4-7 Matter Cycling in Ecosystems

What Are Biogeochemical Cycles? Going in Circles

All organisms are interconnected by vast global recycling systems made up of nutrient cycles, or biogeochemical cycles. In these cycles, nutrient atoms, ions, and molecules that organisms need to live, grow, and reproduce are continuously cycled among air, water, soil, rock, and living organisms.

How Is Water Cycled in the Biosphere? The Water Cycle

The hydrologic cycle, or water cycle, recycles the Earth’s fixed supply of water. Solar energy evaporates water found on the Earth’s surface into the atmosphere.

Some of this water returns to the ground as rain or snow, passes through living organisms, flows into bodies of water, and eventually is evaporated again to continue the cycle.

The main processes in this water recycling and purifying cycle are evaporation (conversion of water into water vapour), transpiration (evaporation from plant leaves after water is extracted from soil by roots and transported throughout the plant), condensation (conversion of water vapour into droplets of liquid water), precipitation (rain, sleet, hail, and snow), infiltration (movement of water into soil), percolation (downward flow of water through soil and permeable rock formations to ground- water storage areas called aquifers), and runoff (surface movement down slopes to the sea to resume the cycle).

Throughout the hydrologic cycle, many natural processes purify water. Evaporation and subsequent precipitation act as a natural distillation process that removes impurities dissolved in water.

Water flowing above ground through streams and lakes and below ground in aquifers is naturally filtered and purified by chemical and biological processes, mostly by the actions of decomposer bacteria. Thus the hydrologic cycle can also be viewed as a cycle of natural renewal of water quality.

How Are Human Activities Affecting the Water Cycle? Messing with Nature

During the past 100 years, we have been intervening in the Earth’s current water cycle in four major ways:

  1. First, we withdraw large quantities of fresh water from streams, lakes, and underground sources. In some heavily populated or heavily irrigated areas, withdrawals have led to groundwater depletion or intrusion of ocean salt water into underground water supplies.

  2. Second, we clear vegetation from land for agriculture, mining, road and building construction, and other activities; sometimes we cover the land with buildings, concrete, or asphalt. This increases runoff, reduces infiltration that recharges groundwater sup-plies, increases the risk of flooding, and accelerates soil erosion and landslides.

  3. Third, we modify water quality by adding nutrients (such as phosphates and nitrates found in fertilizers) and other pollutants.

  4. Fourth, according to a 2003 study by Ruth Curry and her colleagues, the Earth’s water cycle is speeding up as a result of a warmer climate caused partially by human inputs of carbon dioxide and other greenhouse gases into the atmosphere.

How Is Carbon Cycled in the Biosphere? Carbon Dioxide in Action

Carbon is the basic building block of the carbohydrates, fats, proteins, DNA, and other organic compounds necessary for life. It is circulated through the biosphere by the carbon cycle. This cycle is based on carbon dioxide gas, which makes up about 0.038% of the volume of the troposphere and is also dissolved in water.

Aerobic respiration in the cells of oxygen-using producers, consumers, and decomposers breaks down glucose and other complex organic compounds and converts the carbon back to CO2, which is released into the troposphere and in water for reuse by producers.

This linkage between photosynthesis in producers and aerobic respiration in producers, consumers, and decomposers circulates carbon in the biosphere and a major part of the global carbon cycle. Oxygen and hydrogen, the other elements in carbohydrates, cycle almost in step with carbon.

Oceans play important roles in the carbon cycle. Some of the atmosphere’s carbon dioxide dissolves in ocean water, and the ocean’s photosynthesizing producers remove some. On the other hand, as ocean water warms, some of its dissolved CO2 returns to the atmosphere.

Some ocean organisms build their shells and skeletons by using dissolved CO2 molecules in seawater to form carbonate compounds such as calcium carbonate (CaCO3). When these organisms die, tiny particles of their shells and bone drift slowly to the ocean depths. Geological processes may eventually expose the limestone to the atmosphere and acidic precipitation and make its carbon available to living organisms once again.

How Are Human Activities Affecting the Carbon Cycle? Messing with Nature’s Thermostat

Since 1800 and especially since 1950, we have been intervening in the Earth’s carbon cycle in two ways that add carbon dioxide to the atmosphere:

  1. First, in some areas we clear trees and other plants that absorb CO2 through photosynthesis faster than they can grow back.

  2. Second, we add large amounts of CO2 by burning fossil fuels and wood.

How Is Nitrogen Cycled in the Biosphere? Bacteria in Action

Nitrogen is a crucial component of proteins, many vitamins, and the nucleic acids DNA and RNA. However, N2 cannot be absorbed and used (metabolized) directly as a nutrient by multicellular plants or animals. Fortunately, two natural processes convert N2 gas in the atmosphere into compounds that can enter food webs as part of the nitrogen cycle.

One of these processes is atmospheric electrical dis- charge in the form of lightning. This causes nitrogen (N2) and oxygen (O2) in the atmosphere to react and produce nitrogen oxide (NO). The other process is carried out by certain types of bacteria in aquatic systems, in the soil, and in the roots of some plants that can convert or “fix” N2 into compounds useful as nutrients for plants and animals.

Ammonia not taken up by plants may undergo nitrification. In this process, specialized aerobic bacteria convert most of the ammonia in soil to nitrite ions (NO −), which are toxic to plants, and nitrate ions (NO −), which are easily taken up by plants as a nutrient.

Plants and animals return nitrogen-rich organic compounds to the environment as wastes, cast-off particles, and dead bodies. In the ammonification step, vast armies of specialized decomposer bacteria convert this detritus into simpler nitrogen-containing inorganic compounds.

How Are Human Activities Affecting the Nitrogen Cycle? Altering Nature

In the past 100 years, human activities have had several effects on the Earth’s current nitrogen cycle:

  1. First, we add large amounts of nitric oxide (NO) to the atmosphere when we burn any fuel

  2. Second, we add nitrous oxide (N2O) to the atmosphere through the action of anaerobic bacteria on livestock wastes and commercial inorganic fertilizers applied to the soil. This gas can warm the troposphere and deplete ozone in the stratosphere.

  3. Third, we release large quantities of nitrogen stored in soils and plants as gaseous compounds into the troposphere through destruction of forests, grasslands, and wetlands.

  4. Fourth, we upset aquatic ecosystems by adding excess nitrates in agricultural runoff and dis- charges from municipal sewage systems

  5. Fifth, we remove nitrogen from topsoil when we harvest nitrogen-rich crops, irrigate crops, and burn or clear grasslands and forests before planting crops.

  6. Sixth, inputs of nitrogen into the air, soil, and water mostly from our activities is beginning to affect the biodiversity of terrestrial and aquatic systems by shifting their species composition toward species that can thrive on increased supplies of nitrogen nutrients.

How Is Phosphorus Naturally Cycled in the Biosphere? Slow Cycling without Using the Atmosphere

Phosphorus circulates through water, the Earth’s crust, and living organisms in the phosphorus cycle.

Phosphorus is typically found as phosphate salts containing phosphate ions (PO 3−) in terrestrial rock formations and ocean bottom sediments. The slow weathering and erosion of phosphorus- containing rocks releases phosphorus into soil water, lakes, and rivers as phosphate ions, which are taken up by plant roots.

Phosphate can be lost from the cycle for long periods when it washes from the land into streams and rivers and is carried to the ocean. There it can be deposited as sediment on the sea floor and remain for millions of years.

Because most soils contain little phosphate, it is often the limiting factor for plant growth on land unless phosphorus (as phosphate salts mined from the ground) is applied to the soil as a fertilizer.

How Are Human Activities Affecting the Phosphorus Cycle? More Messing with Nature

We intervene in the Earth’s phosphorus cycle in three ways:

  1. First, we mine large quantities of phosphate rock to make commercial inorganic fertilizers.

  2. Second, we reduce the available phosphate in tropical soils when we cut down areas of tropical forests.

  3. Third, we disrupt aquatic systems with phosphates from runoff of animal wastes, fertilizers, and discharges from sewage treatment systems

How Is Sulphur Cycled in the Biosphere? The Sulphur Cycle

Sulphur circulates through the biosphere in the sulphur cycle. Much of the Earth’s sulphur is stored underground in rocks and minerals, including sulphate (SO 2−) salts buried deep under ocean sediments. Sulphur also enters the atmosphere from several natural sources.

How Are Human Activities Affecting the Sulphur Cycle? Overloading Nature

We add sulphur dioxide to the atmosphere in three ways:

  1. First, we burn sulphur-containing coal and oil to produce electric power.

  2. Second, we refine sulphur- containing petroleum to make gasoline, heating oil, and other useful products.

  3. Third, we convert sulphur-containing metallic mineral ores into metals such as copper, lead, and zinc.