Keywords: 
• allelopathy /
• vanillin /
• Chinese fir (Cunninghamia lanceolata) /
• ecological biochemistry /
• nitrogen

Abstract: Chinese fir (Cunninghamia lanceolata)is a fast-growing tree species in the south forest area of China . Planting Chinese firs in succession can result in the degeneration of soil fertility , caused by the accumulation of vanillin in soil .When forest soil is short of nitrogen , the precursor and the energy directly flow down into the combination of sub-metabolites , and soil toxicity will strengthen with the decrease of soil fertility .The generation of the biochemical soil humus can reduce poison .The generation of eco-chemical allelopathic substances can produce poison.Both of them theoretically demonstrate that shortness of nitrogen is a critical factor affecting the two biochemistry processes .Furthermore , the broad-leaved trees in the mixed forest of Chinese firs and broad-leaved trees can supply nitrogen , which can be proved by the normal growth of Chinese fir , no incidence of toxicity and and the increase in the content of humus in soil .The degeneration point of soil fertility in pure Chinese fir planted in succession is identical with the action point of broad-leaved trees in mixed forest of Chinese fir and broad-leaved tree .[ Ch , 1 fig .23 ref .]