Will changes in soil organic carbon act as a positive or negative feedback on

global warming?By: Miko U.F. Kirschbaum

Christie LightfritzAtmospheric Sciences

‘Lil Bit ‘O Background• 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

What are we really trying to do here?• 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 turn-over, or death of plants which allows for the formation of SOC.

• Carbon is returned to atmosphere by decomposition of SOC

Equations, duhInstead 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: because we can assume steady-state.

• Then, after taking derivative with respect to temperature, equation 2 simplifies finally to:

If d(O/k) dT

< 0, there is positive feedback in the global carbon cycle

Oh, how scientists love to use their models

• 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

4 approaches• 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

Estimates due to different approaches

• 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

So what were the results of all this?• 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

More results…

Guess what?....More results• 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 from20⁰C down to 0⁰C

• Findings were consistent to morerecent study of Burke in same area

• An equation Jenny used basicallysays that over the area, the temp sensitivity of SOC decomposition exceeds that of NPP

….and More• 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

Sorry, not done yet…• 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 describedunless the interaction with moisture is explicitly included

…..

You would think this would be common sense…

• 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

Laboratory Incubation• 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.

Summarizing all of that craziness we just went over

• 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 least-biased 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

& more summary…• 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

Just some images(1)

(2)

So what does this even say about global warming?

• 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.

My 2 Cent…..• 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.

…..not 50 Cent

AnyQuestions?