The document discusses the nitrogen cycle, which is the biological process by which nitrogen is converted between its various chemical forms and circulated between living and nonliving matter. Key points include: Winogradsky discovered the organisms responsible for soil nitrification in 1891; nitrogen fixation converts atmospheric nitrogen to biologically available forms; nitrification converts ammonia to nitrite and nitrate; denitrification converts nitrates back to nitrogen gas; and ammonification releases inorganic nitrogen during decomposition.