life at sea but migrate up rivers to breed.

When they are abundant, fish such as salmon import substantial quantities of organic phosphorus to the higher reaches of rivers, where it is decomposed to phosphate after the fish spawn and die. Fish-eating marine birds are also locally important in returning oceanic phosphorus to land through their excrement. Soil is the principal source of phosphorus uptake for terrestrial vegetation. The phosphate ion PO 4 3— is the most important form of plant-available phosphorus. Although phosphate ions typically occur in small concentrations in soil, they are constantly produced from slowly dissolving minerals such as calcium, magnesium, and iron phosphates Ca 3 PO 4 2 , Mg 3 PO 4 2 , and FePO 4.

Phosphate is also produced by the microbial oxidation of organic phosphorus, a component of the more general process of decay. Water-soluble phosphate is quickly absorbed by microorganisms and by plant roots and used in the synthesis of a wide range of biochemicals.

Aquatic autotrophs also use phosphate as their principal source of phosphorus nutrition. In fact, phosphate is commonly the most important limiting factor to the productivity of freshwater ecosystems. This means that the primary productivity will increase if the system is fertilized with phosphate, but not if treated with sources of nitrogen or carbon unless they first have sufficient PO 4 3— added; see Chapter Lakes and other aquatic ecosystems receive most of their phosphate supply through runoff from terrestrial parts of their watershed, and by the recycling of phosphorus from sediment and organic phosphorus suspended in the water column.

Humans are greatly affecting the global phosphorus cycle by mining it to manufacture fertilizer, and applying that material to agricultural land to increase its productivity.

For some time, the major source of phosphorus fertilizers was guano, the dried excrement of marine birds. Guano is mined on islands, such as those off coastal Chile and Peru, where breeding colonies of seabirds are abundant and the climate is dry, allowing the guano to accumulate. During the twentieth century, however, deposits of sedimentary phosphate minerals were discovered in several places, such as southern Florida.

Phosphorus had become geologically concentrated in sedimentary deposits in these places through the deposition of marine organisms over millions of years. These deposits are now being mined to supply mineral phosphorus used to manufacture agricultural fertilizer. What are some of the challenges in determining cause and effect relationships within biogeochemical cycles? Earth systems are incredibly complex and interconnected meaning that one change can trigger multiple abiotic and biological responses and feedbacks.

This, for example, can make studying effects of climate change on organic carbon preservation and cycling in soils challenging. How do human activities affect, and how are they affected by, biogeochemical cycles? It is hard to name something in the environment that is not influenced by humans, including biogeochemical cycles. Climate change is a big concern now and effects of climate change and feedbacks are particularly dramatic in the regions of the permafrost.

Herndon et al. One of the biggest concerns is positive feedback on the global warming due to the release of CO 2 and methane, but many other elements, such as P, N, S, and Fe, are affected.

Why is there urgency in studying the interconnectedness of different ecosystems? Earth is one system with numerous subsystems that are continuously interacting. Different ecosystems cannot be fully understood if they are studied in isolation because they are not closed systems.

Role of Water in the Ecosystem. Why Are Ecosystems So Important? Elements in the Biosphere. Four Components of an Ecosystem. Things That Makes Up an Ecosystem. The Three Cycles of the Ecosystem. Why Is the Food Web Important? Is Grass a Producer or Consumer?