Research review on the pattern of carbon allocation to roots and environmental factors in forest ecosystem
树木根系碳分配格局及其影响因子

Tree root has a function of the source for nutrients and water and the sink for carbon due to fine root production, mortality and decomposition. Although its importance for carbon allocation and nutrient cycling in terrestrial ecosystems has been known during past decades, physiological mechanism of carbon costs and pattern of carbon allocation in roots are poorly understood, because physiological and ecological processes in root dynamics appear the complexity and estimating carbon allocation reveals the uncertainty. Recently, more studies focused on the mechanism of carbon allocated below ground associated with individual root level, whole tree and ecosystem levels. There were four hypotheses for the control of carbon acquisition by roots: 1) root carbon acquisition is determined by the inflow of carbon from shoot; 2) root carbon acquisition is controlled by demand from roots; 3) net acquisition of carbon by roots is determined by their functional equilibrium with shoots; and 4) the control of carbon acquisition by roots is distributed around the plant in shoot and root. However, there were few evidences or data for supporting the first two hypotheses. Most recent evidences concluded that carbon allocated to roots may be more dependent upon soil resource availability. Fine root production and turnover, root respiration, secreting organic matter from root, and foraging of soil herbivory were four important components of carbon allocation in tree roots. The flux of carbon through the production and turnover of fine roots in forest ecosystems has been recognized as the first major component of terrestrial carbon and nutrient cycles. Many methods have been developed to estimate fine root production and turnover, but they may arrive at different results. The second major component of carbon allocation to below ground was root respiration associated with mycorrhizal fungi. The carbon costs of root respiration were, approximately 50% of total carbon allocated to roots, depended on root growth rate, efficiency of ion uptake and transport processes, and as well as biomass maintenance. The third component was secretion or exudation of organic matters from roots, such as sugars, amino acids and organic acids, and etc. Finally, soil herbivory should be an important source of root death and carbon cost, although there were very few data available in study cases. Soil resource availability was an important factor that regulated the flux of carbon from leaves to roots and possibly to soil. Despite of greater carbon is allocated to root system, accurate estimate in the flux of carbon from leaves to roots is limited because of uncertainties in predicting below ground carbon cycling in forest ecosystems. However, there are many opportunities as well as challenges for future belowground carbon allocation research in China. It is suggested that studying pattern of carbon allocated to root system in different forest ecosystem in China and developing accurate estimat