| Chinese fir is the leading timber species in China and is most widely planted for wood production in south of China. By 2004, about 9.213 5 million hectares of Chinese fir plantations that constitute 30.4% of the total nation's plantations have been established. Consequently, Chinese fir plantations have contributed a great deal to satisfy expanding timber demand from the current rapid economic growth and to take pressure off natural forests. However, the possibility whether the traditional management system of industrial forest can sustain long-term productivity of Chinese fir plantation has been questioned.Clear-cutting, prescribed harvest residues burning and site preparation are the common silvicultural treatments employed in Chinese fir plantation management. To determine short-term impacts of these treatments on the soil physical and chemical properties of Chinese fir plantation, soil samples were sequentially collected from 0-15 cm and 15-30 cm depths before and after harvest with residues remained, prescribed residues burning, and site preparation and tree replanting. Our results showed that all treatment together significantly affected soil pH value, organic matter and carbon at 0-15 cm depth, and available N. Soil pH value decreased with the treatments. While residues remaining after clear-cutting increased soil organic carbon, prescribed residues burning reduced soil organic carbon. Due to soil displacement and burning ash incorporation, site preparation made soil organic carbon content return to the pre-harvest value. Available N concentration decreased significantly after clear-cutting and residues burning, but the reduction was almost offset after site preparation. No significant effects on soil bulk density, total N and P, available P were observed for the treatments. Total K and Mg concertrations significantly deceased after harvest at depths of 0-15 cm and 15-30 cm. before and after burning, the content of total K slightly increased, then decreased again with the site preparation treatment. And prescribed burning and site preparation had no significant effect on the content of total Mg. With manual operation and deliberate avoidance of rainy season, increase in soil bulk density and loss of soil organic matter and carbon, N and P were ultimately elusive after all treatments in Chinese fir plantation. Soil pH value increased during the first year after replantation. Bulk density and the nutrient concertration also increased during the former several months, then the effect of eluviation made them decreased at different degrees in the rainseason, but the effect on total P was insignificant.Logging residues with a great of nutrients was the resources of soil organic matter and other nutrients, and also was an important storage for nutrient cycling in forest ecosystem. Research about logging residues'contribution to soil nutrient was conducted from the standpoint of composition and nutrient release, in order to discuss the sustainable management problem in Chinese fir plantation. The results showed that Clear-cutting of Chinese fir plantation produced about 65.91 t.hm-2 harvest residues that contained substantial amount of nutrients, and different components (branch (<0.5 cm) with needle, branch (>0.5 cm) with needle, needle, litter and cone) account for different proportion, and the biomass of branch (<0.5 cm) with needle has the largest proportion, and its nutrient contents also account for the largest proportion of total nutrient in all logging residues. In the composition research, needle has the fast composition ratio, and decomposition ratio in other components was in the order as follows:branch (<0.5 cm) with needle> bark> cone> branch (>0.5 cm) with needle. All the logging residues have faster decomposition ratio during the period between the 122nd d and 246th d. The simulation results performed by regression equations of dry weight remaining rates of different logging residues components varying with decomposition time were that it needs 17.99 a for needle to reach the 50% of decomposition ratio,36.10 a for branch (>0.5 cm) with needle,36.16 a for cone, and 24.07 a for total logging residues; it needs 25.88 a for needle to reach the 95% of decomposition ratio,51.90 a for branch (>0.5 cm) with needle,51.93 a for cone, and 35.58 a for total logging residues. At the end of the research, there were great amount of C, P, Ca and M released from logging residues, and their amount reached 3 996.14 kg.hm-2. The nutrient release rate for logging residues were in the following orders: P>Mg>Ca>C, and branch (<0.5 cm) with needle was the major source; but N accumulation in logging residues happened, though there was a small quantity of N released from branch (<0.5 cm) with needle. Needle was the largest sink among logging residues, bark followed. Critical C/N and C/P ratios varied widely among logging residue fractions, but N/P ratio varied narrowly.
|
PDF Full Text Request