Fire decline in dry tropical ecosystems enhances decadal land carbon sink

The terrestrial carbon sink has significantly increased in the past decades, but the underlying mechanisms are still unclear. The current synthesis of process-based estimates of land and ocean sinks requires an additional sink of 0.6 PgC yr⁻¹ in the last decade to explain the observed airborne fraction. A concurrent global fire decline was observed in association with tropical agriculture expansion and landscape fragmentation. Here we show that a decline of 0.2 ± 0.1 PgC yr⁻¹ in fire emissions during 2008–2014 relative to 2001–2007 also induced an additional carbon sink enhancement of 0.4 ± 0.2 PgC yr⁻¹ attributable to carbon cycle feedbacks, amounting to a combined sink increase comparable to the 0.6 PgC yr⁻¹ budget imbalance. Our results suggest that the indirect effects of fire, in addition to the direct emissions, is an overlooked mechanism for explaining decadal-scale changes in the land carbon sink and highlight the importance of fire management in climate mitigation

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

Fixing tropical forests

An extensive dataset indicates that nitrogen-fixing trees are most abundant in young, dry tropical forests. The finding expands the potential for natural nitrogen fertilization and carbon dioxide sequestration in areas recovering from land use

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

Fire frequency drives decadal changes in soil carbon and nitrogen and ecosystem productivity

Fire frequency is changing globally and is projected to affect the global carbon cycle and climate. However, uncertainty about how ecosystems respond to decadal changes in fire frequency makes it difficult to predict the effects of altered fire regimes on the carbon cycle; for instance, we do not fully understand the long-term effects of fire on soil carbon and nutrient storage, or whether fire-driven nutrient losses limit plant productivity. Here we analyse data from 48 sites in savanna grasslands, broadleaf forests and needleleaf forests spanning up to 65 years, during which time the frequency of fires was altered at each site. We find that frequently burned plots experienced a decline in surface soil carbon and nitrogen that was non-saturating through time, having 36 per cent (±13 per cent) less carbon and 38 per cent (±16 per cent) less nitrogen after 64 years than plots that were protected from fire. Fire-driven carbon and nitrogen losses were substantial in savanna grasslands and broadleaf forests, but not in temperate and boreal needleleaf forests. We also observe comparable soil carbon and nitrogen losses in an independent field dataset and in dynamic model simulations of global vegetation. The model study predicts that the long-term losses of soil nitrogen that result from more frequent burning may in turn decrease the carbon that is sequestered by net primary productivity by about 20 per cent of the total carbon that is emitted from burning biomass over the same period. Furthermore, we estimate that the effects of changes in fire frequency on ecosystem carbon storage may be 30 per cent too low if they do not include multidecadal changes in soil carbon, especially in drier savanna grasslands. Future changes in fire frequency may shift ecosystem carbon storage by changing soil carbon pools and nitrogen limitations on plant growth, altering the carbon sink capacity of frequently burning savanna grasslands and broadleaf forests

Fire as a fundamental ecological process: Research advances and frontiers

Fire is a powerful ecological and evolutionary force that regulates organismal traits, population sizes, species interactions, community composition, carbon and nutrient cycling and ecosystem function. It also presents a rapidly growing societal challenge, due to both increasingly destructive wildfires and fire exclusion in fire‐dependent ecosystems. As an ecological process, fire integrates complex feedbacks among biological, social and geophysical processes, requiring coordination across several fields and scales of study. Here, we describe the diversity of ways in which fire operates as a fundamental ecological and evolutionary process on Earth. We explore research priorities in six categories of fire ecology: (a) characteristics of fire regimes, (b) changing fire regimes, (c) fire effects on above‐ground ecology, (d) fire effects on below‐ground ecology, (e) fire behaviour and (f) fire ecology modelling. We identify three emergent themes: the need to study fire across temporal scales, to assess the mechanisms underlying a variety of ecological feedbacks involving fire and to improve representation of fire in a range of modelling contexts. Synthesis: As fire regimes and our relationships with fire continue to change, prioritizing these research areas will facilitate understanding of the ecological causes and consequences of future fires and rethinking fire management alternatives