Cycles of Life Chapter 2

Key Concepts How carbon, oxygen, and nitrogen are cycled through an ecosystem How the composition of soil affects an ecosystem How human activities affect natural cycles What is the meaning of sustainability

Life and Death Leaves fall, decay, and new plant matter reappears the next spring-decomposers break down dead leaves and plant matter Every organism is made up of chemical elements such as carbon, oxygen, hydrogen, and nitrogen Nutrients are chemicals used by organisms to build, grow, and maintain their bodies These nutrients are obtained from the surrounding environment e.g. soil, air, water, other organisms. Nutrients cycle through the environment

Life and Death Planet Earth is a closed system in which there is little if any input of new materials from outside the system Life is sustained by recycling these nutrients Life requires energy and nutrients As nutrients cycle, sometimes they are in the biotic (living) environment and at other times the abiotic(non-living) part. Nutrients are input into organisms, then throughput as they are processed, and then output back to the environment

The Carbon Cycle Plants produce carbohydrates through photosynthesis which can be summarized by 6CO 2 + 6H 2 O + energy → C 6 H 12 O 6 + 6O 2 Where do the reactants come from?

Photosynthesis, Cellular Respiration, and the Atmosphere Photosynthesis involves the capture of sunlight by green plants and the fixation of atmospheric carbon into carbohydrate molecules i.e chemical energy The carbon is present in CO 2 When consumers eat plants, the carbohydrates are broken down and recombined to build animal tissues. It must be returned to the atmosphere. This is accomplished through cellular respiration 6O 2 + C 6 H 12 O 6 → 6CO 2 + 6H 2 O + energy

Photosynthesis and cellular respiration go together

Photosynthesis uses carbon dioxide and produces oxygen whereas cellular respiration uses oxygen and produces carbon dioxide. In this way, carbon is cycled between living things and their environment Note that only producers carry out photosynthesis but all organisms carry out cellular respiration When organisms die, the remaining carbon in their bodies is released when bacteria and fungi carry out cellular respiration

Disrupting the Carbon Cycle Recall from “An Inconvenient Truth” that CO 2 levels remained fairly constant for a long time How has the Carbon Cycle been disrupted? Much of the carbon cycle takes place in the oceans and lakes i.e 50X as much carbon dioxide is held in the oceans and lakes as in the atmosphere. Carbon dioxide dissolves in water to form carbonic acid which aquatic plants use as their source of carbon. It has been estimated at 100 billion tonnes. Carbon is an ingredient in the calcium carbonate shells of organisms with shells. In this way, some carbon is removed from the cycle. See graphic p. 50.

The Nitrogen Cycle When we want our lawns and gardens to grow and maintain good health, we fertilize them. Nitrogen, phosphorus, and potassium are three elements needed for growth and are found in most fertilizers. Most plants are not fertilized so they must draw their nitrogen from other sources. Where would you think they could access a ready supply of nitrogen?

Nitrogen Cycle Atmospheric nitrogen is easily accessible but not usable in this gaseous form. It must be fixed into compounds usable by plants. Nitrogen fixation refers to the process of taking nitrogen atoms and bonding them chemically to other elements to form compounds. Two common polyatomic ions usable by plants are ammonium NH 4 + and nitrate NO 3 -

Nitrogen Cycle Nitrogen-fixing bacteria such as Rhizobia (found in nodules of roots of legumes such as peas, beans, alfalfa, and clover) perform the task of nitrogen fixation Once nitrogen fixation has taken place, it enters the soil and water for uptake and movement through the food chain.. Nitrogen returns to the soil through dead organisms and waste material Nitrogen can also cycle in an aquatic ecosystem. See graphic.

Nitrogen Cycle Animals can only obtain nitrogen by eating plant material or other animals. Refer to the graphic Denitrifying bacteria carry out denitrification in which gaseous nitrogen is removed from nitrates and ammonia and is returned to the atmosphere. Decomposers break down waste and dead materials often releasing ammonia- a strong smelling gas found in chicken coops, outhouses, commercial cleaners etc.

Altering the balance As elements move through a nutrient cycle, they move through the abiotic environment, through living organisms and through waste products or dead matter. The rate at which the nutrient cycles is balanced at each step. If additional nutrients are added at any step, the balance is altered and subsequent stages are impacted.

Human Impacts on Nitrogen Cycle In the early 20 th Century artificial fertilizers were added to crops resulting in a dramatic increase in food production. Increasing the amount of nitrates for plants greatly increased the growth of the plants. Combined with the increased combustion of fossil fuels, and other human activities, there is approximately 140 million tonnes of extra nitrogen in the environment. This is nitrogen overload!

Effects on the Soil and the Atmosphere Soil Plant growth limited by scarcity of resources such as phosphorus, calcium, water Ecosystem saturation leads to run-off into streams and ground water Tree roots damaged Growth stunted Needles on conifers turn yellow and fall Soil acidity increases Reactions with metals in soil further damage roots and organisms Nitrogen oxides lead to acid precipitation Fish, birds, amphibians, other organisms killed by acid precip Sugar maple trees in Ontario and Quebec killed by acid precip

Effects on Fresh Water and Marine Ecosystems In 1970’s many lakes & streams became choked by rapid growth of algae and weeds. Increased growth was due to nitrates and phosphates which washed into waterways. Nitrogen-containing wastes in sewage lead to a buildup of nutrients in aquatic ecosystems. This is called eutrophication. In rural areas, nitrates in the drinking water supply lead to reduction in the ability of blood to carry oxygen producing a form of anemia Increased nitrogen levels find their way to ocean ecosystems. Added nitrates lead to algal blooms which result from a population explosion of algae in the warmer surface waters. Algae are broken down by bacteria. They decompose using up more oxygen in the deeper colder seawater. This depletes the oxygen level and further impacts food chains. End result is significant fish and shellfish kills

Effects on Biodiversity Biodiversity measures the variety of species on Earth. What conditions would favour high biodiversity? Low biodiversity? Biodiversity is lowered by very high level of nutrients as only a few species would benefit and the less dominant ones would be eliminated or significantly reduced

Sustainability Ecosystems are sustainable i.e. they can perpetuate themselves indefinitely requiring little or no additional materials each year. The key to this is recycling. Human activities deplete non-renewable resources and produce polluting wastes which enter nutrient cycles and damage the productivity of ecosystems. We need to implement the concept of sustainable development in which development meets present needs without compromising the ability of future generations to meet their own needs.

Decisions needed for Sustainable Development More efficient and effective use of scarce resources Reducing, re-using, recycling, recovering materials in manufacturing products as well as waste products created Restoring and reclaiming damaged environments Conserving and developing substitutes for scarce resources Changing political and social structures to reduce human impact on the environment Reducing consumption