Simulating carbon dynamics in arid ecosystems: considerations for processbased modelling

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Simulating carbon dynamics in arid ecosystems
considerations for process-based modelling

• Katherine A. Mitchell
• Jornada Experimental Range
• EPA funded project PIs Curtis Monger,
• Deb Peters and Jeff Herrick

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(1) The inclusion of woody plant encroachment
as a carbon storage pool alters sink estimates

• The apparent US sink without woody encroachment
  was 0.25 to 0.58 Pg C year while the sink
  including woody encroachment was 0.37 to 0.71
  Pg.
• S.W. Pacala in Science, 22 June 2001, vol 292
• Consistent Land and Atmosphere Based US Carbon
  Sink Estimates
• Increase in SOC as grassland transitions to
  shrubland
• Mg C/ha 8.7 Jornada, 1.4 Sevilleta (no woody
  roots gt 1 cm were included in these measures)
• R. B. Jackson in Nature, August 2002, vol 418
• Ecosystem Carbon Loss with Woody Plant Invasion
  of Grasslands

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Significant increase in belowground C due to
extensive woody root biomass of shrubs
Exposed mesquite roots in an arroyo
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Needed Quantify amount of root carbon,Model
decomposition of woody C at depth
Root system of a relatively small mesquite
(Prosopis glandulosa) shrub growing in a
coarse-loamy. Plant top is drawn at same scale
as roots. Root systems of some Chihuahuan Desert
plants Robert P. Gibbens James M. Lenz.
Journal of Arid Env (2001) 49 221263
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Century model results for SOC top 20 cmSimulated
soil C trajectory 1850 to today
Grassland module B. eriopoda
Savannah module P. glandulosa
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Conclusion Woody plant encroachment and the
role of coarse woody roots in the carbon cycle of
arid ecosystems

• Woody roots increase C storage in the slowly
  decomposing ie long turnover soil C pool
• The ratios of below groundabove ground biomass
  can be very very large and are relatively unknown
• Ecosystem process models attempting to provide C
  budget amounts must account for this large C
  pool, and must simulate the decomposition process
  of woody roots to depth

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(2) Importance of soil inorganic carbon calcium
carbonate (CaCO3)

• USDA Desert Soil Geomorphology Project
  carbonate carbon exceeds organic carbon about
  10-fold.
• Wtd mean organic C to 1 m depth 3.3 kg
  /m2
• Wrd mean carbonate C (pedogenic) 38.4 kg /
  m2
• Relationship of CaCO3 to atmospheric CO2
• Role as a sink. For example, DSP accumulation
  rate of 0.1 1.4 g CaCO3/m2/yr-1 (rate 1 -
  3 from Schlesinger)
• Role as a source to the atmosphere
• Biotic factors
• Biogenic carbonate
• effects of CaCO3 on plant and water dynamics

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Bowen Ratio tower data shows black grama
grassland as a net CO2 source

• Jornada BREB tower on black grama grassland (IBP
  site) collected 6 years of CO2/H2O flux
• Averaged for the entire measurement period
• Daily 1.6 g m2 d1 (/- 0.44)
• Monthly Positive flux in every month
• Annual 584 g m2 yr1 (/- 159.3) ( !! Net
  Source of CO2)

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BREB data results comparing two years with
contrasting patterns of CO2 flux CaCO3?
J J D
J J D
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Needed Modelling of calcium carbonate
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Conclusion Modelling soil inorganic carbon,
calcium carbonate, important component for
accurate C budgets in arid lands

• Address short term process of CaCO3
  precipitation/dissolution that occurs in periods
  of days, decades to centuries.
• Include biotic processes of CaCO3 formation
  plants role in CO2 production and H20 removal
  thru transpiration. Biogenic carbonate
• Include plant soil feedbacks through effects of
  carbonate on soil water

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Current Research Goals

• Examine thesis that carbon storage shifts from
  aboveground to belowground with shrub invasion of
  grasslands, and what effects this has on net
  ecosystem carbon.
• Working hypothesis Jornada has been a carbon
  sink for the later half of the 20th Century.

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Current Research Goals

• Arid lands have two equally important carbon
  cycles organic and inorganic. Develop a model
  that simulates calcium carbonate precipitation
  and dissolution and can integrate the effects of
  carbonates on soils, plants, and plant-soil
  interactions.
• Working hypotheses significant CO2 flux from
  carbonates and grasslands underlain by carbonates
  may be a net source of CO2 to the atmosphere

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Summary

• Considerable uncertainties remain as to the
  magnitude of CO2 sinks/sources in different
  regions of the world and North America.
• Recent, focus on regional C budgets, interest in
  western US
• Land use change woody plant encroachment
• Uncertainties based on the contribution of
  under-researched processes
• Calcium carbonate arid lands Source? Sink?
• Timely for ecosystem process models designed
  specifically for arid land to address these
  aspects of the carbon cycling science