关键词: 森林生态系统, 凋落物, 分解

Abstract:

Decomposition of forest litter and simultaneous release of bioelements is one of the fundamental pathway of the flow of nutrient and energy in forest ecosystems, and an important component of the global carbon budget. The aim of this review is to summarize the changes of nutrient and chemical components during the decomposition, the factors controlling litter decay rate and its measurement. Decomposition of plant litter refers to the physical and chemical processes involved in reducing litter to its elemental chemical constituents. It includes two stages. First, litter is broken down by detritivores to small pieces which can be chemically reduced. Second, through the activities of micro-organisms (bacteria and fungi) these small pieces of organic matter are further reduced and mineralized into basic inorganic molecules. Also the course of nutrient dynamics during decomposition often can occur in three sequential phases, including an initial nutrient release through leaching, a net immobilization of nutrients by decomposer microorganisms and nutrient release from the litter, usually at a rate paralleling mass loss. However, the pattern of immobilization and release varies among species and ecosystems, and any particular litter type may not show all three phases. Litter decomposition rates are controlled by climate, litter chemistry, and by soil organisms. Climate has a direct effect on litter decomposition due to the effects of temperature and moisture. However, as a result of the climatic control of soil formation and nutrient cycling, it is to be expected that climate also has an indirect effect through the climatic impact on litter chemistry. Actual evapotranspiration (AET) is used as an index for the climatic control on decomposition. Mean AET in the humid tropical region is higher than in both the temperate and the mediterranean region and this results in rapid decomposition of litter. Especially, decomposition of litter (including root litter) contributes about 70% to the total annual carbon flux. Thus, changes in AET due to global warming will affect the global carbon budget through effects on the decomposition pathway. At a global scale, litter chemistry is only of secondary importance in explaining variation in litter decomposition rates. N and P concentrations, C/N and C/P ratios, lignin concentration and lignin/N and lignin/P ratios are considered as primary litter chemistry parameters, of which the lignin/N ratio and P related parameters exert significant influence on litter decomposition rates in the tropics. However, there is no consistent chemical predictor of litter decomposability in the temperate region. Also, there are indications that the chemical control of litter decomposition depends on the phase of the decomposition process. Generally, readily decomposable carbohydrates are lost at the initial stage, subsequently the decay-resistant materials such as lignin and cellulose are accumulated. High initial N, P and K concentrations lead to high initial decay rates, but to low long-term litter decay. Thus, there is clearly a need for more long-term studies on the chemical control of litter decomposition rates in various climatic regions. In addition, physical and chemical properties of soils and microbial communities also affect litter decomposition. Litter bag represent the most standardized method for studying litter decomposition, but it has several limitations. Litter bag may alter decomposition rates by altering litter microclimate, hindering soil contact and excluding litter macrofauna. Nevertheless, litter bags allow comparisons among species and experimental manipulations. Currently, the three most widely used indices of litter decomposition rate in forest ecosystem are release rate of CO₂, decay constant of k value and weight loss rate of litter. Despite intensive research in the last decade, uncertainties of litter decomposition in forest ecosystems still remain for further studies: long-term in situ observation; use of uniform methods to obtain comparable data; research on the mechanism of litter decay using models; the effects of global climatic changes on litter decomposition in forest ecosystems; and the assessment of management activities controls on the litter decay and nutrient release in forests.

Key words: forest ecosystem, litter fall, decomposition