Questions: Thinking back to the activity we just played, what does fixing nitrogen mean? a) Nitrates are converted to nitrogen gas by denitrifying bacteria b) Nitrogenous compounds in the soil are absorbed by plants via their roots c) Organic matter is converted into inorganic nitrogenous compounds d) Nitrogen gas is converted into reactive compounds, such as ammonia

The answer is the fourth one (or D): Nitrogen gas is converted into reactive compounds, such as ammonia.

Explanation for each option:

a) Nitrates are converted to nitrogen gas by denitrifying bacteria - This describes the process of denitrification, not nitrogen fixation. Denitrification is the reduction of nitrates back into nitrogen gas, which is the opposite of nitrogen fixation.

b) Nitrogenous compounds in the soil are absorbed by plants via their roots - This describes nitrogen uptake by plants, not nitrogen fixation. While plants do absorb nitrogenous compounds, this process does not involve the conversion of nitrogen gas into reactive compounds.

c) Organic matter is converted into inorganic nitrogenous compounds - This describes mineralization, a process where organic nitrogen is converted into inorganic forms, but it is not the same as nitrogen fixation.

d) Nitrogen gas is converted into reactive compounds, such as ammonia - This correctly describes nitrogen fixation. Nitrogen fixation is the process by which nitrogen gas (N₂) from the atmosphere is converted into ammonia (NH₃) or other nitrogenous compounds that plants can use.

Summary: Nitrogen fixation is the process by which nitrogen gas is converted into reactive compounds like ammonia, making it available for plant use. This is crucial for plant growth, especially for legumes, which have symbiotic bacteria in their roots that perform this function.