Carbon cycling in forests: Priming of old Soil Organic Matter through plant derived C input

到目前为止，人们已经达成了一个广泛的共识，即大气中二氧化碳浓度的上升会导致地球气候的变化，全球气温上升，许多地区的降水量和模式可能发生变化。由于全球变化，陆地生态系统中大部分C储存在土壤中，土壤中的C释放将导致大气CO2浓度的额外增加，从而通过温室效应加剧气候强迫。考虑到土壤中碳的大量储存，只有进入这个库的碳与以CO2形式离开这个库的碳之间的平衡发生微小变化，才能对全球大气CO2收支产生深远影响。已知植物从大气中固定的一定量的C仅通过土壤循环，因为它要么直接从植物根部以CO2形式排放，要么以不稳定的形式固定，在短时间内分解。正是以柠檬酸形式储存的C，才可能提供与大气隔离的真正的C储存。然而，最近的证据表明，植物对CO2的额外封存不会导致生物量的增加，从而导致可能变得稳定的复杂化合物的增加，而只是增加了作为不稳定化合物从根部进入土壤的C的量。在一个被称为土壤碳启动的过程中，人们早就知道，添加不稳定的分解基质会导致碳以CO2的形式从其他土壤有机质中释放出来。鉴于未来不稳定化合物的输入可能增加，目前尚不清楚土壤引发是否可能是土壤释放CO2的重要来源，减轻甚至逆转气候变化下植物增加CO2螯合的任何潜在益处。菌根真菌是一种与植物根系紧密共生的生物体，它们形成一个密集的菌丝网络，沿着网络进行碳和养分的运输。这些生物体中的C来自植物根系的分布和释放的作用，到目前为止还没有得到足够的重视，让一个完整的了解他们的作用，在总土壤CO2排放动态。由于它们的物理位置在树根和土壤有机质之间，并且由于它们的菌丝的广泛网络，它们可能在潜在的土壤引发效应中发挥关键作用。因此，本建议的主要目的是确定和量化的植物C输入的启动效应在几种气候变化条件下引起的土壤中的CO2排放。这将包括分离不同的土壤表面CO2通量组分，明确分离直接来自植物根系的C和来自菌根的C。此外，它计划在目前的气候条件下，土壤CO2排放组分通量（包括土壤引发）与植物同化通量的相关性，以了解地上和地下C交换过程的直接相互作用。土壤干旱和再湿润对引发和其他土壤CO2排放组分的影响，以及CO2浓度升高的影响，形成了拟议工作下的进一步实验方法。该项目的结果将被纳入目前的生态系统模型，以解决目前缺乏植物和土壤碳交换过程之间的正反馈相互作用。这些模型有助于预测陆地生态系统中碳储存的未来变化，因此也有助于减缓气候变化的潜力。通过调查土壤CO2排放组分对土壤类型的依赖性，研究结果还将直接为林业管理决策者提供信息，因为如果植树造林实际上导致C的净释放，则旨在长期螯合C的植树造林可能会受到当地土壤条件的显著限制。

There is by now a broad consensus that rises in atmospheric CO2 concentrations result in changes to the Earth's climate, with globally increased temperatures, and likely changes in precipitation amount and patterns for many regions. A release of C from soils, where most of C in terrestrial ecosystems are stored, as a consequence of global change would lead to an additional increase of atmospheric CO2 concentrations, thus aggravating climatic forcing through the greenhouse effect. Given the large store of C in soils, only a small change in the balance between C entering this pool, or leaving it as CO2 can have a profound effect on the global atmospheric CO2 budget. A certain amount of C fixed by plants from the atmosphere is known to simply cycle through soils, since it is either emitted from plant roots directly as CO2, or is fixed in labile forms that decompose over short periods. It is C stored in recalcitrant forms that may provide a true store of C sequestered from the atmosphere. However, recent evidence indicates that additional sequestration of CO2 by plants does not result in an increase in biomass and thus complex compounds that are likely to become stabilised, but simply in an increase in the amount of c entering the soil from roots as labile compounds. In a process called soil C priming, it has long been known that the addition of a labile substrate for decomposition can result in the additional release of C as CO2 from other soil organic matter. in the light of likely increases in the input of labile compounds in the future, it is not known if soil priming may be a significant source of CO2 release from soils, mitigating or even reversing any potential benefit from increased CO2 sequestration by plants under a changed climate. Mycorrhizal fungi, organisms that live in close association with plant roots in a symbiotic relationship, form a dense network of hyphae along which the transport of C and nutrients takes place. The role of these organisms in the distribution and release of C derived from plant roots has so far received insufficient attention to allow a complete understanding of their role in total soil CO2 efflux dynamics. Due to their physical location between tree roots and the soil organic matter, and owing to the extensive network of their hyphae, they are however likely to play a key role in potential soil priming effects. The main aim of this proposal is therefore to identify and quantify the CO2 efflux from soils caused by the priming effect of plant C input under several climate change conditions. This will include the separation of different soil surface CO2 flux components, explicitly separating C derived from plant roots directly and that derived from mycorrhizas. Further it is planned to correlate soil CO2 efflux component fluxes (including soil priming) with plant assimilation fluxes under present climatic conditions, in order to understand the direct interaction of aboveground and belowground C exchange processes. The effect of soil drought and rewetting on priming and other soil CO2 efflux components, as well as the effect of elevated CO2 forms a further experimental approach under the proposed work. Results of this project will be incorporated into current ecosystem models in order to address the present lack of positive feedback interactions between plant and soil C exchange processes. These models are instrumental in predicting future changes in the storage of c in terrestrial ecosystems, and therefore also the potential of mitigating climatic change. By investigating the dependence of soil CO2 efflux components on soil types, the results will also directly inform decision makers in forestry management, since tree plantations aimed at sequestering C in the long term may be significantly constrained by local soil conditions, if tree plantations actually result in a net release of C.