UNIT 7 Ecosystems II: matter and energy THE NITROGEN CYCLE Biology and Geology 4. Secondary Education UNIT The nitrogen cycle 7 • Even though air contains 78% nitrogen (N2) by volume, this gas is unavailable for use by most living things. Only certain kinds of soil bacteria can harness it. ATMOSPHERE N2-fixing bacteria NO3- Nitrates Excretion Absorption Feeding Decomposition by bacteria Denitrifying bacteria Nitrogen (N2) Nitrifying bacteria NO2- Nitrites Biology and Geology 4. Secondary Education NH3 Ammonia • Producers can only use nitrogen in the form of soluble mineral salts (nitrates). Consumers and decomposers take in nitrogen with their food. • Atmospheric nitrogen is transformed into nitrates that producers can use through lighting and through fixation carried out by different kinds of soil bacteria. • Nitrates can leave the cycle when water transports them to a sedimentary environment, where they will precipitate and form sedimentary rocks.