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The age of C respired from ecosystems

Explore the age of carbon respired from ecosystems with radiocarbon dating, modeling comparisons, and implications for understanding carbon cycling dynamics. Discover the significance of heterotrophic and autotrophic sources in soil respiration and the potential biases in measurement methods.

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The age of C respired from ecosystems

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  1. The age of C respired from ecosystems Susan Trumbore, Jim Randerson, Claudia Czimczik, Nicole Nowinski (UC Irvine) Jeff Chambers (Tulane) Eric Davidson (Woods Hole Research Center) Ted Schuur (Univ. Florida) Simone Vieira, Plínio Camargo (Centro de Energía Nuclear na Agricultura, USP, Brazil)

  2. The age of C respired from ecosystems Photosynthesis Ecosystem respiration leaf Soil respiration Allocation Fire stem Root Respiration Litter and SOM decomposition storage root Microbial community Stabilized SOM Loss by leaching, erosion

  3. Why this might be interesting • Measure of the capacity for storage of C and interannual variability in C balance • Isodisequilibrium; improve 13C deconvolutions • Potential for direct comparison of data with models Deviation from mean C lost C stored C stored Time

  4. Radiocarbon of soil-respired CO2 provides a direct measure of isodisequilibrium “mean age” of several years up to a decade D14C D14C Atmosphere- north hemisphere from Levin and Hessheimer2000

  5. Soil respired CO2 is a mix of heterotrophic and autotrophic sources Heterotrophic values – from models CASA Empirical Local atmosphere (Autotrophic respiration) Data from Harvard Forest, MA

  6. Heterotrophic Respiration can be measured by putting litter and 0-5 cm soil cores in sealed jars, then measuring the rate of CO2 evolution and the isotopic signature of evolved CO2.

  7. Soil respired CO2 is a mix of heterotrophic and autotrophic sources CO2 evolved in incubation O horizon Flux weighted mean A horizon Local atmosphere (Autotrophic respiration) Data from Harvard Forest, MA

  8. DD14C = D14C of CO2 evolved in incubation - D14C of CO2 in atmosphere Boreal forest Temperate forest 3 age stands Manitoba* Harvard, Bear Brook Tropical forest Joshua Tree* Desert shrub Manaus, Santarem Latitude of site *see posters by Czimczik, Nowinski

  9. Flux-weighted total heterotrophic respiration for comparison with model Temperate forest Boreal forest Harvard, Bear Brook 3 age stands Manitoba* Tropical forest Joshua Tree* Desert shrub Manaus, Santarem Latitude

  10. Determine age of respired CO2 using pulse-response function for CASA Tropical forest Desert shrub Temperate forest CO2 respired Boreal forest Thompson,and Randerson, Global ChangeBiol., 1999. Years since pulse

  11. Mean age of heterotrophically respired C from CASA range from 10- 40 years Boreal forest Temperate forest Age of respired C (yr) Tropical forest Desert shrub Fraction of total respiration

  12. 14C from CASA X X Boreal forest DD14C Temp. forest Tropical forest Desert shrub Fraction of total respiration

  13. Model-data comparison initially disappointing at low latitudes ~ 4 years 20 years CASA Prediction Latitude

  14. Agreement improves if the oldest 25-50% of respired CO2 is removed 100% 75% 50% 25% Latitude

  15. Problems with incubations • Overemphasis of ‘young’ part of the respiration distribution • Exclusion of woody debris from soil sampling will bias against the longer ‘tail’ - Artifacts with incubation in general - Inclusion of roots in incubations emphasizes ‘young’ pools (variation with time)

  16. Potential issues with CASA Too long of a ‘tail’ - treats wood as a single pool with homogeneous turnover (do we need multiple pools to reflect different life strategies in tropical forest? Vieira et al. in revision PNAS) - model may allocate too much NPP to stem growth in tropical forests (stem allocation <1/3 of NPP)

  17. Observations Manaus ZF2 Total ecosystem respiration ~30 (3 – 8 yr) Total autotrophic respiration ~20 (0.01-1 yr) Total heterotrophic respiration ~10 (11-24 yr) Photosynthesis (~30) 3.3leaf litter (2-3 yr) 2.0 dead wood (50-100) yr Fluxes from Chambers et al. Units are MgC ha-1 yr-1 4.7Root/SOM (5-10 yr) Root respiration Dead wood excludes trees <10cm DBH

  18. Bias removes wood component Total ecosystem respiration ~30 (0.9-1.6 yr) Total autotrophic respiration ~20 (0.01-1 yr) Total heterotrophic respiration ~10 (4-7 yr) Photosynthesis (~30) 3.3leaf litter (2-3 yr) 0 dead wood (50-100) yr Fluxes from Chambers et al. Units are MgC ha-1 yr-1 6.7Root/SOM +wood (5-10 yr) Root respiration Dead wood excludes trees <10cm DBH

  19. CASA Total ecosystem respiration ~30 (7-24 yr) Total autotrophic respiration ~20 (0.1-1 yr) Total heterotrophic respiration ~10 (20-37 yr) Photosynthesis (~30) 3.3leaf litter (2-3 yr) 3.3 dead wood (50-100) yr Fluxes from Chambers et al. Units are MgC ha-1 yr-1 3.3Root/SOM (5-10 yr) Root respiration Dead wood excludes trees <10cm DBH

  20. Conclusions • Radiocarbon provides a measure of time elapsed between photosynthesis and respiration • The mean age of C respired from soils ranges from <1 – 20 years • Heterotrophically respired C has ages from 3-40 years • Comparisons of model with data show that either data are skewed ‘young’ or models are skewed ‘old’; largest problem in low latitude forests

  21. Santarém Manaus • Biomass C MgC/ha 180 141 • Growth MgC/ha/yr 1.6-2.1 2.2 -3.0 • MRT of C (yr)86-11247-64 • (stock/growth)** • Mean age of C (yr)* 260 220

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