14C在陆地生态系统碳循环研究中的应用及进展

熊晓虎; 周卫健; 杜花; 程鹏; 吴书刚; 牛振川; 卢雪峰

摘要

深刻理解陆地生态系统碳循环有助于提高未来气候预测的准确性。14C是陆地生态系统碳循环研究中广泛应用的重要工具，本文对14C在土壤有机碳周转时间和平均停留时间的评估、陆地生态系统释放CO2的源解析、河流有机碳的源解析及陆地生态系统碳循环对气候变化和土地利用的响应等四个方面的应用及进展进行了细致的介绍，最后展望了14C应用于陆地碳循环研究发展趋势，并对国内相关研究的开展提出了几点建议。%Background, aim, and scope Terrestrial ecosystems are one of the main sources and sinks of atmospheric CO2 now, however, it is not clear whether it will become a net source or sink in the future under the inlfuence of climate change and human activities. Radiocarbon is an important and widely used tool in the research of terrestrial carbon cycle. In this paper, the research progress in recent years were reviewed and divided in four parts: the calculation of turnover time and mean residence time of soil organic carbon, partitioning sources of CO2 derived from terrestrial ecosystem, partitioning sources of lfuvial organic carbon and the response of terrestrial carbon cycle to climate change and land use. The expanded applications of radiocarbon in terrestrial carbon cycle in future and the deifciencies of related research in China were also discussed.Materials and methods We mainly reviewed the literature those use radiocarbon with alternative method (model) to evaluate the turnover of soil organic carbon, partition sources of different types of C in terrestrial ecosystem or catch the response of terrestrial carbon cycle to climate change and land use, then, we made comparisons between these methods (models).Results The turnover time of soil organic carbon (SOC) is a measuring of its mixing or refresh rate, and is the time it would take for the reservoir to completely empty if there were no further inputs. And the mean residence time (MRT) of C in the reservoir is the average time spent in the reservoir by individual C atoms before leaving. The turnover time and MRT of a homogeneous reservoir at steady state are equal. Decomposition rate is also widely used in soil carbon dynamics, which is reciprocal of the turnover time. To calculate the turnover time or MRT, We need14C activity (generally reported asΔ14C) of SOC and atmospheric CO2, the content of C and a model. If archived samples of a soil are available, the average input rate and decomposition rate can be gained (equations (1)—(2) in this article). However, archived samples are unavailable in most study sites, and another model can be used (equations (3)—(4)). If the lag time between the C ifxed by plants and added to SOM is taken into consideration, which cannot be ignored in some ecosystems, a modiifed equation is needed (equation (5)). Because of the heterogeneity of SOC, the turnover time of different fractions in SOC is interesting, and there are two methods to achieve this aim. One is by the aid of archived samples (equations (6)—(8)), then the turnover time of active pool and its percentage of total carbon pool can be got. The other is with the help of pretreatment technology. At ifrst, different fractions of SOC are separated by physical and / or chemical methods; secondly, the turnover time of different parts are calculated separately. The change of the lfux of CO2 derived from soil respiration and lfuvial degassing likely results in the lfuctuation of atmospheric CO2. Radiocarbon can be used to distinguish the CO2 sources of soil respiration and lfuvial degassing. The CO2 sources of soil respiration usually are divided into two parts, autotrophic respiration (from root) and heterotrophic respiration (from the decomposition of SOC). If the determinedΔ14C of CO2 of total soil respiration, autotrophic respiration and heterotrophic respiration are substituted into the model (equation (11)), the percent of autotrophic respiration and heterotrophic respiration would be got. The heterotrophic respiration can be further divided into leaf litter decomposition and soil decomposition (equation (13)). The models (equations (11)—(13)), which have been widely used in forest case, are not very suitable for the case of peatland and permafrost. Three and four boxes model are chosen for peatland and permafrost, respectively. The models of partitioning sources of CO2 derived from lfuvial degassing have two boxes (young carbon and old carbon) or three boxes (vegetation carbon, SOC and fossil carbon) according to the catchment environment. Fluvial organic carbon is mainly composed of dissolved organic carbon (DOC) and particle organic carbon (POC). The14C activity vary from source to source, therefore it can be used to partition the contribution of every source. There are mainly two kinds of models, two boxes model (equation (16)) and three boxes model (equations (17)—(19)). Two boxes model contains non-fossil carbon (from soil and vegetation) and fossil carbon (from bedrock weathering and erosion), and compared to two boxes model, three boxes model have an additional source, aged soil carbon. To catch the response of terrestrial carbon cycle to climate change and land use, the researchers determined theΔ14C of samples (e.g. SOC, soil respiration CO2, DOC) from different time or different site, and made a comparison. If the results have significant change, then the change may attribute to climate change or land use.Discussion To evaluate the turnover of SOC, there are four different kinds of models depend on whether archive sample is available, lag time is ignored or the heterogeneity of SOC is taken into consideration. Archive samples are very useful to calculate the turnover time, although in most research site it is unavailable. We should note that the SOC is heterogeneous, thus those models containing different carbon pools are more helpful. However, the more detail in turnover time of SOC we need, the more time and cost would consume. There is a common trade-off, only the turnover time of mineral-associated fraction is calculated, because this fraction is more stabilized than the others, then we can estimate the stability of the SOC. Compared to the forest, peatland and permafrost have remarkable different vegetation and climate, and their models have more boxes. The models of partitioning sources of CO2 or organic carbon in river have two boxes or three boxes according to the catchment environment. If aged soils are not prevalent in the interested catchment, two boxes model is appropriate; otherwise, three boxes model is better. Moreover, how to determine theΔ14C of different sources is the key to get reliable results. The utilization ofΔ14C of SOC, soil respiration CO2 and lfuvial organic / inorganic carbon can help us to recognize the response of terrestrial carbon cycle to climate change and land use. The comparison of observed results in different sampling time or sites is an effective mothed; and study area need be chosen carefully to distinguish the inlfuence of climate change from land use, moreover, a continuous observation is necessary to catch the change timely, because sometimes the inlfuence may be subtle in a short time. Conclusions Radiocarbon is a useful tool to help understanding the terrestrial carbon cycle. Although there are many methods (models) when radiocarbon is applied in different cases, there is no easy answer that which is better. Instead, we should choose appropriate method (model) according the actual situation of study area and objective material condition.Recommendations and perspectives The rapid development of graphite preparation method and Accelerator Mass Spectrometry (AMS) analysis technology of small-mass radiocarbon sample (microgram degree) would improve further the application of radiocarbon in terrestrial carbon cycle, for example, SOC dynamics in molecular level. When it comes to related researches in China, there is a lot of room to improve in future by using of radiocarbon tool.

14C在陆地生态系统碳循环研究中的应用及进展

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