Nitrogen is an important element for all living things. Animals and plants need nitrogen to live and grow. Living things use molecules with nitrogen atom, called nitrogen compounds, every day. Compounds are made of atom from different elements.Our bodies need different nitrogen compounds to make many kinds of protein. Protein help build muscels. Muscels allow you to move, run , jump, and lift things.

The Carbon Cycle

All living things are made of carbon. Carbon is also a part of the ocean, air, and even rocks. Because the Earth is a dynamic place, carbon does not stay still. It is on the move!

In the atmosphere, carbon is attached to some oxygen in a gas called carbon dioxide.

Plants use carbon dioxide and sunlight to make their own food and grow. The carbon becomes part of the plant. Plants that die and are buried may turn into fossil fuels made of carbon like coal and oil over millions of years. When humans burn fossil fuels, most of the carbon quickly enters the atmosphere as carbon dioxide.

Carbon dioxide is a greenhouse gas and traps heat in the atmosphere. Without it and other greenhouse gases, Earth would be a frozen world. But humans have burned so much fuel that there is about 30% more carbon dioxide in the air today than there was about 150 years ago, and Earth is becoming a warmer place. In fact, ice cores show us that there is now more carbon dioxide in the atmosphere than there has been in the last 420,000 years.

The main component of the nitrogen cycle starts with the element nitrogen in the air. Two nitrogen oxides are found in the air as a result of interactions with oxygen. Nitrogen will only react with oxygen in the presence of high temperatures and pressures found near lightning bolts and in combustion reactions in power plants or internal combustion engines. Nitric oxide, NO, and nitrogen dioxide, NO2, are formed under these conditions. Eventually nitrogen dioxide may react with water in rain to form nitric acid, HNO3. The nitrates thus formed may be utilized by plants as a nutrient.

Nitrogen in the air becomes a part of biological matter mostly through the actions of bacteria and algae in a process known asnitrogen fixation. Legume plants such as clover, alfalfa, and soybeans form nodules on the roots where nitrogen fixing bacteria take nitrogen from the air and convert it into ammonia, NH3. The ammonia is further converted by other bacteria first into nitrite ions, NO2-, and then into nitrate ions, NO3-. Plants utilize the nitrate ions as a nutrient or fertilizer for growth. Nitrogen is incorporate in many amino acids which are further reacted to make proteins.

Rising air currents take the water, as vapor, up into the atmosphere, along with water from "evapotranspiration," which is water transpired or "breathed out" from plants and evaporated from the soil. The cooler temperatures in the atmosphere cause it to condense into clouds, which float around until the fall from the sky as precipitation. Some precipitation falls as snow and can accumulate as ice caps and glaciers, where it can stay, as frozen water, for thousands of years. In warmer climates, snow melts during the warmer spring and summer months, and that water flows into streams and rivers, which eventually return it to the ocean, or into the groundwater, which eventually reach underground aquifers. Over time, the water continues flowing, some to reenter the ocean, where the water cycle renews itself. There are four basic steps that tie this all together.

Four steps in the water cycle Evaporationoccurs when water transforms from liquid to gas, usually as a result of the sun's warming rays. Evaporation often technically includes transpiration from plants (the vapor the "breathe" out as they grow), though together they are specifically referred to as evapotranspiration.

Condensationoccurs as the vapor rises into the atmosphere, creating clouds and fog. Once clouds are formed, advection -- the movement of water in its various states -- through the atmosphere. Without advection, the cycle would screech to a halt, as the water would evaporate and precipitate (the next step) in the same place.

Precipitationoccurs when the vapor that condensed comes back out of the sky as rain, snow, sleet, hail. Most of it comes back to the ground or body of water, but some of it is intercepted by plant foliage and evaporates back to the atmosphere instead of making it to the ground, in a process called "canopy interception."

Runoffis the process by which water moves across land and includes both surface runoff -- when water travels over land -- and channel runoff -- when it gets into streams and rivers. As is bubbles and rambles along, it can drain into the ground, evaporate into the air, run into and become stored in lakes or reservoirs, or be gathered up for human uses