Periodic Reporting for period 4 - 14Constraint (Radiocarbon constraints for models of C cycling in terrestrial ecosystems: from process understanding to global benchmarking)
Periodo di rendicontazione: 2021-06-01 al 2022-11-30
Predictions of land carbon storage over the next century made by Earth System Models continue to be highly uncertain, reflecting poor understanding of the processes controlling how long carbon persists in soils. Quantifying how long C added to ecosystems resides there before returning to the atmosphere - the transit time - is fundamental to improving models. Understanding what processes govern transit times will be key to designing effective strategies to
“Bomb” radiocarbon added to the atmosphere by nuclear weapons testing in the early 1960s provides a unique tracer for studying carbon flow through ecosystems on decadal to century timescales. Despite general recognition of the utility of radiocarbon as an important constraint for earth system models, its use has been limited because of poor communication as well as a lack of data products at appropriate scales for testing models. The overall goal of 14Constraint was to enhance the availability and use of radiocarbon data as constraints for process-based understanding of the age distribution of carbon in and respired by soils and ecosystems. We did this in three ways: (1) synthesizing data on radiocarbon from many individual studies at specific sites to produce global estimates of the radiocarbon in soil organic matter and in carbon being respired from soils; (2) filling large gaps in radiocarbon data from understudied regions; (3) advancing new tools for the comparison of radiocarbon data in soil fractions and respired from soils with model-predicted distributions of 14C; and (4) training a cohort of junior scientists in the use of radiocarbon to study carbon dynamics.
“Bomb” radiocarbon added to the atmosphere by nuclear weapons testing in the early 1960s provides a unique tracer for studying carbon flow through ecosystems on decadal to century timescales. Despite general recognition of the utility of radiocarbon as an important constraint for earth system models, its use has been limited because of poor communication as well as a lack of data products at appropriate scales for testing models. The overall goal of 14Constraint was to enhance the availability and use of radiocarbon data as constraints for process-based understanding of the age distribution of carbon in and respired by soils and ecosystems. We did this in three ways: (1) synthesizing data on radiocarbon from many individual studies at specific sites to produce global estimates of the radiocarbon in soil organic matter and in carbon being respired from soils; (2) filling large gaps in radiocarbon data from understudied regions; (3) advancing new tools for the comparison of radiocarbon data in soil fractions and respired from soils with model-predicted distributions of 14C; and (4) training a cohort of junior scientists in the use of radiocarbon to study carbon dynamics.
The 14Constraint team catalyzed a community effort to build the International Radiocarbon Database (ISRaD; www.soilradiocarbon.org) and build a completely open-source repository for published soil radiocarbon data. The current version of ISRaD summaries 423 studies, with close to 15,000 radiocarbon measurements made at nearly 1000 different sites. Importantly, the data include not only bulk radiocarbon measurements but also measurements of physically, chemically and thermally separated fractions, soil CO2 fluxes and metadata including a range of soil properties that provide the opportunity to link the age distribution of carbon in soil to various stabilization mechanisms.
One of our goals was to fill gaps in global data coverage. We did this through collaboration with international researchers, especially those from regions like Africa and Latin America without easy access to 14C measurements. In addition we focused our efforts on expanding time series of radiocarbon data to provide strong constraints for modeling carbon cycling in soils and how this is affected by management (e.g. of carbon inputs) or by disturbances like deforestation.
Much effort went into advancing methods to facilitate data-model intercomparison efforts and communicating how radiocarbon data can be most effectively used to evaluate models. Many studies in the past have used a single metric - the mean age of carbon in soil organic matter derived from a bulk radiocarbon value. This provides a measure of the average time since the C in all soil organic matter was originally fixed from the atmosphere. However, it also assumes that soil organic matter is homogeneous, while soil organic matter is highly heterogeneous. We therefore proposed the use of an additional metric - the transit time, a measure of the mean time elapsed between when C was fixed from the atmosphere to when it is returned by respiration or decomposition back to the atmosphere as CO2. Mean soil carbon transit times range from years (tropical forests) to centuries (boreal and arctic regions), much shorter than the mean ages of organic matter (generally hundreds to thousands of years). For models attempting to predict changes in C stores over the next century, the transit time is a more constraining metric than the age as it is a measure of faster-cycling carbon that can respond to global change.
However, we also moved beyond the single metrics of mean age and transit time to take advantage of our ability to use a variety of methods to determine the distributions of ages and transit times. For example, we developed a method to determine the age distribution of C in mineral soil using a physical separation of particulate organic matter followed by thermal ramped oxidation. We used incubation of soils from different depth intervals to help define the shape of transit time distributions. Where available, we could also take advantage of time series of radiocarbon for constraining the age and transit time distributions of carbon. These can then be compared directly with age and transit time distributions predicted by models. Alternatively, model predictions can be used to generate predictions of radiocarbon distributions for any given year since 1960 for comparison with measurements and especially time series. Model-data comparisons then can be focused on what parts of the distribution are not well simulated - and which processes might be responsible.
One of our goals was to fill gaps in global data coverage. We did this through collaboration with international researchers, especially those from regions like Africa and Latin America without easy access to 14C measurements. In addition we focused our efforts on expanding time series of radiocarbon data to provide strong constraints for modeling carbon cycling in soils and how this is affected by management (e.g. of carbon inputs) or by disturbances like deforestation.
Much effort went into advancing methods to facilitate data-model intercomparison efforts and communicating how radiocarbon data can be most effectively used to evaluate models. Many studies in the past have used a single metric - the mean age of carbon in soil organic matter derived from a bulk radiocarbon value. This provides a measure of the average time since the C in all soil organic matter was originally fixed from the atmosphere. However, it also assumes that soil organic matter is homogeneous, while soil organic matter is highly heterogeneous. We therefore proposed the use of an additional metric - the transit time, a measure of the mean time elapsed between when C was fixed from the atmosphere to when it is returned by respiration or decomposition back to the atmosphere as CO2. Mean soil carbon transit times range from years (tropical forests) to centuries (boreal and arctic regions), much shorter than the mean ages of organic matter (generally hundreds to thousands of years). For models attempting to predict changes in C stores over the next century, the transit time is a more constraining metric than the age as it is a measure of faster-cycling carbon that can respond to global change.
However, we also moved beyond the single metrics of mean age and transit time to take advantage of our ability to use a variety of methods to determine the distributions of ages and transit times. For example, we developed a method to determine the age distribution of C in mineral soil using a physical separation of particulate organic matter followed by thermal ramped oxidation. We used incubation of soils from different depth intervals to help define the shape of transit time distributions. Where available, we could also take advantage of time series of radiocarbon for constraining the age and transit time distributions of carbon. These can then be compared directly with age and transit time distributions predicted by models. Alternatively, model predictions can be used to generate predictions of radiocarbon distributions for any given year since 1960 for comparison with measurements and especially time series. Model-data comparisons then can be focused on what parts of the distribution are not well simulated - and which processes might be responsible.
By increasing the amount and utility of available radiocarbon data, we have added new constraints to carbon cycle modeling and developed new tools for comparing model predictions with measurable properties in soil fractions and ecosystem respiration. Our results have advanced understanding of the factors controlling C stabilization in soils and the role of soils in the global carbon cycle in the following ways:
- Demonstrating that current Earth System models underestimate the age of soil C and predict too much C storage in soils over the next century.
- Using radiocarbon profiles to improve a 14C-enabled global carbon-climate model (ESM3).
- Development of tools to use output from any carbon cycle model to predict 14C age and transit time distributions through time, meaning that models do not need to implement isotopes in their code to take advantage of 14C constraints.
- Demonstration that understanding of the climatic and mineralogical controls of soil C amount and age developed from site-level studies also hold at subcontinental scales using data from sub-Saharan Africa obtained in collaboration with the African Soil Information Service.
- Using time series of radiocarbon data to demonstrate that increased soil inputs with management do not store additional carbon but instead increase the speed with which soils return carbon to the atmosphere. This supports model structures that require microbial adaptation to altered inputs.
- Demonstrating the importance and timescales associated with different soil mineral stabilization processes by comparing ages and transit time distributions of surface litter with mineral soils from a range of ecosystems and mineralogical/pedogenic settings.
- Expanding relationships between soil mineralogy and the age to include the chemistry of mineral associated soil carbon, while reinforcing the importance of overall pedogenic setting as the main control of the age of soil organic matter.
Finally, by hosting international researchers, teaching short courses on the interpretation and use of radiocarbon in modeling, and holding regular ‘Hackathons’, we have greatly expanded the international community using bomb 14C as a constraint for the timescales of global carbon cycling.
- Demonstrating that current Earth System models underestimate the age of soil C and predict too much C storage in soils over the next century.
- Using radiocarbon profiles to improve a 14C-enabled global carbon-climate model (ESM3).
- Development of tools to use output from any carbon cycle model to predict 14C age and transit time distributions through time, meaning that models do not need to implement isotopes in their code to take advantage of 14C constraints.
- Demonstration that understanding of the climatic and mineralogical controls of soil C amount and age developed from site-level studies also hold at subcontinental scales using data from sub-Saharan Africa obtained in collaboration with the African Soil Information Service.
- Using time series of radiocarbon data to demonstrate that increased soil inputs with management do not store additional carbon but instead increase the speed with which soils return carbon to the atmosphere. This supports model structures that require microbial adaptation to altered inputs.
- Demonstrating the importance and timescales associated with different soil mineral stabilization processes by comparing ages and transit time distributions of surface litter with mineral soils from a range of ecosystems and mineralogical/pedogenic settings.
- Expanding relationships between soil mineralogy and the age to include the chemistry of mineral associated soil carbon, while reinforcing the importance of overall pedogenic setting as the main control of the age of soil organic matter.
Finally, by hosting international researchers, teaching short courses on the interpretation and use of radiocarbon in modeling, and holding regular ‘Hackathons’, we have greatly expanded the international community using bomb 14C as a constraint for the timescales of global carbon cycling.