The average global surface temperature is predictedto increase by between 1.1 and 6.4 °C by 2100 (REF. 21),and this might also have an effect on soil carbon seques-tration by potentially accelerating heterotrophic micro-bial activity. The sensitivity of stable and labile fractionsof soil organic carbon to temperature change is thoughtto vary greatly. For example, increased thaw rates anddepths in high-latitude permafrost render the large stocksof organic carbon in these soils (400 Petagrams (Pg);that is, 4,000 million tonnes) vulnerable to increaseddecomposition rates48 . Without the balancing effect oforganic carbon input from above-ground primary pro-duction, this could result in a large and uncontrollablepositive-feedback effect 49.Overall, increased temperature has been directlylinked to increased soil respiration, and a global averagetemperature increase of 2 °C is predicted to increase soilcarbon release by 10 Pg, mainly owing to increases inmicrobial activity 50–52. This is thought to be because theincreased temperature will stimulate the use of labile car-bon; however, recalcitrant carbon is diverse and complexin structure, so its temperature sensitivity is uncertain.This scenario is further complicated by the role of envi-ronmental constraints in organic carbon decomposi-tion, including physical and chemical protection againstenzymatic activity, and the impact of drought, floods andtemperature on enzymatic activity and on the availabilityof oxygen52. moreover, these environmental constraintsare themselves affected by climate change. Therefore,predicting the effect of temperature increases on carbonstock has been difficult. In some cases, increased tem-perature may lead to a loss of soil organic carbon, espe-cially in temperate ecosystems53,54. Indeed, a recent studyshows that even subtle warming (by approximately 1 °C)can increase the ecosystem respiration rates in a subarc-tic peatland, particularly in the subsurface layers55. Thisis indicative of a large and long-lasting positive feedbackof the organic carbon stored in northern peatlands to theglobal climate system, although the mechanism of thisresponse remains unclear56.because different microbial groups have distinctoptimal temperature ranges for growth and activity,increased temperature can affect the composition ofthe microbial community, which in some cases couldreduce the release of soil organic carbon owing to theloss of acclimatized microbial groups 57 . For example,an increase in temperature in a high-latitude ecosystemresulted in an up to 50% decrease in bacterial and fun-gal abundance and soil respiration, as well as a phylo-genetic shift in the fungal community 58, suggesting thatincreased temperature does not always lead to enhancedcarbon loss to the atmosphere. To complicate mattersfurther, these changes in respiration could be causedby shifts in the composition and activities of microbialcommunities or by changes in the quality and quantity ofsoil organic carbon59,60. specifically, there is evidence thatwarming of soils leads to a decreased relative abundanceof fungi and to changes in bacterial community structurein arctic ecosystems 61 , but the long-term reduction insoil respiration due to warming could also be caused bythe sequential removal of easily decomposable organiccarbon that results from an initial stimulation of decom-position. It is also possible that some soil organic carbonis physically and chemically protected from microbialdecomposition59,62. because there are so many variables,the estimation of carbon loss by climate change is unre-liable 63 , and reducing this uncertainty will be a majoradvancement.Another key determinant of the terrestrial microbialcommunity structure and the decomposition rate of soilorganic carbon is soil moisture, which will be affectedby the 20% increase or decrease in precipitation ratethat has been predicted by the Intergovernmental Panelon Climate Change 21 . microbial communities respondto moisture levels directly, because they require waterfor physiological activities, and indirectly, owing to theeffect of changing soil moisture on gas diffusion ratesand oxygen availability. The effect of changing precipita-tion on the feedback responses of soil microorganismsto climate change may therefore be due to the directeffect on microbial physiology and community struc-ture. long periods of drier conditions may limit micro-bial growth and decomposition 64 and may consequentlyhave a negative-feedback effect on carbon fluxes in someecosystems. However, soil drying may increase oxygenavailability and enhance carbon cycling in wetlands andpeatlands, thereby having a positive-feedback effect onCO2 fluxes65
Question
The average global surface temperature is predictedto increase by between 1.1 and 6.4 °C by 2100 (REF. 21),and this might also have an effect on soil carbon seques-tration by potentially accelerating heterotrophic micro-bial activity. The sensitivity of stable and labile fractionsof soil organic carbon to temperature change is thoughtto vary greatly. For example, increased thaw rates anddepths in high-latitude permafrost render the large stocksof organic carbon in these soils (400 Petagrams (Pg);that is, 4,000 million tonnes) vulnerable to increaseddecomposition rates48 . Without the balancing effect oforganic carbon input from above-ground primary pro-duction, this could result in a large and uncontrollablepositive-feedback effect 49.Overall, increased temperature has been directlylinked to increased soil respiration, and a global averagetemperature increase of 2 °C is predicted to increase soilcarbon release by 10 Pg, mainly owing to increases inmicrobial activity 50–52. This is thought to be because theincreased temperature will stimulate the use of labile car-bon; however, recalcitrant carbon is diverse and complexin structure, so its temperature sensitivity is uncertain.This scenario is further complicated by the role of envi-ronmental constraints in organic carbon decomposi-tion, including physical and chemical protection againstenzymatic activity, and the impact of drought, floods andtemperature on enzymatic activity and on the availabilityof oxygen52. moreover, these environmental constraintsare themselves affected by climate change. Therefore,predicting the effect of temperature increases on carbonstock has been difficult. In some cases, increased tem-perature may lead to a loss of soil organic carbon, espe-cially in temperate ecosystems53,54. Indeed, a recent studyshows that even subtle warming (by approximately 1 °C)can increase the ecosystem respiration rates in a subarc-tic peatland, particularly in the subsurface layers55. Thisis indicative of a large and long-lasting positive feedbackof the organic carbon stored in northern peatlands to theglobal climate system, although the mechanism of thisresponse remains unclear56.because different microbial groups have distinctoptimal temperature ranges for growth and activity,increased temperature can affect the composition ofthe microbial community, which in some cases couldreduce the release of soil organic carbon owing to theloss of acclimatized microbial groups 57 . For example,an increase in temperature in a high-latitude ecosystemresulted in an up to 50% decrease in bacterial and fun-gal abundance and soil respiration, as well as a phylo-genetic shift in the fungal community 58, suggesting thatincreased temperature does not always lead to enhancedcarbon loss to the atmosphere. To complicate mattersfurther, these changes in respiration could be causedby shifts in the composition and activities of microbialcommunities or by changes in the quality and quantity ofsoil organic carbon59,60. specifically, there is evidence thatwarming of soils leads to a decreased relative abundanceof fungi and to changes in bacterial community structurein arctic ecosystems 61 , but the long-term reduction insoil respiration due to warming could also be caused bythe sequential removal of easily decomposable organiccarbon that results from an initial stimulation of decom-position. It is also possible that some soil organic carbonis physically and chemically protected from microbialdecomposition59,62. because there are so many variables,the estimation of carbon loss by climate change is unre-liable 63 , and reducing this uncertainty will be a majoradvancement.Another key determinant of the terrestrial microbialcommunity structure and the decomposition rate of soilorganic carbon is soil moisture, which will be affectedby the 20% increase or decrease in precipitation ratethat has been predicted by the Intergovernmental Panelon Climate Change 21 . microbial communities respondto moisture levels directly, because they require waterfor physiological activities, and indirectly, owing to theeffect of changing soil moisture on gas diffusion ratesand oxygen availability. The effect of changing precipita-tion on the feedback responses of soil microorganismsto climate change may therefore be due to the directeffect on microbial physiology and community struc-ture. long periods of drier conditions may limit micro-bial growth and decomposition 64 and may consequentlyhave a negative-feedback effect on carbon fluxes in someecosystems. However, soil drying may increase oxygenavailability and enhance carbon cycling in wetlands andpeatlands, thereby having a positive-feedback effect onCO2 fluxes65
Solution
The text you provided is a detailed discussion on the potential effects of global warming on soil carbon sequestration. It highlights the complexity of predicting these effects due to the numerous variables involved, including the sensitivity of different fractions of soil organic carbon
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