Characteristics of nutrient cycling in first and second rotations of Chinese fir plantations

Nutrient cycling is one of the most important functional processes in forest ecosystems.Nutrient contents and its biogeochemistry cycle directly affect forest productivity and soil fertility.It has been widely reported that timber yield was apparently declined and soil fertility was greatly degraded on the successive rotations of Chinese fir plantations.Therefore,it is a significance to examine the relationships between the characteristics of nutrient cycling and the decline of site productivity in the successive rotations of Chinese fir plantations.The pattern of nutrient biogeochemical cycle and nutrient utilizations efficiency were investigated and compared in the first and second rotations of Chinese fir plantations using the data collected from long term projects over past 30 years.The results showed that about 1.58—3.29kg and 4.23—5.92kg more nutrients were required to produce every one tons dry matter in the fast-growing stage and timbering stage,respectively,in the second rotations than in the first rotations.Nutrient utilizations coefficient and nutrient cycling coefficient reduced 19.7%—22.8%,12.8%—15.6% and 35.3%—36.2%,23.2%—27.0%,respectively in the second rotations when comparing with those in the first rotations.When the stands reached in the mature stage,the ability of nutrient accumulation declined in the second rotations.The nutrient loss rate through runoff in the second rotations was about two times high as in the first rotations.The nutrient accumulation rate decreased about 40% in the second rotations than in the first rotations.The results suggest that the changes in nutrient biogeochemical cycling in the second rotations had negative effects on tree growth and site productivity.The research provides scientific references in making reasonable management practices and policies on the plantations in Southern China.