Explanation:

Source of Nitrogen:
Nitrogen is an essential element required for the growth and development of living organisms. It is a major component of proteins, nucleic acids, and other important biomolecules. Nitrogen is obtained by organisms through various sources in the environment. Among the given options, the source of nitrogen that is NOT correct is option 'B', which is xylem.

Automobile Exhaust:
Automobile exhaust is a major source of nitrogen pollution in the environment. Nitrogen oxides (NOx) are released from the combustion of fossil fuels in vehicles, which includes nitrogen as one of its components. These nitrogen oxides contribute to air pollution and can have detrimental effects on human health and the environment.

Industrial Combustion:
Industrial combustion processes, such as those in power plants and factories, release nitrogen-containing compounds into the atmosphere. These compounds are often in the form of nitrogen oxides (NOx), which are emitted during the burning of fossil fuels. The release of nitrogen oxides from industrial combustion contributes to air pollution and can lead to the formation of smog and acid rain.

Forest Fire:
Forest fires can release large amounts of nitrogen into the atmosphere. Nitrogen is present in vegetation and organic matter, and when these materials burn, nitrogen compounds are released as gases. These nitrogen compounds can contribute to air pollution and can have both short-term and long-term effects on the environment.

Xylem:
Xylem is a vascular tissue in plants responsible for the transport of water and minerals from the roots to the rest of the plant. While xylem plays a crucial role in water and mineral transport, it is not a source of nitrogen. Nitrogen is typically obtained by plants through the roots in the form of nitrates or ammonium ions from the soil.

In conclusion, among the given options, xylem is NOT a source of nitrogen. Nitrogen can be obtained from automobile exhaust, industrial combustion, and forest fires, but not from xylem.