Afield is abandoned, and an invasive plant that can live in nutrient-poor soil moves into the field. if the land is later cleared of this invasive species and it is discovered that the soil has an abundance of nitrogen compounds, what conclusion can best be made

a. the plants used up all of the phosphorus and potassium and left the nitrogen behind.
b. primary succession always produces an abundance of nitrogen.
c. bacteria in soil and in root nodules converted free nitrogen into nitrogen compounds.
d. the plants were able to produce nitrogen in their leaves.
e. more rock was weathered and broken down to release nitrogen compounds.

The conclusion that can be best made is that "bacteria in soil and in root nodules converted free nitrogen into nitrogen compounds".

NITROGEN FIXATION:

Certain bacteria species are capable of converting atmospheric nitrogen into usable form of nitrogen by plants such as nitrates etc. These group of bacteria are called nitrogen fixing bacteria. Nitrogen-fixing bacteria e.g. Azotobacter, Clostridium etc. live in the root nodules of leguminous plants and help the plant fix atmospheric nitrogen into the soil. According to this question, an invasive plant species colonizes a field and after clearing the species, it was discovered that the soil has an abundance of nitrogen compounds. The conclusion that can be best made is that "bacteria in soil and in root nodules converted free nitrogen into nitrogen compounds".

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