Modelling DOC production, transport and retention in forest soils, exploiting recent 14C enrichment at Oak Ridge, USA

将植物凋落物的碳并入森林土壤，以及它在不同土层(土层)中停留的时间长短，是全球碳循环的重要因素，因此也是气候变化的速度和程度的重要因素。在许多土壤中，渗滤水中溶解有机碳(DOC)的下移是碳转移到土壤深层的主要途径，在那里可以发生长期的碳储存。此外，DOC还是土壤中金属、营养物质和有机污染物以及从土壤到水的重要运输工具。DyDOC是一个基于过程的模型，旨在模拟土壤碳循环，包括DOC的生成和运输。该项目旨在通过在美国田纳西州橡树岭保护区附近的焚烧炉释放碳-14而收集的独特数据集上对该模型进行校准，该数据集是在大气中二氧化碳的局部浓缩之后收集的，因此森林植被也是如此。数据收集是由美国同事在一个名为EBIS(丰富背景同位素研究)的项目中进行的。碳-14是一种天然的放射性同位素，因其在“碳测年”中的应用而广为人知，测定的年代从100年到数万年不等。然而，将其用于生态系统碳循环研究的第二种方式是通过人类活动对大气中的二氧化碳进行浓缩。直到最近，这只意味着‘炸弹碳’，即20世纪中叶大气武器试验造成的浓缩，大约是全球14C水平的两倍。“炸弹标记”的碳可以在不同的土壤池中追踪，并通过使用适当的模型，可以推断其吸收和损失的速度。这已经被用来在欧洲的地点校准DyDOC。但现在，由于1999年橡树岭焚烧炉排放的碳-14，产生了进一步的本地化标签，由于当地科学家在EBIS中迅速采取行动建立了监测计划和实验研究，因此可以获得更详细的同位素数据来进行建模。这里提出的工作是利用EBIS数据来更广泛和更精确地校准DyDOC，而不是使用其他方法。该项目的工作将首先包括整理EBIS关于土壤碳14C、不同土壤库中的碳量和DOC浓度的数据，以及其他表征橡树岭森林的当地数据(土壤深度、降雨量、温度、凋落物等)。然后，我们将运行模型，调整其参数，以尽可能接近测量值。将进行额外的模型运行，以查看输出对参数值变化的敏感度。校准后的模型将被用来预测(A)土壤碳库和DOC的碳14标记的演变，以便在未来收集更多数据时进行进一步测试，以及(B)土壤碳循环和DOC的形成和运输对外部环境因素的依赖，如温度、降雨量和凋落物输入。研究结果应为评估森林土壤对未来气候和其他环境条件变化的可能反应提供有用的信息。该模型在一个独特的研究地点的“一次性”应用将提供信息，这些信息可用于限制模型应用到其他地点的参数值范围，从而最终对森林土壤碳行为进行更可靠的区域尺度预报。

The incorporation of the carbon of plant litter into forest soils, and how long it spends in different soil layers (horizons), are important factors in the global carbon cycle, and therefore in the rate and extent of climate change. In many soils, downward movement of dissolved organic carbon (DOC) in percolating water is the main means by which carbon is transferred to deeper soil layers, where long-term carbon storage can occur. Moreover, DOC is an important transporter of metals, nutrients and organic contaminants in soils, and from soils to water. DyDOC is a process-based model designed to simulate the soil carbon cycle, including DOC generation and transport. This project aims to improve the model by calibrating it on a unique data set that is being collected following the localised enrichment of atmospheric CO2, and hence forest vegetation, due to the release of carbon-14 from an incinerator close to the Oak Ridge Reservation in Tennessee USA. The data collection is being carried out by American colleagues in a project called EBIS (Enriched Background Isotope Study). Carbon-14 is a natural radioactive isotope, well-known for its application in 'carbon dating', for samples of ages in the range 100 to tens of thousands of years. However, a second way in which it can be used in ecosystem research into the carbon cycle is through the enrichment of atmospheric CO2 by human activities. Until recently, this only meant 'bomb carbon', enrichment caused by atmospheric weapons testing in the middle of the 20th Century, which approximately doubled the worldwide 14C level. The 'bomb-labelled' carbon can be traced in different soil pools and, through the use of an appropriate model, its rate of incorporation and loss can be deduced. This has already been used to calibrate DyDOC at sites in Europe. But now a further localised label has been generated by the emission of carbon-14 from the incinerator at Oak Ridge during 1999, and, thanks to the swift actions of local scientists in setting up a monitoring programme and experimental studies in EBIS, much more detailed isotope data are consequently available for modelling. The work proposed here is to make use of the EBIS data to calibrate DyDOC much more extensively and precisely than would otherwise be possible. The work in the project will firstly comprise the collation of the EBIS data on soil carbon 14C, the amounts of carbon in different soil pools, and DOC concentrations, together with other local data (soil depths, rainfall, temperature, litterfall etc) characterising the Oak Ridge forests. We will then run the model, adjusting its parameters in order to match as closely as possible the measured values. Additional model runs will be done to see how sensitive the outputs are to changes in the parameter values. The calibrated model will be used to forecast (a) the evolution of the carbon-14 labelling of soil carbon pools and DOC, in readiness for further testing as more data are collected in the future, and (b) the dependence of soil carbon cycling and DOC formation and transport on external environmental factors such as temperature, rainfall and litter inputs. The results should provide useful information to assess the possible responses of forest soils to future variations in climate and other environmental conditions. This 'one-off' application of the model to a unique research site will provide information that can be used to limit the ranges of parameter values permissible in model applications to other sites, and therefore ultimately to more reliable regional-scale forecasting of forest soil carbon behaviour.