陆地生态系统碳平衡主要研究方法评述

陆地生态系统碳平衡是全球变化科学中的核心问题之一，目前也是生态科学中的前沿与热点问题，而陆地生态系统的复杂性与不确定性决定了对陆地生态系统碳平衡估测的复杂性和不确定性。为研究这一复杂性问题，已发展了许多研究方法。可分为“自下而上”与“自上而下”两种，各种方法都有其自身的优势和劣势。相关方向也已经有了大量的研究报道，但是，不同的研究由于在方法、时间与空间尺度等存在的差异，使得许多研究结果和预测很难被有效的整合或适用于大范围甚至全球水平。综述了陆地碳平衡的主要研究方法，分析和比较了各方法的特点，指出在研究中对不同方法的结果进行分析和比较，以及采用综合方法的必要性。

An overview of current methods to estimate carbon budget of terrestrial ecosystems

The continued increase in atmospheric carbon dioxide (CO_2) due to anthropogenic emissions (mostly by fossil burning and land use and land cover change) is believed to have caused significant changes in global climate and biology. Because terrestrial ecosystems play an important but ambiguous role in the global carbon budget, a prime focus of much research in the global change studies is on the carbon accounting of the terrestrial ecosystems. Despite carbon sequestration by terrestrial ecosystems has been thought to occur predominantly in the Northern Hemisphere, the magnitude, longitudinal distribution and dynamics of carbon sinks remain uncertain. To fully balance carbon budget, uncertainties in estimating carbon sinks / sources must be quantified that include soil respiration variations with temperature, terrestrial responses to climate change, major disturbances in forest ecosystems, and total amount of sequestrations by the global oceans and terrestrial ecosystems. Most of these uncertainties mainly result from the complexity of terrestrial ecosystems. To reduce uncertainties in carbon budget estimation, many methods have been developed in the past several decades. However, these methods may not necessarily result in uncertainties reduction, instead, they often constrain the generality of carbon estimates. The purpose of this study is to provide comprehensive review of current methods, specifically focusing on terrestrial ecosystems. Eight methods commonly used in estimating terrestrial ecosystem carbon budget were reviewed with respects to their merits and limitations that include 1) the whole-plant or soil chamber method, 2) forest inventory, 3) the eddy covariance techniques, 4) plant-soil C isotopes techniques, 5) land use and land cover change, 6) process-based modeling, 7) atmospheric tracer-transport modeling, and 8) remote sensing techniques. These methods were divided into two groups: bottom-up (or land-based) and top-down (or atmosphere-based). Generally, the bottom-up approaches are more appropriate in understanding and predicting changes in carbon balance of terrestrial ecosystems. However, they are difficult to be scaled up to represent the regional or global terrestrial ecosystems. The top-down approaches, similarly, have the advantage of estimating carbon budget over large spatial areas, but provide little information about the processes. As each approach has its own limitations, results from each study often vary significantly and difficult to compare. It is suggested that a study be made to compare and analyze the results from different methods using a single dataset, which will provide some quantitative assessment of uncertainties from each method.