Cycles of Matter: Water, Carbon and Nitrogen

Older than the dinosaurs

The water in your glass today may once have fallen as snow on a mountain, flowed down a river, been sipped by a dinosaur and floated in a cloud over the ocean. The carbon atoms in your body may once have been in a maple leaf, a bison or the air above the Pacific.

Earth gets a constant supply of energy from the Sun. But it gets almost no new matter. Living things have been reusing the same water, carbon and nitrogen for billions of years. That only works because matter cycles.

Energy flows, matter cycles

• Energy moves one way: Sun → producers → consumers, and most of it is lost as heat at each step (last lessons).
• Matter is reused. Atoms are not used up. They move from the air to plants, to animals, to decomposers, to the soil and back again.

When a living thing dies, its matter does not disappear. Decomposers break it down, and the same atoms become part of new living things. You will look at three important cycles: water, carbon and nitrogen.

The water cycle

You learned about the water cycle in earlier grades. Here is the full cycle, including the parts living things play:

1. Evaporation: the Sun warms lakes, rivers and oceans, and water becomes invisible water vapour.
2. Transpiration: plants pull water up from the soil through their roots and release water vapour from their leaves. A large tree can release hundreds of litres of water on a hot summer day.
3. Condensation: water vapour cools high in the air and forms tiny droplets that make clouds.
4. Precipitation: rain, snow, sleet or hail falls.
5. Collection and runoff: water soaks into the ground (groundwater) or runs over the surface into streams and lakes.

Living things are part of the water cycle. Animals drink water and release it when they breathe out, sweat and get rid of wastes. Wetlands and forests soak up rain like sponges and release it slowly.

The carbon cycle

Carbon is in every living thing: in sugars, fats, proteins and the wood of trees. Carbon moves like this:

1. Photosynthesis: plants and algae take carbon dioxide from the air (or water) and build it into sugar.
2. Cellular respiration: plants, animals and decomposers break down sugar for energy and release carbon dioxide back into the air.
3. Eating: carbon moves from plants into animals when animals eat them.
4. Death and wastes: dead plants, dead animals and wastes pass carbon to decomposers.
5. Decomposition: decomposers use the dead matter and release carbon dioxide.

Some carbon gets stored for a long time. Forests store carbon in wood. Ontario's Hudson Bay Lowlands have huge peatlands, where cold, wet conditions slow decomposition so dead plants pile up over thousands of years. The oceans absorb carbon dioxide. And over millions of years, some buried remains of living things became fossil fuels: coal, oil and natural gas.

The nitrogen cycle

Living things need nitrogen to make proteins and DNA. The air is about 78% nitrogen gas, but plants and animals cannot use nitrogen gas directly. Here is how nitrogen gets into living things:

1. Nitrogen fixation: certain bacteria in the soil, and bacteria living in bumps (nodules) on the roots of beans, peas, clover and alfalfa, change nitrogen gas into forms that plants can use. Lightning also fixes a little nitrogen.
2. Plants take up these nitrogen compounds from the soil through their roots.
3. Animals get nitrogen by eating plants (or other animals).
4. Decomposers break down dead things and wastes and release nitrogen compounds back into the soil, where plants use them again.
5. Other bacteria change some nitrogen compounds back into nitrogen gas, which returns to the air.

Remember the Three Sisters from Grade 6? The beans add nitrogen to the soil for the corn and squash. That is the nitrogen cycle at work, used by First Nations farmers for many centuries.

How cycling matter promotes sustainability

• Nothing is wasted. One organism's waste is another's food. Dead leaves become soil nutrients for next year's leaves.
• Ecosystems can last for thousands of years without running out of materials, because the same materials keep being reused.
• Balance: in a natural cycle, carbon dioxide released by respiration and decomposition is roughly balanced by carbon dioxide taken in by photosynthesis.

Humans can upset the cycles:

• Burning fossil fuels releases carbon that was stored for millions of years, much faster than plants and oceans can take it back. Extra carbon dioxide in the air traps more heat and is the main cause of climate change.
• Cutting or burning forests and draining peatlands releases stored carbon and removes carbon sinks.
• Too much fertilizer adds extra nitrogen and phosphorus. Rain washes it into lakes, where it can cause huge algae blooms.
• Sending food scraps and leaves to landfills buries them without oxygen. They break down slowly and release methane, a gas that traps even more heat than carbon dioxide. Composting and green bin programs return these nutrients to the soil instead.