Will changes in soil organic carbon act as
a positive or negative feedback on
global warming?
By: Miko U.F. Kirschbaum
Christie Lightfritz
Atmospheric Sciences
•So what the heck is Soil Organic Carbon?
•The Carbon occurring in soil due to soil organic matter
•What are soil organic matter then?
•The organic constituents in the soil, due to dead plants and
animals, which play a key roll in decomposition
•Carbonate
•a neutral ionic compound of the inorganic compound,
carbonic acid
•Carbonates might be lost with climate change if currently dry
environments were to become wetter
•If Greenhouse gas induced climate change caused even small
changes in organic carbon, there would be significant feedback on
Greenhouse gases in the atmosphere
•A change by just 10% in soil organic carbon would be
equivalent to all the anthropogenic CO2 emitted over 30 years
•Figure out if decomposition rate or net primary production (NPP) are more
responsive to increasing temperature
•Whether changes in soil organic carbon would act as a positive or negative
feedback on climate change based on temperature increases
•If soil organic carbon increased with warming, there would be a negative
feedback. Decreasing organic carbon with warming, would have a positive
feedback by further adding to the build-up of CO2 in the atmosphere
•Changes in soil organic carbon have a positive or negative feedback depending on
the relative temperature dependencies of NPP and decomposition rates
•Carbon enters through photosynthesis, the net
gain of Carbon is NPP.
•Carbon is transferred to soil via litter, root turnover, or death of plants which allows for the
formation of SOC.
•Carbon is returned to atmosphere by
decomposition of SOC
Instead of explaining what every little detail is (because it would take
me forever) I’ll summarize it:
•You start off with this equation:
•Which simplifies to this:
state.
because we can assume steady-
•Then, after taking derivative with respect to temperature, equation 2
simplifies finally to:
If d(O/k) < 0, there is positive feedback in the global carbon cycle
dT
•Experiments and modeling can be done to identify the
temperature dependence of organic carbon decomposition
•The rest of the paper reviews multiple experiments to try to determine the
temperature dependence of organic carbon decomposition
•Ultimately, their goal was to figure out if decomposition of soil organic
carbon would positively or negatively affect global warming
•1st Approach: Based on biomes at the Last Glacial Maximum. To get the total
of carbon storage, the carbon stored by each biome was multiplied by the
area covered by that biome
•Prentice & Fung, King & Neilson, Smith, Friedlingstein, Prentice, van
Campo, Cramer & Solomon, Solomon, Neilson, Prentice & Sykes
•2nd Approach: Uses palaeoecological (fossil animals and plants) information
to infer the distribution of vegetation types
•Adams, Crowley
•3rd Approach: Modeled the response of carbon fixation and decomposition
rates to carbon dioxide concentration and temperature for each biome, then
they summed this over all different biomes
•King
•4th Approach: Constrained the total carbon by the observed
changes in carbon isotopes in the oceans and atmosphere
•Crowley, Bird
•All studies estimate an increase
in terrestrial carbon storage from
the Last Glacial Maximum to
present
•The top is total carbon and the
bottom is SOC only
•Adams
•Had the largest increase in carbon storage
•Used the most thorough data for changing vegetation types, but used
rather extreme carbon densities
• Crowley
•Repeated Adam’s analysis, but re-evaluated it and used better
palaeovegetation distribution
•Resulted in a slightly lower estimate of carbon storage
•Estimated greater carbon gain than those experiments that used the
carbon isotope record that modeled the distribution of biomes
•Prentice & Fung, King & Neilson, Cramer & Solomon, Neilson, Prentice & Sykes
•For a 2xCO2 future, most modeling studies indicated little change
•There was a slight increase in total carbon storage, but a slight loss of soil
carbon
•The fact that many people got the same estimates was largely because
they were all based on simulation altered biome distributions
•Only King based the analysis on a ecosystem that responded to climatic
factors, but they messed up because they didn’t include vegetation
•Prentice & Fung, Friedlingstein, Van Campo, Prentice, Cramer & Solomon,
Prentice & Sykes, and Adams
•Soil Carbon storage increased, although it is based on weaker info
•Changes in soil carbon were only estimated through modeling or
through guessing the distribution of vegetation
•Assumed that there was no change in soil carbon storage for each
biome type
•The land area at the Last Glacial Max is assumed to be similar to its’
present area
•Kern & Schlesinger, Schlesinger
•There were more cool deserts at the Last Glacial Max
•The effect of deserts on total terrestrial carbon is GREATLY affected
by including or excluding carbonates.
•In deserts, most carbon is in inorganic form which exceeds the
amount stored in organic carbon
•Past studies have not included inorganic carbon
•This may account for the differences between isotope-based
analyses and studies based on the distribution of biomes
•Harradine & Jenny, Jenny, Burke, Carter
•Reported a strong effect of temperature on SOC or Nitrogen – decreasing
amounts with decreasing temp
•Jenny
•Covered the Great Plains
•Data
• For constant moisture,
there was a large increase in soil
organic nitrogen for temps from
20⁰C down to 0⁰C
•Findings were consistent to more
recent study of Burke in same area
•An equation Jenny used basically
says that over the area, the temp
sensitivity of SOC decomposition exceeds that of NPP
•Post
•Analyzed global patterns in SOC and nitrogen
•Represented findings via Holdridge diagrams
•Amounts of SOC were expressed as
functions of mean annual temp and the
ratio of potential evapotranspiration to
precipitation
•For a constant ratio of potential
evapotranspiration to precipitation, data
indicated a trend of decreasing SOC with
increasing temp
•However, trends were weak, biomes
with intermediate moisture levels were
inconsistent
•Unlike studies focusing on a specific small area, global studies implied that
organic carbon decomposition has marginally greater temp sensitivity than NPP
•But, it is still difficult to draw firm conclusions because solar radiation and
vegetation types varied with temp as well
•O’Brien & Stout, Anderson & Paul, Jenkinson
•The relative abundance of the stable carbon isotopes, 12C and13C, and the
radioactive isotope 14C can be used to infer turn-over rates of carbon in the soil
•The atmospheric abundance of 13C is decreasing due to the dilution with 12C from
the burning of fossil fuels
•Used that trend to infer the turn-over times of soil organic carbon components,
but the observed trend was weak and affected by other factors so it could not be
used to relate temperature and soil organic carbon turn-over times
•14C is better to use.
•14C concentration in total organic carbon can be used to infer its turn-over time.
•Trumbore, Townsend
•Used the previous info to infer turn-over
times and express it as a function of temp
•This means, warmer sites generally have
lower moisture levels.
•Hence, the warmer temperatures might
have been partly negated by the effect of
reduced soil moisture
•Keith
•Working in Australia, measured soil respiration over one year and
expressed measured fluxes as a function of temperature and moisture.
•The data in the figure shows as a function of temperature both at the
observed field moisture levels (Figure (a)) and as rates corrected for
moisture (Figure (b)).
•Measurements at higher temperatures are typically associated with lower
moisture which tends to reduce respiration rates.
•Data corrected for the effect of moisture
showed stronger temperature dependence
•Temperature dependence of soil
respiration cannot be well described
unless the interaction with moisture
is explicitly included
•The most direct way to measure the effect of warming on soil processes is by
experimentally heating patches of soil under otherwise normal field conditions
•Lükewille & Wright, Peterjohn, McHale, and Rustad and Fernandez did this
•Other studies are being done but results aren’t available yet
•The studies that have enough detail were compared and represented in the
figure
•The figure shows the soil respiration per degree warming at different
background temperatures
•But of course, there are problems
•If the decomposition of organic
carbon and litter is stimulated by
warming, it gets rid of its own
supply of readily decomposable
material
•So, relative stimulation of soil
respiration diminishes over time
as the supply of substrate is lost
•Basically, people have no idea
how to interpret and generalize
the findings
•The previous methods can’t directly assess the effect of temperature on
organic carbon decomposition without the effects of changing soil moisture,
litter quantity and quality, or root respiration.
•These effects can be minimized by experimenting under controlled
laboratory incubations if moisture levels are controlled and respiration from
any roots can be prevented
•The down side: Laboratory incubations are conducted under artificial
conditions and generally exclude important interacting Factors including
wetting/drying cycles
•Kirschbaum
•Summarized the temperature
sensitivity of organic carbon
decomposition obtained in various
laboratory incubation studies
•Very high temperature sensitivities
have been reported at low
temperatures, and moderately high
values were maintained at higher
temperatures.
•All the evidence above gives different estimates of the temperature sensitivity of soil
organic carbon decomposition (a)
•The temp sensitivity obtained under lab
conditions is considered to give the leastbiased estimation of the true temperature
dependence of organic carbon decomposition
•(b) compares the temp dependence of NPP
with the temp dependence of SOC decomp
obtained from lab incubations
•NPP has a weaker temp dependence than
decomp rates at all temps, with the largest
difference at lowest temperatures.
•This suggests that with increasing temp,
SOC decomp should be stimulated more
than NPP. This is consistent with the
observation that total amounts of SOC
generally decrease with increasing temp
•The figure shows the % carbon loss per
degree warming for soils at different
initial temps
•The figure indicates the potential for
large losses of organic carbon with
warming.
•As soils in cool regions tend to have
higher soil carbon contents, a given % of
carbon loss corresponds to a higher
absolute loss in cooler regions
•Hence, soil carbon losses alone could
provide a powerful positive feedback
on global warming
(1)
(2)
•All experiments agreed by indicating a strong temperature sensitivity that greatly
exceeded that of NPP.
•The strongest evidence came from lab incubations which most factors could be excluded.
•Resulted in highest temp sensitivity
•Soil warming experiments, field measurements of soil respiration ,and inference of organic
carbon turn-over times from carbon isotope ratios, gave similar results
•Resulted in, however, lower temperature sensitivity
•With the temp sensitivity of organic carbon decomposition being significantly greater than
NPP, changes in soil carbon could result in a significant positive feedback.
•However, because of negative feedback within the plant-soil system, these changes
are likely to be slow, requiring many centuries before approaching a new equilibrium
•Also, because of the additional effect of CO2 concentration on plant production and soil
organic carbon storage, the overall feed-back from SOC on the atmosphere is likely to be
small.
•This paper was very confusing
•They negated almost everything they tried to convey
•They needed to dumb it down a bit. Unless you have a lot background on the
topic, it will not make much sense.
•King’s experiment would have been the most accurate had he used vegetation
•I honestly think that they should stop doing experiments until the models are
redesigned or updated. They do not seem to be getting anywhere.