Abstract: Investigations on the populations of major insect pests in the mixed forest of Chinese fir and Homana and pure Chinese fir plantations in Liuwan state forest farm of Guangxi were made. Insect pests harming Chinese fir are mainly Polychrosis cunninghamiacola and Mourotermes barneyi, and Caloptilia sp.and Homana coffecerta are the major insects attacking Homana. The rates of attacked trees and branches by Polyehrosis cunningharniacola are 23.7% and 3.3% in the mixed forest respectively lower than that in pure stands. The rate of attacked trees by termite in mixed forests is 4—32% lower than that in the pure stands too. The populations of Caloptilia sp. and Homana coffeceria were very low.

Key words: Denitrification, N₂O reduction, Greenhouse gases, N₂O/N₂ ratio