Chapter 3, Problem 9CR

To determine

The key concept under Section 3-4. The effect on matter in an ecosystem. The definition of a nutrient cycle. The way in which nutrient cycles connect the past, present, and future life. The description for hydrologic or water cycle. The three major processes involved in the water cycle. The definition of surface runoff. The definition for groundwater. The definition for an aquifer. The percentage of the Earth’s water supply that is available to humans and other species as liquid freshwater. The unique properties of water. Three ways through which the humans alter the water cycle. The way in which clearing a rain forest can affect local weather and climate.

Answer to Problem 9CR

The main concept dealt under Section 3-4 is movement of matter throughout the ecosystems and biosphere by means of various biogeochemical cycles along with the human impact on these cycles. The flow of matter and nutrient within and in between ecosystems takes place through different biogeochemical or nutrient cycles. Nutrient cycles enable the sustainable use of matter and nutrients. The cycling of water among the different spheres of the Earth is known as hydrologic or water cycle. Surface runoff is the flow of water over the surface into the Earth’s water bodies when the rate of precipitation exceeds the rate of infiltration. Groundwater is the water that is present below the Earth’s surface. The porous and permeable rock unit within which the groundwater gets stored is known as an aquifer. Out of the 2 percent freshwater on the Earth, only 0.024% water is available for humans and animals as liquid freshwater for drinking and other activities. The attractive forces between the molecules, the high specific heat capacity, the expansion on freezing, the ability to filter the harmful UV wavelengths, and the capability to dissolve a large number of substances are some of the unique characteristics of water. Humans alter the water cycle by extracting more water from the rivers, lakes, and aquifers at a rate that surpasses their replenishing capacity. Moreover, increased deforestation that triggers a higher amount of runoff, polluting water by dumping of industrial wastes and sewages, and the damaging of wetlands that can increase future flood risk are also some of the ways in which the water cycle can get altered. The degradation of tropical forests may lead to the extinction of many wildlife species, and an increase in the atmospheric carbon dioxide content eventually leads to global warming. Moreover, the regional weather can also be affected by clearing of the forests. Carbon cycle involves the cycling of carbon dioxide in different forms throughout the Earth’s spheres. In the nitrogen cycle, nitrogen gets transferred in various forms throughout the spheres of the Earth. In phosphorus cycle, different forms of phosphorus get cycled through the hydrosphere, crust, and biosphere of the Earth. Sulphur cycle involves the circulation of sulfur within the biosphere. Developmental activities, intensive agricultural practices, and use of fossil fuels, fertilizers, and pesticides by the humans can cause disruption in each of these cycles.

Explanation of Solution

The main concepts of Section 3-4 include matter and nutrients in the environment, their cycling through the ecosystems and biosphere by means of various biogeochemical cycles, and the type of anthropogenic activities changing these chemical cycles. Matter moves continuously through air, water, rocks, soil, and other living organisms within an ecosystem or from one ecosystem to another, and this movement is known as nutrient cycle or biogeochemical cycle.

The nutrient cycling is to ensure the equal supply of nutrients from time to time to maintain the continuity of life forms. The nutrient cycles that take place on the Earth are water (hydrologic) cycle, nitrogen cycle, oxygen cycle, phosphorous cycle, carbon cycle, and sulfur cycle.

Water cycle or hydrologic cycle can be defined as the cyclic movement of water between the different spheres of the Earth. The cycle is directly or indirectly powered by solar energy and the Earth’s surface. It is very important for the living organisms.

The unique properties of water are enlisted below:

1. i. Water can exist in the form of liquid over an extensive range of temperature due to the attractive forces between its molecules. Moreover, it can also take the form of solid (ice) and gas (water vapor).
2. ii. The change in the temperature of liquid water is gradual as it is able to store a huge quantity of heat with no significant fluctuation in its own temperature. Thus, the specific capacity of water is very high due to which it absorbs and releases the heat very slowly.
3. iii. The attractive forces between the molecules in water require a high quantity of energy for evaporation. The phase change of water will require huge quantities of energy.
4. iv. Liquid water is capable of dissolving a number of compounds.
5. v. Water can filter out wavelengths of the solar ultraviolet radiation that would otherwise harm the aquatic life forms.
6. vi. The process of freezing of water causes its expansion. This is why ice occupies more space than water. However, water is denser than ice due to which it floats over water surface.

The three main phenomena involved in water cycle are evaporation, precipitation, and runoff. Condensation, infiltration, and transpiration are also important in water cycle. Once water gets evaporated from the water bodies on the Earth, it rises up to form clouds, which then condenses and falls down as precipitation of any form (rain, snow, or hailstones depending on the physical conditions). The precipitation that reaches the ground can runoff to surface water bodies, can infiltrate down adding to groundwater, can be absorbed by plants, and can again be evaporated into the atmosphere directly or through transpiration. Thus, the cycle goes on.

Surface runoff refers to the process by which the precipitation water that reaches the ground flows over the surface to join the waterbodies such as lakes, ponds, rivers, and oceans. Runoff occurs when the soil is saturated and cannot hold more water due to an increased precipitation rate than infiltration rate.

The water stored in the subsurface of the Earth is known as groundwater. The groundwater may be stored in the aquifers. An aquifer is a rock unit with porosity and permeability and an impermeable layer at the base. There is a total of 2% freshwater available on the Earth; however, the percentage of water available to the humans and other creatures in the form of liquid fresh water is just about 0.024 percent. The rest of the fresh water is frozen and locked up within the glaciers.

Nowadays, the water cycle is being adversely affected by anthropogenic activities mainly in three ways. The disruption of water cycle can cause climatic instability. Sometimes, humans withdraw water from the lakes, rivers, and aquifers at a faster rate than the rate at which water gets naturally replenished. This can lead to the depletion of some aquifers and disappearance of certain rivers, which makes it unavailable for the generation yet to come.

When the vegetation on the land is removed for agriculture, road construction, mining, or other activities and when the land gets covered with concrete, asphalt, and buildings, the precipitation falling on the surface would not infiltrate down and would rather run off. It brings a decline in the groundwater recharge due to which the water table drops. The future generation would face scarcity of good quality groundwater due to this reason.

Moreover, the wetlands may be drained and filled for cultivation or urban development. Wetlands act as good flood controls as they absorb and retain water overflows acting like a sponge. Therefore, when wetlands are disturbed, the chance of flooding also increases in the future.

The tropical rain forests help in controlling the carbon dioxide concentration of the Earth’s atmosphere, which in turn is helpful to reduce global warming. Therefore, clearing of those forests would cause an increase in the atmospheric carbon content and thus initiates global warming. The rain pattern will also be affected by clearing the forests since forests maintain homogenous weather. Therefore, the local patterns of weather may be affected by the large-scale loss of tropical forests. In certain cases, those changes would restrict the regrowth of rain forests in the degraded areas and convert such regions into tropical grasslands.

Carbon cycle:

The carbon cycle includes the cycling of carbon that forms the fundamental building unit of carbohydrates, proteins, fats, and other organic compounds. The carbon cycle is based on carbon dioxide. The atmospheric carbon dioxide may be absorbed by the plants for utilizing it for photosynthesis. When these plants decay, they get converted into organic carbon and later form fossil fuels. The combustion of fossil fuels again releases CO₂ into the atmosphere.

Carbon dioxide may be added to the atmosphere from the respiration of animals, plants, and also from the emissions of industries and automobiles. When the organisms die, they also gradually form a part of the fossil fuels. The entire cycle continues.

However, humans are changing the normal or permissible amount of atmospheric carbon dioxide generally by the overuse of fossil fuels and by damaging the carbon-absorbing vegetation.

Nitrogen cycle:

The Earth’s atmosphere holds about 78% of nitrogen. It forms an important constituent of proteins, several vitamins, and nucleic acids such as DNA. In the nitrogen cycle, the atmospheric free nitrogen gets fixed by certain bacteria into ammonia. The process of lightening converts nitrogen into a usable form. The ammonia in the soil is then transformed into nitrite and nitrate ions by some specific bacteria by the process of nitrification.

Nitrates are absorbable forms of nitrogen for plants; hence, they absorb them and utilize them for their growth and productivity. The detritus and dead remains of the plants may be converted by some decomposer bacteria into ammonia (ammonification process). The ammonia and the ammonium ions found in the soil get converted by some denitrification bacteria into nitrite and nitrate ions and then into nitrogen gas and nitrous oxide gas, which are then liberated into the atmosphere. The whole cycle repeats.

The addition of nitrogen from the land into the remaining part of the environment during a year has more than doubled due to human activities such as the largely increased utilization of inorganic fertilizers in agriculture. Such an unnecessary addition of nitrogen into the water and air might cause pollution and other problems.

Phosphorus cycle:

In the phosphorous cycle, different compounds of phosphorus circulate through hydrosphere, biosphere, and crust of the Earth. The cycling of phosphorus does not take place through the Earth’s atmosphere. The ocean floor sediments and the rock formations form the principal phosphorus reservoirs of the Earth. They are circulated through food webs and form a significant constituent of various biological molecules. They also act as limiting factor in plant growth.

Phosphate is removed at a high rate from the Earth by the humans for making fertilizers, and the phosphate concentration in tropical soils is also decreased by destructing forests. The runoff from agricultural lands carry large quantities of phosphate into the water bodies, where they can use algal bloom or eutrophication due to which the cycling of chemicals and other phenomena get disrupted.

Sulfur cycle:

The sulfur cycle encompasses the process of circulation of sulfur components through the biosphere. Sulfur forms a necessary constituent of the macromolecules of biotic components as it controls the 3-D folding patterns of proteins. Much of the sulfur may be stored underground within the rocks and minerals as sulfate ions and salts. The sulfate salts might be buried within the ocean sediments.

On land, sulfur reaches the atmosphere through acid rain, weathering of rocks, organism material decomposition, geothermal vents, and fallouts. Sulfur reaches the ocean through runoff, subaquatic geothermal vents, and fallout. Certain oceanic ecosystems also depend on chemoautotrophs for sulfur. The combustion of fossil fuels causes an increase in the quantity of sulfide in the atmosphere and triggers acid rain. In turn, acid rain destructs the aquatic ecosystems and corrodes man-made buildings and structures.