Testing the role of nutrient input thresholds in governing microbial-mediated carbon sequestration for temperate peatlands

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Our research will shed new light on the ways nutrient input (primarily phosphorus and nitrogen) controls how efficiently temperate, ombrotrophic peatlands store carbon. Our recent synthesis of the few peatlands with well-dated and parallel core profiles of phosphorus, nitrogen and carbon underpinned a new conceptual model of long-term peatland carbon cycling (Schillereff et al. 2021). Ombrotrophic (rain-fed) peatlands derive nutrients primarily via atmospheric deposition, so nutrient input maintains the tight balance between primary productivity and the decomposition of organic matter by microbes. We therefore hypothesised that nutrient input thresholds govern how efficiently and how much carbon becomes sequestered in peatlands over decades, centuries and millennia. Our study highlighted two research gaps preventing our conceptual model being tested: i) measurements of the activity and diversity of microbial communities are strikingly absent from palaeoenvironmental peat research and (ii) surprisingly few peatlands have parallel measurements for all nodes of the peatland carbon cycle: climate, vegetation, nutrients, microbes and carbon. Our proposal will implement an innovative research design to test our conceptual model. Working at four carefully selected temperate, ombrotrophic peatlands in the UK and Sweden where we have established research portfolios, we will integrate some of the first DNA characterisation of down-core microbial dynamics with high-resolution reconstructions of each other node of the peatland carbon cycle spanning the last 2000 years. This will enable our hypothesis that nutrient input thresholds govern long-term peatland carbon sequestration to be empirically tested for the first time. To deliver these objectives, we will: (i) collect new peat cores from four sites and perform DNA characterisation of their microbial communities at regular depths; (ii) complete high-resolution measurements of each other node at King's College London, University of Liverpool, Stockholm University and the NERC Environmental Omics Facility; (iii) apply statistical modelling to quantify the role of and interplay between each driver of peatland carbon cycling. Site selection (Holcroft and May Moss, England; Store and Draftinge Mosse, Sweden) is guided by our extensive baseline data and strategically captures gradients of nutrient deposition, altitude and land-use. By combining in an innovate way cutting-edge metagenomic characterisation of microbial activity with conventional measurements of other drivers, we will produce a more complete picture of peatland carbon cycling. Globally, peatlands are a significant carbon store, containing one-third of the soil carbon pool. Peatlands sequester carbon efficiently because their waterlogged, nutrient impoverished conditions slow the decomposition of organic matter relative to the productivity of surface vegetation. This also means small changes in nutrient input can trigger significant shifts in carbon storage. Human activities have amplified P and N deposition in recent decades, which has changed the nutrient limitation status and carbon cycling in other terrestrial ecosystems. The implications for the future peatland carbon sink remain unclear. As well as establishing a testbed for our conceptual model, the findings should stimulate deeper integration between the peatland ecology, soil microbiology and global carbon cycling research communities. We intend our findings to underpin ambitious future research aimed at better understanding the resilience of temperate peatlands to both 21st-century climate and biogeochemical change. This will involve parameterising new numerical models of peatland development that encapsulate nodes for climate, vegetation, nutrients, microbes and carbon. Subsequent integration into Earth System models should produce more representative trajectories for peatland carbon through the 21st-century. Schillereff et al. 2021, Comms. Earth & Env. 2:1-10