Which of these statements is false?

Which of these statements is false?

Let's analyze each statement regarding the nitrogen cycle:

• A. Nitrogen fixing bacteria increases soil fertility: This statement is true. Nitrogen-fixing bacteria convert atmospheric nitrogen gas ($\text{N}_2$) into ammonia ($\text{NH}_3$), which can then be used by plants. This process makes nitrogen available in a usable form, thereby enriching the soil and increasing its fertility.
• B. Most living things cannot use nitrogen directly: This statement is true. Atmospheric nitrogen ($\text{N}_2$) is very stable and cannot be directly utilized by most organisms, including plants and animals. They require nitrogen in "fixed" forms, such as nitrates or ammonium ions, which are produced by nitrogen-fixing bacteria or through other processes in the nitrogen cycle.
• C. Nitrogen fixing bacteria is found in the air we breathe: This statement is false. Nitrogen-fixing bacteria are microorganisms that primarily live in the soil or in symbiotic relationships within the root nodules of certain plants (like legumes). They are not found in the air we breathe.
• D. Nitrogen gas is produced from the decay of plants and animals: This statement is false. The decay of plants and animals (decomposition) by decomposers releases nitrogen in the form of ammonia ($\text{NH}_3$) or ammonium ions ($\text{NH}_4^+$), a process called ammonification. Nitrogen gas ($\text{N}_2$) is primarily produced by denitrifying bacteria, which convert nitrates ($\text{NO}_3^-$) back into atmospheric nitrogen gas. While decay provides the nitrogen compounds that eventually lead to $\text{N}_2$ through denitrification, decay itself does not directly produce nitrogen gas.

Comparing C and D, statement C is a more direct and fundamental falsehood about the habitat of nitrogen-fixing bacteria. They are unequivocally not found in the air we breathe.

The final answer is $\boxed{\text{C. Nitrogen fixing bacteria is found in the air we breathe.}}$