These are ways in which humans alter N cycle, not really increase N-fixation. The ozone shield played large role in ability of life to invade land. Ozone reacts with and degrades chlorophyll and photosynthetic enzymes, and thus lowers the plant's productivity.
Ozone reduces photosynthetic efficiency by competing with carbon dioxide.
During the last 100 million yr there has been a general upward trend in CO2 concentrations. The fluctuations in CO2 concentrations match the glacial cycles. All of the statements are true.