Forest Carbon Cycle

Terrestrial carbon stocks above- and belowground (in humus and litter layers, woody debris, and mineral soil) are not only sensitive to physical environmental controls (e.g., temperature, precipitation, soil moisture) but also to land use history/management, disturbance, "quality" of carbon input (a reflection of plant carbon allocation and species controls), and the microbial community.

The relative importance of these controls on soil carbon storage and flux can be assessed with radiocarbon-based turnover times and SOM distribution for locations of interest. Information on changes in the rate of exchange between belowground pools are not otherwise discernible: net loss may be determined but the loss could be from a fast or slow turnover pool or the movement of carbon between pools. This information can be coupled with other observations to understand and model the mechanisms for soil organic matter (SOM) protection and the transformation of recently fixed labile carbon into more stable carbon that is sequestered in the biosphere.

Schematic of the belowground movement and sources/reservoirs of carbon.
Figure 1. Schematic of the belowground movement and sources/reservoirs of carbon.

In the context of terrestrial carbon cycle studies, radiocarbon measurements can be used to determine the 'age' and rate of change of carbon stocks or as a biogeochemical tracer to elucidate processes and pathways. It is this dual nature that can be exploited across scales (individual plant, microbial community, plot or research site, ecosystem, regional, and global) and time (days to millennia). Globally, more carbon is stored in soils as soil organic matter than in terrestrial vegetation and the atmosphere combined [e.g.,]. Soil is the largest single reservoir for carbon stored in the terrestrial system; nearly three quarters of all carbon in the terrestrial reservoir resides in the soil carbon pool including peat and permafrost stores. Soil organic matter is heterogeneous, consisting of components that turn over on timescales ranging from days to thousands of years with input from litter, roots, and other allochtonous detrital components. These sub-pools can be physically separated in the laboratory to determine the distribution of soil carbon amongst labile, intermediate, and stable pools. Radiocarbon measurements for these sub-pools coupled with carbon stock estimates allow for the calculation of soil organic matter turnover times, which also yield estimates of carbon loss. Differences in SOM distribution or turnover times amongst sites or over time can indicate factors controlling SOM storage, stabilization, and loss.

Net (biome or ecosystem) uptake of carbon is the difference of two large fluxes: photosynthesis and respiration. Carbon fixation by photosynthesis is, to a large extent, a single process with theoretical underpinnings. On the other-hand, net ecosystem or biome respiration integrates microbial (heterotrophic) and plant (autotrophic) respiration. Eddy covariance methods can be used to estimate bulk CO2 fluxes but they cannot discriminate the process nor the source of the respired CO2. It is these processes that are parameterized in predictive models and contribute to the uncertainty in the climate forcing effect of the carbon cycle in the future. Across regional scales, Δ14C measurements of atmosphere CO2 can be used to attribute carbon dioxide to sources (e.g., respiration vs. fossil fuel emissions) or sinks (e.g,. photosynthesis), which cannot be readily inferred from concentration, net flux measurements, or δ13CO2. At smaller scales, similar analyses can be used to elucidate the source, and 'age' of the below ground component undergoing heterotrophic respiration.

Because 14C is much more rare than 13C (~1:1010) radiocarbon can be used as an isotopic tracer in labeling studies using much lower levels of label than the rare stable isotope (13C, which is ~1:100 relative to 12C). This allows for the addition of very low amounts of added carbon and for the ability to measure the added label over a wider range of time scales compared to stable isotope labeling techniques which are rapidly diluted. Low-level 14C pulse-chase experiments provide valuable information on carbon cycling patterns across temporal scales ranging from a few hours to months, shorter timescales than can be investigated using carbon stock measurements. This type of experiment has been used to study photosynthate allocation patterns and determine mean residence time for plant photosynthate in plants, to partition ecosystem respiration sources (e.g., plants vs. soil heterotrophs, above- vs. belowground plant tissue), and to investigate soil microbial food webs, specifically consumption and subsequent respiration of labile soil organic matter compounds by saprotrophic (decomposer) fungi.

14C as an ecosystem level carbon cycle tracer was utilized by the Enriched Background Isotope Study: a collaborative national multi-lab and university project. In this study, an inadvertent release of 14C effectively labeled a portion of the Oak Ridge reservation in Tennessee; a semi-deciduous hardwood forest. Labeled litter was collected, and applied to explicitly track above ground (leaf litter) carbon through the soil and incorporation into mineral bound carbon. Through the application of labeled litter to non-labeled sites at the reservation, the belowground (root) input was able to be distinguished. A key finding of this study was that over the 6-year duration of the experiment the vast majority of below-ground carbon was directly sourced from root input (fine roots, root exudates) and not from leaf litter. Effectively, the litter layer was remote from the below ground carbon stocks and was subject to significant loss through heterotrophic decomposition and export as dissolved organic carbon.

To further elucidate the climatic, edaphic, and biological controls on the rate of carbon flux from litter sources to mineral soil sinks, beginning in 2007 labeled litter was applied to a series of plots at four Ameriflux sites that span the climatic extent of the eastern deciduous hardwood forests. Hypotheses that are being explicitly tested include:

  • H1 – Carbon sources from leaf-litter, surface humus, or fine-root organic sources will exhibit distinctly different accumulation rates into distinct mineral soil C pools.
  • H2 – Carbon transport from organic to mineral soil carbon pools will be slower in colder and/or drier environments.
  • H3 – The carbon from litter of constant composition will be apportioned differently among (1) losses via CO2 and DOC, (2) incorporation into the microbes and microbial byproducts, and (3) persistence as undecomposed material in AmeriFlux sites with different climates and microbial community composition.
  • H4 – Variation in the population densities of macrobiota (esp. earthworms and millipedes) will impact the rate of surface to mineral soil incorporation across the eastern United States.