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1. Scientific Communities Striving for a Common Cause: Innovations in Carbon Cycle Science

   https://doi.org/10.1175/BAMS-D-19-0306.1  

   Whelan, Mary E.; Anderegg, Leander D.; Badgley, Grayson; Campbell, J. Elliott; Commane, Roisin; Frankenberg, Christian; Hilton, Timothy W.; Kuai, Le; Parazoo, Nicholas; Shiga, Yoichi; et al (September 2020, Bulletin of the American Meteorological Society)

   Abstract Where does the carbon released by burning fossil fuels go? Currently, ocean and land systems remove about half of the CO 2 emitted by human activities; the remainder stays in the atmosphere. These removal processes are sensitive to feedbacks in the energy, carbon, and water cycles that will change in the future. Observing how much carbon is taken up on land through photosynthesis is complicated because carbon is simultaneously respired by plants, animals, and microbes. Global observations from satellites and air samples suggest that natural ecosystems take up about as much CO 2 as they emit. To match the data, our land models generate imaginary Earths where carbon uptake and respiration are roughly balanced, but the absolute quantities of carbon being exchanged vary widely. Getting the magnitude of the flux is essential to make sure our models are capturing the right pattern for the right reasons. Combining two cutting-edge tools, carbonyl sulfide (OCS) and solar-induced fluorescence (SIF), will help develop an independent answer of how much carbon is being taken up by global ecosystems. Photosynthesis requires CO 2 , light, and water. OCS provides a spatially and temporally integrated picture of the “front door” of photosynthesis, proportional to CO 2 uptake and water loss through plant stomata. SIF provides a high-resolution snapshot of the “side door,” scaling with the light captured by leaves. These two independent pieces of information help us understand plant water and carbon exchange. A coordinated effort to generate SIF and OCS data through satellite, airborne, and ground observations will improve our process-based models to predict how these cycles will change in the future. 

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2. Plant diversity decreases greenhouse gas emissions by increasing soil and plant carbon storage in terrestrial ecosystems

   https://doi.org/10.1111/ele.14469  

   Dang, Pengfei; Zhang, Miaomiao; Chen, Xinli; Loreau, Michel; Duffy, J Emmett; Li, Xin'e; Wen, Shuyue; Han, Xiaoqing; Liao, Lechen; Huang, Tiantian; et al (July 2024, Ecology Letters)

   Abstract The decline in global plant diversity has raised concerns about its implications for carbon fixation and global greenhouse gas emissions (GGE), including carbon dioxide (CO₂), nitrous oxide (N₂O) and methane (CH₄). Therefore, we conducted a comprehensive meta‐analysis of 2103 paired observations, examining GGE, soil organic carbon (SOC) and plant carbon in plant mixtures and monocultures. Our findings indicate that plant mixtures decrease soil N₂O emissions by 21.4% compared to monocultures. No significant differences occurred between mixtures and monocultures for soil CO₂emissions, CH₄emissions or CH₄uptake. Plant mixtures exhibit higher SOC and plant carbon storage than monocultures. After 10 years of vegetation development, a 40% reduction in species richness decreases SOC content and plant carbon storage by 12.3% and 58.7% respectively. These findings offer insights into the intricate connections between plant diversity, soil and plant carbon storage and GGE—a critical but previously unexamined aspect of biodiversity–ecosystem functioning. 

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3. Legacy Effects of Plant Community Structure Are Manifested in Microbial Biofilm Development With Consequences for Ecosystem CO₂ Emissions

   https://doi.org/10.1111/gcb.17603  

   Wyatt, Kevin_H; Cieslik, Jillian; Dieleman, Catherine_M; Kane, Evan_S; Rober, Allison_R; Sullivan, Barrett; Turetsky, Merritt_R (November 2024, Global Change Biology)

   ABSTRACT To better understand linkages between hydrology and ecosystem carbon flux in northern aquatic ecosystems, we evaluated the relationship between plant communities, biofilm development, and carbon dioxide (CO₂) exchange following long‐term changes in hydrology in an Alaskan fen. We quantified seasonal variation in biofilm composition and CO₂exchange in response to lowered and raised water table position (relative to a control) during years with varying levels of background dissolved organic carbon (DOC). We then used nutrient‐diffusing substrates (NDS) to evaluate cause–effect relationships between changes in plant subsidies (i.e., leachates) and biofilm composition among water table treatments. We found that background DOC concentration determined whether plant subsidies promoted net autotrophy or heterotrophy on NDS. In conditions where background DOC was ≤ 40 mg L⁻¹, plant subsidies promoted an autotrophic biofilm. Conversely, when background DOC concentration was ≥ 50 mg L⁻¹, plant subsidies promoted heterotrophy. Greater light attenuation associated with elevated levels of DOC may have overwhelmed the stimulatory effect of nutrients on autotrophic microbes by constraining photosynthesis while simultaneously allowing heterotrophs to outcompete autotrophs for available nutrients. At the ecosystem level, conditions that favored an autotrophic biofilm resulted in net CO₂uptake among all water table treatments, whereas the site was a net source of CO₂to the atmosphere in conditions that supported greater heterotrophy. Taken together, these findings show that hydrologic history interacts with changes in dominant plant functional groups to alter biofilm composition, which has consequences for ecosystem CO₂exchange. 

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4. The Southern Ocean Carbon Cycle 1985–2018: Mean, Seasonal Cycle, Trends, and Storage

   https://doi.org/10.1029/2023GB007848  

   Hauck, Judith; Gregor, Luke; Nissen, Cara; Patara, Lavinia; Hague, Mark; Mongwe, Precious; Bushinsky, Seth; Doney, Scott C; Gruber, Nicolas; Le_Quéré, Corinne; et al (November 2023, Global Biogeochemical Cycles)

   Abstract We assess the Southern Ocean CO₂uptake (1985–2018) using data sets gathered in the REgional Carbon Cycle Assessment and Processes Project Phase 2. The Southern Ocean acted as a sink for CO₂with close agreement between simulation results from global ocean biogeochemistry models (GOBMs, 0.75 ± 0.28 PgC yr⁻¹) andpCO₂‐observation‐based products (0.73 ± 0.07 PgC yr⁻¹). This sink is only half that reported by RECCAP1 for the same region and timeframe. The present‐day net uptake is to first order a response to rising atmospheric CO₂, driving large amounts of anthropogenic CO₂(C_ant) into the ocean, thereby overcompensating the loss of natural CO₂to the atmosphere. An apparent knowledge gap is the increase of the sink since 2000, withpCO₂‐products suggesting a growth that is more than twice as strong and uncertain as that of GOBMs (0.26 ± 0.06 and 0.11 ± 0.03 Pg C yr⁻¹ decade⁻¹, respectively). This is despite nearly identicalpCO₂trends in GOBMs andpCO₂‐products when both products are compared only at the locations wherepCO₂was measured. Seasonal analyses revealed agreement in driving processes in winter with uncertainty in the magnitude of outgassing, whereas discrepancies are more fundamental in summer, when GOBMs exhibit difficulties in simulating the effects of the non‐thermal processes of biology and mixing/circulation. Ocean interior accumulation of C_antpoints to an underestimate of C_antuptake and storage in GOBMs. Future work needs to link surface fluxes and interior ocean transport, build long overdue systematic observation networks and push toward better process understanding of drivers of the carbon cycle. 

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5. Maximum carbon uptake rate dominates the interannual variability of global net ecosystem exchange

   https://doi.org/10.1111/gcb.14731  

   Fu, Zheng; Stoy, Paul C.; Poulter, Benjamin; Gerken, Tobias; Zhang, Zhen; Wakbulcho, Guta; Niu, Shuli (July 2019, Global Change Biology)

   Abstract Terrestrial ecosystems contribute most of the interannual variability (IAV) in atmospheric carbon dioxide (CO₂) concentrations, but processes driving the IAV of net ecosystem CO₂exchange (NEE) remain elusive. For a predictive understanding of the global C cycle, it is imperative to identify indicators associated with ecological processes that determine the IAV of NEE. Here, we decompose the annual NEE of global terrestrial ecosystems into their phenological and physiological components, namely maximum carbon uptake (MCU) and release (MCR), the carbon uptake period (CUP), and two parameters, α and β, that describe the ratio between actual versus hypothetical maximum C sink and source, respectively. Using long‐term observed NEE from 66 eddy covariance sites and global products derived from FLUXNET observations, we found that the IAV of NEE is determined predominately by MCU at the global scale, which explains 48% of the IAV of NEE on average while α, CUP, β, and MCR explain 14%, 25%, 2%, and 8%, respectively. These patterns differ in water‐limited ecosystems versus temperature‐ and radiation‐limited ecosystems; 31% of the IAV of NEE is determined by the IAV of CUP in water‐limited ecosystems, and 60% of the IAV of NEE is determined by the IAV of MCU in temperature‐ and radiation‐limited ecosystems. The Lund‐Potsdam‐Jena (LPJ) model and the Multi‐scale Synthesis and Terrestrial Model Inter‐comparison Project (MsTMIP) models underestimate the contribution of MCU to the IAV of NEE by about 18% on average, and overestimate the contribution of CUP by about 25%. This study provides a new perspective on the proximate causes of the IAV of NEE, which suggest that capturing the variability of MCU is critical for modeling the IAV of NEE across most of the global land surface. 

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