Shrub-driven transformation of the alpine soil carbon cycle

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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.