Arctic Soil Governs Whether Climate Change Drives Global Losses or Gains in Soil Carbon

Key uncertainties in terrestrial carbon cycle projections revolve around the timing, direction, and magnitude of the carbon cycle feedback to climate change. This is especially true in carbon-rich Arctic ecosystems, where permafrost soils contain roughly one third of the world's soil carbon stocks, which are likely vulnerable to loss. Using an ensemble of soil biogeochemical models that reflect recent changes in the conceptual understanding of factors responsible for soil carbon persistence, we quantify potential soil carbon responses under two representative climate change scenarios. Our results illustrate that models disagree on the sign and magnitude of global soil changes through 2100, with disagreements primarily driven by divergent responses of Arctic systems. These results largely reflect different assumptions about the nature of soil carbon persistence and vulnerabilities, underscoring the challenges associated with setting allowable greenhouse gas emission targets that will limit global warming to 1.5°C

Multiple models and experiments underscore large uncertainty in soil carbon dynamics

Soils contain more carbon than plants or the atmosphere, and sensitivities of soil organic carbon (SOC) stocks to changing climate and plant productivity are a major uncertainty in global carbon cycle projections. Despite a consensus that microbial degradation and mineral stabilization processes control SOC cycling, no systematic synthesis of long-term warming and litter addition experiments has been used to test process-based microbe-mineral SOC models. We explored SOC responses to warming and increased carbon inputs using a synthesis of 147 field manipulation experiments and five SOC models with different representations of microbial and mineral processes. Model projections diverged but encompassed a similar range of variability as the experimental results. Experimental measurements were insufficient to eliminate or validate individual model outcomes. While all models projected that CO efflux would increase and SOC stocks would decline under warming, nearly one-third of experiments observed decreases in CO flux and nearly half of experiments observed increases in SOC stocks under warming. Long-term measurements of C inputs to soil and their changes under warming are needed to reconcile modeled and observed patterns. Measurements separating the responses of mineral-protected and unprotected SOC fractions in manipulation experiments are needed to address key uncertainties in microbial degradation and mineral stabilization mechanisms. Integrating models with experimental design will allow targeting of these uncertainties and help to reconcile divergence among models to produce more confident projections of SOC responses to global changes. 2

A trade-off between plant and soil carbon storage under elevated CO2

This is the author accepted manuscript. The final version is available from Nature Research via the DOI in this record.Data availability: All the empirical data that support the main findings of this study have been deposited in Figshare: (https://figshare.com/account/home#/projects/74721) and GitHub (https://github.com/cesarterrer/SoilC_CO2). FACE-MDS data can be accessed at https://www.osti.gov/dataexplorer/biblio/dataset/1480327. CMIP5 data can be accessed at https://esgf-index1.ceda.ac.uk/search/cmip5-ceda/. TRENDY data can be requested at http://dgvm.ceh.ac.uk/index.html.Code availability: The R code used in the analysis presented in this paper is available in GitHub and can be accessed at https://github.com/cesarterrer/SoilC_CO2Terrestrial ecosystems remove about 30% of the CO2 emitted by human activities each year, yet the persistence of this carbon sink partly depends on how plant biomass and soil carbon stocks respond to future increases in atmospheric CO2. While plant biomass often increases in elevated CO2 (eCO2) experiments, soil carbon has been observed to increase, remain unchanged, or even decline. The mechanisms driving this variation across experiments remain poorly understood, creating uncertainty in climate projections. Here, we synthesized data from 108 eCO2 experiments and found that the effect of eCO2 on soil carbon stocks is best explained by a negative relationship with plant biomass: when plant biomass is strongly stimulated by eCO2, soil carbon accrual declines; conversely, when biomass is weakly stimulated, soil carbon accumulates. This trade-off appears related to plant nutrient acquisition, whereby enhanced biomass requires mining the soil for nutrients, which decreases soil carbon accrual. We found an increase in soil carbon stocks with eCO2 in grasslands (8±2%) and no increase in forests (0±2%), even though plant biomass in grassland responded less strongly (9±3%) than in forest (23±2%). Ecosystem models do not reproduce this trade-off, which implies that projections of soil carbon may need to be revised.Lawrence Livermore National Laboratory (LLNL).U.S. Department of Energy, Terrestrial Ecosystem Science ProgramNAS

Arctic Soil Governs Whether Climate Change Drives Global Losses or Gains in Soil Carbon

Key uncertainties in terrestrial carbon cycle projections revolve around the timing, direction, and magnitude of the carbon cycle feedback to climate change. This is especially true in carbon-rich Arctic ecosystems, where permafrost soils contain roughly one third of the world's soil carbon stocks, which are likely vulnerable to loss. Using an ensemble of soil biogeochemical models that reflect recent changes in the conceptual understanding of factors responsible for soil carbon persistence, we quantify potential soil carbon responses under two representative climate change scenarios. Our results illustrate that models disagree on the sign and magnitude of global soil changes through 2100, with disagreements primarily driven by divergent responses of Arctic systems. These results largely reflect different assumptions about the nature of soil carbon persistence and vulnerabilities, underscoring the challenges associated with setting allowable greenhouse gas emission targets that will limit global warming to 1.5°C

Land Use and Land Cover Affect the Depth Distribution of Soil Carbon: Insights From a Large Database of Soil Profiles

Soils contain a large and dynamic fraction of global terrestrial carbon stocks. The distribution of soil carbon (SC) with depth varies among ecosystems and land uses and is an important factor in calculating SC stocks and their vulnerabilities. Systematic analysis of SC depth distributions across databases of SC profiles has been challenging due to the heterogeneity of soil profile measurements, which vary in depth sampling. Here, we fit over 40,000 SC depth profiles to an exponential decline relationship with depth to determine SC concentration at the top of the mineral soil, minimum SC concentration at depth, and the characteristic “length” of SC concentration decline with depth. Fitting these parameters allowed profile characteristics to be analyzed across a large and heterogeneous dataset. We then assessed the differences in these depth parameters across soil orders and land cover types and between soil profiles with or without a history of tillage, as represented by the presence of an Ap horizon. We found that historically tilled soils had more gradual decreases of SC with depth (greater e-folding depth or Z∗), deeper SC profiles, lower SC concentrations at the top of the mineral soil, and lower total SC stocks integrated to 30 cm. The large database of profiles allowed these results to be confirmed across different land cover types and spatial areas within the Continental United States, providing robust evidence for systematic impacts of historical tillage on SC stocks and depth distributions

Emergent temperature sensitivity of soil organic carbon driven by mineral associations

Acknowledgements: K.G. was supported as a Lawrence Fellow at Lawrence Livermore National Laboratory (LLNL) by the LLNL-LDRD Program under Project No. 21-ERD-045 and 24-LW-053. J.P.-R. and E.W.S. were supported by the US Department of Energy (DOE) Office of Science, Office of Biological and Environmental Research, Genomic Science Program as part of the LLNL Microbes Persist Scientific Focus Area, SCW1632. Work at LLNL was conducted under the auspices of DOE Contract DE-AC52-07NA27344. W.R.W. was supported by National Science Foundation Grants 1926413 and 2031238 and by USDA NIFA-AFRI Grant 2020-67019-31395. C.D.K., W.J.R. and Q.Z. were supported by the US DOE Biological and Environmental Research (BER) Program at LBNL under DOE Contract DE-AC02-05CH11231 through the Regional and Global Model Analysis Program (RUBISCO SFA). N.J.B. was supported by the US DOE BER Early Career Research Program under Contract FP00005182. R.Z.A. and B.N.S. were supported by the US DOE BER at Oak Ridge National Laboratory under DOE Contract DE-AC05-00OR22725. A.A. acknowledges the research environment Biodiversity and Ecosystem Services in a Changing Climate (BECC) at Lund University and funding from the Swedish Research Council (2021-05344).AbstractSoil organic matter decomposition and its interactions with climate depend on whether the organic matter is associated with soil minerals. However, data limitations have hindered global-scale analyses of mineral-associated and particulate soil organic carbon pools and their benchmarking in Earth system models used to estimate carbon cycle–climate feedbacks. Here we analyse observationally derived global estimates of soil carbon pools to quantify their relative proportions and compute their climatological temperature sensitivities as the decline in carbon with increasing temperature. We find that the climatological temperature sensitivity of particulate carbon is on average 28% higher than that of mineral-associated carbon, and up to 53% higher in cool climates. Moreover, the distribution of carbon between these underlying soil carbon pools drives the emergent climatological temperature sensitivity of bulk soil carbon stocks. However, global models vary widely in their predictions of soil carbon pool distributions. We show that the global proportion of model pools that are conceptually similar to mineral-protected carbon ranges from 16 to 85% across Earth system models from the Coupled Model Intercomparison Project Phase 6 and offline land models, with implications for bulk soil carbon ages and ecosystem responsiveness. To improve projections of carbon cycle–climate feedbacks, it is imperative to assess underlying soil carbon pools to accurately predict the distribution and vulnerability of soil carbon.</jats:p