Shrub-driven transformation of the alpine soil carbon cycle

全球生态系统正在经历植被的快速变化，以应对气候变化。世界上最普遍但研究较少的植被变化发生在高山地区，那里的气候变化速度几乎是北半球平均速度的两倍。这与土地利用的变化相结合，导致木质白垩灌木迅速而广泛地向上扩展到高山草原，对这些脆弱的高海拔生态系统的功能可能产生深远的影响，但人们对其知之甚少。一个紧迫的不确定性涉及到白垩灌木改变高寒草原土壤C循环过程的潜力，以及对土壤C储存和持久性的影响。考虑到高寒草地土壤代表着全球主要的碳储存，而且即使土壤碳储存能力的微小变化也可能对碳循环对气候变化的反馈产生重大影响，解决这种不确定性的必要性再紧迫不过了。应对这一挑战需要我们对灌木驱动的土壤有机质（SOM）转化及其持久性的机制的理解发生重大变化，土壤有机质是决定土壤C对全球变化响应的关键生态系统属性。我们认为，在未来气候变化下，灌丛及其根相关的灌丛菌根真菌在高寒草地上广泛而快速的向上扩张触发了不同的根际途径，从而抑制了土壤有机质的分解和持久有机质的形成，从而稳定了土壤C库，减少了土壤C的损失。我们还预计，这些土壤碳增加和稳定的途径将超过导致土壤碳流失的相反根际途径，从而导致净碳增加。我们计划利用先进的稳定同位素、基因组学和生化方法，结合景观、地块和实验室研究来验证我们的新假设，以探究高山草地中SOM分解和C稳定的根际驱动途径的相对作用。此外，我们在过去NERC资助的高山草原研究的基础上，包括在奥地利阿尔卑斯山的一个长期实验平台，我们将利用有关根际驱动过程调节土壤C持久性的途径的新概念和新发现。我们的研究将通过确定快速和广泛的白垩灌木扩张改变调节高寒草原土壤C增益和损失的根际途径平衡的新机制开辟新的领域，最终决定土壤C储存和C循环反馈。但同时，它也将推动理解根际驱动过程作为土壤碳储存和持久性调节者的作用的前沿，这是一个快速发展但知之甚少的科学领域，对全球碳平衡和减缓气候变化至关重要。

Ecosystems worldwide are experiencing rapid shifts in vegetation in response to climate change. One of the most pervasive, but poorly studied, vegetation shifts is occurring in alpine regions of the world, where climate change is taking place at almost double the rate of the northern hemisphere average. In combination with changes in land use, this is leading to the rapid and widespread upward expansion of woody ericaceous shrubs into alpine grasslands, with potential far-reaching, but poorly understood, consequences for the functioning of these fragile high-altitude ecosystems. A pressing uncertainty concerns the potential for ericaceous shrubs to transform processes of soil (C) cycling in alpine grasslands, with implications for soil C storage and persistence. The need to address this uncertainty couldn't be more urgent given that alpine grassland soils represent a major global C store, and because even small changes in their soil C storage capacity could have major implications for C-cycle feedbacks to climate change. Tackling this challenge requires a step change in our understanding of the mechanisms underpinning shrub-driven transformations of soil organic matter (SOM) and its persistence, a critical ecosystem property determining the response of soil C to global change.We posit that the widespread and rapid upward expansion of ericaceous shrubs and their root-associated ericoid mycorrhizal fungi into alpine grassland triggers distinct rhizosphere pathways that lead to the suppression of SOM decomposition and formation of persistent SOM, thereby stabilising the soil C pool and reducing soil C loss under future climate change. We also expect these pathways of soil C gain and stabilisation to outweigh opposing rhizosphere pathways that cause soil C loss, thereby leading to net C gain. We plan to test our novel hypotheses using a powerful combination of landscape, plot, and laboratory studies with advanced stable-isotope, genomics, and biochemical approaches to interrogate the relative roles of contrasting rhizosphere-driven pathways of SOM decomposition and stabilisation of C in alpine grassland. Moreover, we build on our past NERC funded research in alpine grasslands, including a long-term experimental platform in the Austrian Alps, and we draw on novel concepts and discoveries concerning the pathways by which rhizosphere-driven processes regulate the persistence of soil C. Our study will break new ground by identifying novel mechanisms by which rapid and widespread ericaceous shrub expansion alters the balance of rhizosphere pathways that regulate soil C gain and loss in alpine grasslands, ultimately determining soil C storage and C-cycle feedbacks. But also, it will push the frontiers of understanding the role of rhizosphere-driven processes as regulators of soil C storage and persistence, which is a fast moving, but poorly understood area of science of central importance to the global C balance and mitigation of climate change.