Turbo-charging the mycorrhizosphere - Could more productive ecosystems threaten soil carbon stocks in boreal and sub-arctic zones of transition?

研究背景——许多引人注目的研究论文和综合研究将北部高纬度地区植被生产力的提高和植被类型的变化与大气中净碳(C)固存的增加等同起来。尽管合乎逻辑且直观，但这在很大程度上忽略了这些地区原有土壤有机碳（SOC）的潜在命运。这是一个问题，因为高纬度地区的土壤明显富含碳（仅在北方针叶林/针叶林和苔原土壤中就含有~570 Pg C；注意，1 Pg (Peta-gram) = 1000,000,000吨），而且这个库是动态的，与植被覆盖和气候内在地相互作用。尽管调查具有挑战性，但如果我们要了解与气候紧急情况有关的时间尺度上的净C预算，我们就不能忽视地下过程。了解控制土壤有机质（SOM）积累、稳定和损失的基本机制对预测地球未来气候的重要性不亚于了解光合作用和植物生产力。然而，我们对SOM动态的理解和建模能力远远落后于初级生产力。此外，北纬高纬度地区的快速变暖增加了了解对整个生态系统碳循环的控制、生态系统与大气之间的二氧化碳净通量以及SOM对气候和管理变化（例如为碳封存而植树）的脆弱性的紧迫性。目的和目标-在MYCONET中，我们关注北部高纬度地区富碳土壤的“菌根圈”（直接受根及其菌根真菌影响的土壤和生物）及其对提高植物生产力和向多木灌木和乔木群落转移的潜在反应。我们假设菌根圈的相关变化可能在与气候紧急情况相关的时间尺度上（即几十年）导致土壤C的净损失而不是收益，这是自相矛盾的。这将代表对气候变化的“正反馈”（即，由于SOM分解，二氧化碳排放到大气中的速率超过了通过光合作用吸收二氧化碳的净速率）。我们将通过在使用和创新的实验部署中应用突破性技术来推动前沿-自然丰度（和耗尽）放射性碳（14C），以及宏基因组学，土壤和根尖酶测定和SOM化学，量化和了解菌根圈的过程和动态以及这些过程和动态如何影响SOC储量。我们详细地关注“启动”过程（当添加到土壤中的物质影响SOM的分解速度时，无论是积极的还是消极的），以及菌根真菌在这个过程和相关过程中的具体作用。我们将就地测量这些过程（在北极和英国高地）和对照实验（使用树木、灌木和菌根共生体的特定组合），作为机械研究、土壤剖面分析和动态SOM建模的综合包的一部分，以量化和理解启动如何起作用，以及对SOM动力学、生态系统C通量和养分循环的影响。潜在的应用和好处-通过应用突破性的技术，MYCONET将改变我们对植物-土壤相互作用和菌根真菌在SOM动力学中的作用的理解。获得的基础新知识将显著改善气候-生物地球化学相互作用的区域和全球模型，特别关注植物群落转移的间接影响。该项目与泛北极“灌木林化”以及为碳封存或“重新野生化”而管理的生态系统有关。考虑到主要的政策重点和公众对碳封存植树的兴趣，这个项目尤其及时。

The Context of the Research - Many high-profile research papers and syntheses have equated increased vegetation productivity and shifting vegetation types in northern high latitudes with increased net carbon (C) sequestration from the atmosphere. Although logical and intuitive, this largely overlooks the potential fate of pre-existing soil organic carbon (SOC) in these regions. This is a problem because soils at high latitudes are notably C-rich (containing ~570 Pg C in boreal/taiga forest and tundra soils alone; note, 1 Pg (Peta-gram) = 1,000,000,000 tonnes) and this pool is dynamic, intrinsically interacting both with vegetation cover and with climate.Although challenging to investigate, we cannot overlook below-ground processes if we are to understand net C budgets on timescales relevant to the Climate Emergency. Understanding the fundamental mechanisms controlling the accumulation, stability, and loss of soil organic matter (SOM) is as essential for predicting the Earth's future climate as understanding photosynthesis and plant productivity. However, our understanding of, and ability to model, SOM dynamics lags far behind that of primary productivity. Furthermore, rapid warming at high northern latitudes adds urgency to understanding controls on whole-ecosystem C cycling, net fluxes of CO2 between ecosystems and the atmosphere, and the vulnerability of SOM to changes in both climate and management (for example, tree planting for C-sequestration).Aims and Objectives - In MYCONET we focus on the 'mycorrhizosphere' (the soil and organisms directly influenced by roots and their mycorrhizal fungi) of C-rich soils of northern high latitudes and its potential response both to increasing plant productivity and to shifts to woodier shrub and tree communities. We hypothesise that associated changes in the mycorrhizosphere could, paradoxically, result in net losses, rather than gains, of soil C over timescales (i.e. several decades) of relevance to the Climate Emergency. This would represent a 'positive feedback' on climate change (i.e. when the rates of CO2 emission to the atmosphere, due to SOM decomposition, exceed net rates of CO2 uptake via photosynthesis).We will push the frontiers by applying ground-breaking techniques in the use - and innovative experimental deployment - of natural abundance (and depleted) radiocarbon (14C), together with metagenomics, soil and root-tip enzyme assays and SOM chemistry, to quantify and understand the processes and dynamics of the mycorrhizosphere and how these affect SOC stocks. We focus, in detail, on the process of 'priming' (which occurs when material added to soil affects the rate of decomposition of SOM, either positively or negatively), and the specific role of mycorrhizal fungi in this, and related, processes. We will measure these processes both in situ (in the Arctic and the UK uplands) and in controlled experiments (using specific combinations of tree, shrub and mycorrhizal symbionts), as part of an integrated package of mechanistic studies, soil profile analysis and dynamic SOM modelling, to quantify and understand how priming works, and the implications for SOM dynamics, ecosystem C fluxes, and nutrient cycling. Potential applications and benefits - By applying ground-breaking techniques MYCONET will transform our understanding of plant-soil interactions and the role of mycorrhizal fungi in SOM dynamics. The fundamental new knowledge gained will significantly improve regional and global modelling of climate-biogeochemical interactions, with a particular focus on the indirect effects of shifting plant communities. The project has relevance for the pan-Arctic 'shrubification', as well as for ecosystems being managed for C-sequestration or 're-wilding'. This project is especially timely, given the major policy emphasis and public interest in tree planting for C sequestration.