Global Warming’s Effect on Soil Carbon Microbial DecompositionTABLE OF CONTENTS   Page NumberSUMMARY           1-2KEYWORDS (climate change, carbon cycling, Arrhenius equation, organic   decomposition, thermal adaptation, microbial soil respiration)INTRODUCTION         2-3SOIL-CARBON DECOMPOSTION      3-5 Factors of organic decomposition      3-4 Formulas involved in decomposition rates     4-5PERMAFROST         5-8 Subarctic soil microbial biomass      6-7 Carbon exchange and release from thawing permafrost   7-8MICROBIAL SOIL RESPIRATION      8-11 Thermal adaptation        9-10 Environmental soil classes       10-11CONCLUSION         11-13REFERENCES         13[Cite your source here.]Summary In theory if the base formulas of microbial decomposition were the only elements at play, climate change and microbial decomposition would be involved in a global positive feedback loop.  With climate change increasing temperature, decomposition rates will also increase and in turn provide additional CO2 that will perpetuate climate change.  Terrestrial organic carbon being such a large portion of the global carbon supply, any increase in current global decomposition rates could have a profound effect.  There are many other environmental components involved that prevent this from being quite as large of a positive feedback loop as theoretically possible.IntroductionClimate change has been a topic on people’s tongues for years.  From the factors affecting climate change, to the effect’s climate change has on the Earth.  There is an incredible amount of research being done on the subject.  Aboveground climate change effects have become more well known, while underground parts of the ecosystem have been neglected.  The main effect terrestrial organisms can have on climate change is through carbon cycling.  Soil flora reduce atmospheric carbon, while root respiration and microbe aided decomposition adds to atmospheric carbon (1).The processes involved in plants participation of organic carbon cycling is fairly well understood.   Soil-carbon decomposition aided by microbes has many factors that are starting to be researched more heavily.  A core aspect of decomposition that is uniquely interesting to climate change is its sensitivity to increases in temperature (1,2).  In theory if the rate of decomposition increases with increases in temperature; then a positive feedback loop may occur as the CO2 produced through decomposition will help raise earth’s temperature, in turn raising decomposition rates (3).  Studies have shown however that this increased rate of carbon release into the atmosphere reduces back to normal levels after several years (4).  Measuring the effect different processes have on the climate is already difficult due to the duration it takes to see real changes.  As we learn more about decomposition’s place in this process, more questions get raised at the same time.Soil-Carbon Decomposition In the global organic carbon supply, a large percentage of it exists in the world’s wetlands, peatlands, permafrost, and upland soil (1).  There is a significant amount more organic carbon in the world’s soil than in the world’s atmosphere.  Soil contains several thousand various organic carbon compounds in a variety of combinations (1).  Different types of soil contain various amounts of the global carbon as seen in Table 1 (1).  These carbons compounds decompose as part of the global carbon cycle, thus playing a part in climate change.  Due to differences in soil conditions upland soil has more involved decomposition; while wetlands, peatlands, and permafrost have lower rates of decomposition leading to higher carbon densities (1).  Warming temperatures can lead to increases in decomposition rates, affecting permafrost and peatlands due to their higher latitudes (1). There are many factors at play with decomposition rates.Factors of organic decomposition Decomposition of organic carbon is a complex process involving many different factors.  Much of the carbon in the decomposition cycle is imputed by leaf and root detritus, during decomposition CO2 and CH4 is outputted (1).  While root respiration makes much more CO2, microbial decomposition of organic matter is a temperature dependent process that produces a large amount of global CO2 (1,3,4). At the core of microbial decomposition of organic carbon is the microbe’s ability to get past the activation energy needed to decompose the various organic carbon molecules.  The process is temperature dependent as an increase in temperature provides more energy needed to get past the activation energy.  The activation energy is related to the molecular structure of the organic carbon and its ambient temperature (1).  Decomposition is temperature sensitive, as molecular complexity of organic carbon increases the temperature sensitivity of decomposition will also increase (1).  Activation energy becomes greater as molecular complexity increases, making the process of decomposition more sensitive to temperature changes (1).Formulas involved in decomposition rates-Davidson There are several formulas involved in measuring the rate changes of biological systems.  With decomposition being temperature sensitive, the Arrhenius equation is used to find the relative reaction rate.  The Arrhenius equation says changes in relative reaction rates is a function of temperature (1).   Relative rate of decomposition of soil organic matter with high activation energy might be sensitive to temperature, but changes in absolute rate will most likely be hard to detect (1).  The basis of the Arrhenius equation was used to form the formula for Q10 to find the absolute rate.k=Ae(-Ea/RT)Arrhenius equationk is the reaction rate constantA is the frequency factor (theoretical reaction rate constant in the absence of activation energy)Ea is the required activation energyR is the gas constant (8.314 J K-1mol-1)T is temperature in kelvinsQ10 is the temperature coefficientR is the rateT is the temperature in kelvins  Q10 is a necessary factor to use when evaluating rate changes.  Q10 is the factor that reaction rates increase with a 10° rise in temperature.  For most systems the Q10 is 2-3, and according to Davidson’s article “A rule of thumb widely accepted in the biological research community is that the rate of decomposition of SOM, like any other biological reaction rate, tends to double for every 10o rise in temperature” (1). According to the Arrhenius equation, the Q10 should decrease as the temperature increases, which is shown in the top half of Figure #.  The theory for why this happens is the lack of molecules that need the higher temperature to react (1).  There is a visible increase in the slant of the data in Figure 1, showing that as activation energy increases the sensitivity to temperature also increases.Permafrost There has been a marked trend over the past century of warming throughout artic regions (5).  The Arctic Climate Impact Assessment (ACIA) projects 4-7°C temperature increase by year 2100 (5).  This warming is projected to promote soil organic matter (SOM) decomposition.  With permafrost containing a large concentration of organic carbon, this projected accelerated decomposition could have profound effects on the earth.  Thawing permafrost shows most potential for positive climate feedback.Subarctic soil microbial biomass There was a long-term study done in a subarctic heath ecosystem of northern Sweden where they collected data for 15 years since 1989 (5).  They manipulated the environment to simulate potential factors of climate change.  Passive open-top greenhouses to simulate predicted warming, NPK fertilization to simulate increased nutrient availability, and shading cloths to simulate increased cloudiness (5).  A variety of above and belowground reactions including plant species abundances, plant biomass, plant and soil nutrient content and C pools, microbial biomass, fluxes of CO2 and CH4 were studied (5).  There was evidence that microbial biomass was more responsive to addition of carbon than fertilization, showing that carbon is potentially a limitation of microorganisms. As shown in Figure 2, there was statistically significant drop in soil carbon at depths 5 and 10 cm after being exposed to warming over 15 years.  Showing evidence that as temperatures rise, so does the rate of decomposition.  Shading also had a slight affect on decomposition, but much less statistically significant.  The affect of warming wasn’t notable until at least a decade had passed.As one could expect, fertilization had a large impact on the carbon quantity of the top soil, with some effect on the deeper soil.  Adding fertilizer will obviously greatly increase the total nutrient amount if more nutrients than the microbes can physically process.  This large increase was then counteracted by both warming and shading.  With the increase of decomposition rates due to the warmer temperatures, a higher percentage of the added nutrients will be decomposed.  On the top soil there was a larger decrease in carbon in the fertilized soil vs the control group.  This may be due to nutrition being a limiting factor in the reaction. It can be inferred that the activation energy of the added carbon sources must be higher than the original carbon as the additional nutrients require warming to be decomposed.  Both nutrient and temperature increases will increase the rate of decomposition.   Although there was no data collected on CO2 production from decomposition, it can be assumed that with an increase in decomposition rates an increase in CO2 production will follow.Carbon exchange and release from thawing permafrost Permafrost soil is a large piece of relatively untouched organic carbon storage.  The total carbon storage of the world’s permafrost is twice what the world’s atmosphere contains (6).  To study the carbon exchange and release from thawing permafrost, a team measured an Alaskan tundra site over several years.Figure 3 is interesting in that it provides information that at first glance might appear to contradict previous theories.  In reality it actually helps confirm the theory that global warming is increasing decomposition rates.  Since decomposition increases with a temperature increase, one might be confused by the loss of atmospheric carbon during the summer months.  But the greatly increased growth rates during the summer counteract the increased decomposition rates.  While in the winter months there is near zero growth, but still warm enough for microbial reactions to occur.  During warmer years, as identified by the dark bar showing an extensive thaw, there is larger amounts of atmospheric carbon intake.  This trait is also evident during the early and late months of the growing season.  Showing that it takes a much higher temperature for plants to begin growth, but warm enough for microbes to thrive.  Microbes do especially well during the warmer years, plants however do better during the moderate years.  This carbon exchange battle between plants and microbes shows the potential negative and positive feedbacks that will affect the earth.Microbial Soil Respiration Microorganisms’ decomposition of soil organic carbon produces atmospheric CO2 through respiration.  The decomposition rates of soil organic carbon in the short term are dependent on temperature (4).  If this temperature dependence was extended to the long term, the decomposition rates would keep increasing as world temperatures increased.  Field experiments have shown that respiration rates enduring soil warming. return to normal after a few years (4).  Two potential reasons for this rate return is substrate-depletion of carbon supply, or the microbes adapt to increased temperatures with thermal adaptation (4).  There is obviously reason to believe that the carbon supply might be depleting faster than it can replenish itself, which would decrease the reaction rates.  To properly test thermal adaptation without substrate-depletion being a factor, an excess of substrate can be added to the experiments.Thermal Adaptation The changes in respiration that occur due to temperature changes in microbes is defined by thermal adaptation.  The term “thermal adaptation” could be used describe responses organisms show, from instantaneous temperature compensation to mutation selection.  Bradford defines “thermal adaptation as a decrease in heterotrophic soil respiration rates per unit microbial biomass in response to sustained increase in temperature” (4).  The same also applies to extended decreases in temperature.  He normalizes for biomass because temperature change responses affect mass specific respiration rates (Rmass).  Responses to instantaneous temperature changes don’t affect Rmass,In Figure 4, three thermal adaptations of Rmass are graphically described.  Type I adaptation shows soils adapted to higher temperatures, which exhibit a decrease in the Q10 of Rmass. In the heated soils here is no change in Rmass at low temperatures, and lower Rmass at intermediate and high temperatures.  Type II adaptation has lower Rmass values at every temperature, with no change in Q10.  Type III adaptation depicts cold adapted microbes being heated and thermal adaptation increases the optimal temperature for Rmass.  It is possible for all of the above adaptations to occur at the same time in one soil microbial community (4).Environmental Soil Classes While most models for carbon decomposition use a fixed Q10 value, it is known to vary region to region.  This is impart due to different environmental soil classes (ESCs).  It is accepted that Q10 decreases as regions become closer to the equator; soil moisture also impacts soil respirations sensitivity to temperature. (2)In Figure 5 the Q10 was on average lowered by 5% when water holding capacity (WHC) was increased from 30% to 75%.  There is also evidence that the soils pH value regulates the effect soil moisture has on Q10 (2).  Q10 greatly increased with soil moisture when in high pH soil, while the opposite occurred to Q10 when in low pH soil (2).  It can also be noted that soil moistures effect on Q10 is more noticeable when moving from very dry soil to intermediate soil moisture levels (2).  This may be in part due to the near lack of moisture being a very limiting factor to decomposition.  Although there is apparent variability within individual environmental soil classes, there is obvious trends between the different classes.  This may be explained by pH or available organic carbon.Conclusion It is common knowledge that CO2 plays a large part in climate change.  Organic carbon is constantly being moved between the atmosphere and the earth.  A natural source of this atmospheric organic carbon comes from carbon cycling, and more specifically the decomposition of soil organic carbon step in carbon cycling.  Carbon cycling is crucial aspect of climate change, in theory both can impact the other due to decomposition’s sensitivity to increased temperature and its production of CO2. Decomposition rates is proven to increase with increased temperatures, this is in part to increasing temperature being the simplest way to overcome the activation energy of more complex carbon compounds.  Doing this increases the amount of organic carbon available for the reaction.  Q10 is the value used to describe how much the rate increases with a temperature increase.  Different sources of research have looked into the various factors that also have an effect on Q10 besides just temperature. One of the classic examples of how as the earth warms it increases the amount of organic carbon to cycle into the atmosphere is the earths permafrost.  What makes permafrost unique from other warming regions is not only are complex carbons having their activation energy achieved, but due to the thawing of the soil simple carbons become available for decomposition.  These simpler carbons, with low activation energies, tend to exist in larger quantities providing the process of decomposition with much more substrate per degree change; as compared to regions fighting for the energy to react with more complex organic carbon.  Permafrost’s carbon exchange operates at such low levels that its decomposition is in a constant tug of war with the plant’s removal of organic carbon from the atmosphere. At its core, decomposition rate is affected by temperature fluctuations.  Several studies have shown that there are additional factors that impact decompositions sensitivity to temperature, including thermal adaptation and different environmental soil classes.  Although decomposition rates increase with a temperature increase, field studies show that decomposition rates return to normal after several years of increased temperature.  This may be due to substrate depletion and the thermal adaptation of the microbes.  Studies showed that when substrate depletion is removed as a cause, decomposition rates still even out due to thermal adaptation.  The Q10 and mass specific respiration rates are affected differently by temperature change depending on what temperature it is currently adapted to. The region in which the soil exists also impacts how Q10 changes with temperature changes.  This is due to differences in soil organic carbon composition, soil moisture levels, and pH levels.  Soil organic carbon composition is a simple factor as reaction rates increase with more available substrate.  If there aren’t any carbons with activation energy within the temperature change range available for the reaction then an increased temperature won’t increase the decomposition rate.  Increasing soil moisture is shown to slightly decrease Q10; this effect is stronger when in a low pH, with the opposite happening in high pH soil. Microbial decomposition is shown to be a highly complicated process, that is part of an even more complicated global system.  Although the reaction of decomposition is similar across the world, no region is affected similarly by the multitude of factors at play.  There is a possibility that microbial decomposition could be involved in a massive global positive feedback loop.  Much more research and long-term data accumulation must occur before a more concise answer can be given on that theory.ReferencesDavidson, E. A. and I. A. Janssens (2006). “Temperature sensitivity of soil carbon decomposition and feedbacks to climate change.” Nature 440: 165.Meyer, N., et al. (2018). “The Temperature Sensitivity (Q10) of Soil Respiration: Controlling Factors and Spatial Prediction at Regional Scale Based on Environmental Soil Classes.” Global Biogeochemical Cycles 32(2): 306-323.Allison, S. D., et al. (2010). “Soil-carbon response to warming dependent on microbial physiology.” Nature Geoscience 3: 336.Bradford, M. A., et al. (2008). “Thermal adaptation of soil microbial respiration to elevated temperature.” Ecology Letters 11(12): 1316-1327.Rinnan, R., et al. (2006). “Fifteen years of climate change manipulations alter soil microbial communities in a subarctic heath ecosystem.” Global Change Biology 13(1): 28-39.Schuur, E. A. G., et al. (2009). “The effect of permafrost thaw on old carbon release and net carbon exchange from tundra.” Nature 459: 556.