Abstract
AbstractCarbon cycle feedbacks from permafrost ecosystems are expected to accelerate global climate change. Shifts in vegetation productivity and composition in permafrost regions could influence soil organic carbon (SOC) turnover rates via rhizosphere (root zone) priming effects (RPEs), but these processes are not currently accounted for in model predictions. We use a radiocarbon (bomb‐14C) approach to test for RPEs in two Arctic tall shrubs, alder (Alnus viridis (Chaix) DC.) and birch (Betula glandulosa Michx.), and in ericaceous heath tundra vegetation. We compare surface CO2 efflux rates and 14C content between intact vegetation and plots in which below‐ground allocation of recent photosynthate was prevented by trenching and removal of above‐ground biomass. We show, for the first time, that recent photosynthate drives mineralization of older (>50 years old) SOC under birch shrubs and ericaceous heath tundra. By contrast, we find no evidence of RPEs in soils under alder. This is the first direct evidence from permafrost systems that vegetation influences SOC turnover through below‐ground C allocation. The vulnerability of SOC to decomposition in permafrost systems may therefore be directly linked to vegetation change, such that expansion of birch shrubs across the Arctic could increase decomposition of older SOC. Our results suggest that carbon cycle models that do not include RPEs risk underestimating the carbon cycle feedbacks associated with changing conditions in tundra regions.
Highlights
Current estimates of net CO2 release from permafrost ecosystems vary from around 20 billion to more than 200 billion tonnes of C by the end of this century (Gasser et al, 2018; Natali et al, 2019; Schuur et al, 2015; Turetsky et al, 2019 )
In this study we ask: do Rhizosphere priming effects (RPEs) influence soil organic carbon (SOC) mineralization rates in permafrost ecosystems? If so, do RPEs differ between vegetation types, which may shift in abundance with Arctic greening? We develop the radiocarbon approach of Hartley et al (2012) and Gavazov et al (2018) to compare RPEs in two species of tall woody shrub vegetation versus ericaceous heath tundra, using a novel Bayesian mixing model approach to examine potential positive and negative RPEs
Average soil pore space 14CO2 in intact plots was depleted relative to ambient atmospheric values for all vegetation types and >100%modern only in birch plots (Figure 5a)
Summary
Current estimates of net CO2 release from permafrost ecosystems vary from around 20 billion to more than 200 billion tonnes of C by the end of this century (Gasser et al, 2018; Natali et al, 2019; Schuur et al, 2015; Turetsky et al, 2019 ). We develop the radiocarbon (bomb-14C) approach of Hartley et al (2012) and Gavazov et al (2018) to compare RPEs in two species of tall woody shrub vegetation versus ericaceous heath tundra, using a novel Bayesian mixing model approach to examine potential positive and negative RPEs. We test whether the allocation of recent photosynthate C (RPC) below-ground influences SOC mineralization by comparing the 14C content of soil CO2 effluxes with and without an intact plant canopy, using two-source mixing models: (a) a null model in which there are no priming effects and (b) a model in which either positive or negative RPEs can impact both the 14C content of respired SOC and the rate of SOC mineralization. We expect RPEs to be greater in birch vegetation compared to alder because of greater labile C inputs to the soil under birch
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