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Radiocarbon constraints for models of C cycling in terrestrial ecosystems: from process understanding to global benchmarking

Periodic Reporting for period 2 - 14Constraint (Radiocarbon constraints for models of C cycling in terrestrial ecosystems: from process understanding to global benchmarking)

Reporting period: 2018-06-01 to 2019-11-30

The overall goal of 14Constraint is 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.

Predictions of land carbon storage over the next centures made by Earth System Models (ESMs) are highly uncertain, largely because of a lack of basic understanding of processes that control the amount of time carbon resides in soils. Our goal is to improve this situation by applying the best available tracer for studying carbon flow through ecosystems on decadal timescales – a global isotope label in the form of 14C added to the atmosphere by weapons testing in the 1960s. The flow of this “bomb” radiocarbon from the atmosphere through plants to soils and into respiration over subsequent decades provides a way to directly measure timescales of C cycling. For example, using bomb radiocarbon in the oceans has been a critical part of three global ocean measurement campaigns, most recently WOCE (world ocean circulation experiment). These data sets have proven extremely valuable for understanding the ventilation and circulation of the oceans. However, we have no equivalent global summary of radiocarbon for the terrestrial C cycle. This is partly because of the greater spatial heterogeneity found on land, but also because there has to date been no comprehensive effort on the scale of the ocean surveys. Indeed our best global understanding of the transit of bomb radiocarbon through terrestrial ecosystems at the global scale comes from a top-down calculation that estimates the terrestrial biosphere bomb 14C inventory through time from the residual of atmosphere and ocean inventories (just as we currently do for global C budgets). While modelers have generally recognized that 14C can provide an important constraint for ESMs and in particular for understanding soil C, there is a critical lack of data except for a few select sites, and no product with which to compare patterns on regional to global scales.

14Constraint will fill this gap. We will synthesize the existing radiocarbon data and develop a common framework for comparing these data with models. To fill gaps in under-sampled biomes, we will strategically expand the total number of 14C measurements in plant litter, soil C and microbially respired CO2. These data will be used to construct models of belowground C dynamics that will serve as benchmarks against which to compare ecosystem or global C cycle models. To provide tests for expanding from well-characterized sites to the globe, we will sample along carefully selected gradients to test our understanding of how minerals, vegetation and climate combine to control the age of litter, soil and respired C at global scales. By collaborating with ESM modelers, we will work to improve the characterization of soil processes and thereby the predictions of how much C can be stored in soils over the next century.
To date, our major achievements are structured among the three work packages.

WP1 aims to engage the community in the construction of a comprehensive database to synthesize existing radiocarbon data in vegetation, soils and respired CO2. WP1 has benefited from other efforts in the soil carbon community to create an openly available database, but the commission of dedicated resources will accelerate the process. WP1 also increases the number of openly available tools for interpretation of radiocarbon data (e.g. through continued development of modeling tools by collaborator Carlos Sierra) and use short courses to train researchers and students in the details of measuring radiocarbon and applying 14C data constraints.
Our major result to date is the construction of the ISRad database, hosted on Github (https://international-soil-radiocarbon-database.github.io/ISRaD/). A paper announcing the database (developed in collaboration with a group funded by the Powell Center effort in the US) is under revision for publication in Earth System Science Data - the Discussion paper can be found at https://www.earth-syst-sci-data-discuss.net/essd-2019-55/ As it is on Github, the database is completely open sources and available to all.

WP2 adds data from new biomes/soil types to fill in data gaps. We also are actively developing the best methods for comparing available radiocarbon data with model output from both ecosystem-scale and global-scale earth system models. To date, we have put together extensive data sets with radhioarbon measurements in both solid phase organic matter and respired CO2 at selected sites (Tropical to boreal) for comparison with ecosystem models. We have also published an initial comparison of data and model predictions for depth profiles of soil organic matter (Chen et al., 2018) and are preparing a second one that suggests what changes need to be made for improved model performance. Through collaborations begun with researchers who participated in our initial Radiocarbon short course, we are also filling in data gaps in the global database and working on the initial synthesis publications.

WP3 will test approaches to extrapolate from the few well-described WP2 sites to large regions and the globe. These depend on the underlying hypothesis that the age of respired and litter C will depend on vegetation and climate, while the age of C stored in soils will reflect mineralogy. To test and quantify these relationships, we will identify gradients to specifically test vegetation (woody versus nonwoody plants), climate, and mineralogy as controls of C dynamics.
So far, we have initiated collaborations where these gradients (parent material and climate/latitude) are being sampled and compared with models. The partner Instittion (UC Irvine) is taking the lead in producing a global data synthesis product for distribution, hopefully by the end of 2019/start of 2020.
Expected results:

The ISRaD soil radiocarbon database represents an ambitious and unique effort to have an open source, public data base. By requiring that all submitted databases have a doi, we are able to give credit to contributors; thus we hope that the database will be largely self-sustaining into the future beyond the 14Constraint project.

Use of the radiocarbon database to constrain global models. Here we have introduced the concept of transit time in addition to the radiocarbon in soil organic carbon as a constraint for both faster and slower C cycling mechanisms in soils. We have focused on comparisons with the models that include radiocarbon for the CMIP6 comparisons (E3SM from Lawrence Berkeley Lab and the COMMISSION model potential component of JSBACH). To date, we can demonstrate that models need to include a larger 'passive' soil component in order to come closer to the mean 14C content in soils, and have started the first comparison of global transit times.