树木根系碳分配格局及其影响因子

根系作为树木提供养分和水分的“源”和消耗C的“汇”，在陆地生态系统C平衡研究中具有重要的理论意义。尽管20多年来的研究已经认识到根系消耗净初级生产力占总净初级生产力较大的比例，但是，根系（尤其是细根）消耗C的机理以及C分配的去向一直没有研究清楚。主要原因是细根消耗光合产物的生理生态过程相当复杂，准确估计各个组分消耗的C具有很大的不确定性，常常受树种和环境空间和时间异质性、以及研究方法的限制。综述了分配到地下的C主要去向，即细根生产和周转、呼吸及养分吸收与同化、分泌有机物、土壤植食动物，及有关林木地下碳分配机理的几种假说，分析了地下碳分配估计中存在的不确定性。目的是在全球变化C循环研究中对生态系统地下部分根系消耗的C以及分配格局引起重视。

Research review on the pattern of carbon allocation to roots and environmental factors in forest ecosystem

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 estimate technologies will be the main objectives of study on root ecology and as well as carbon cycling in global change projects.