Abstract

Abstract. Despite the importance of carbon (C) pools and CO2 fluxes in terrestrial ecosystems and especially in soils, as well as many attempts to assign fluxes to specific pools, this challenge remains unsolved. Interestingly, scientists investigating pools are not closely linked with scientists studying fluxes. This review therefore focused on experimental approaches enabling soil C pools to be linked with CO2 flux from the soil. The background, advantages and shortcomings of uncoupled approaches (measuring only pools or fluxes) and of coupled approaches (measuring both pools and fluxes) were evaluated and their prerequisites – steady state of pools and isotopic steady state – described. The uncoupled approaches include: (i) monitoring the decrease of C pools in long-term fallow bare soil lacking C input over decades, (ii) analyzing components of CO2 efflux dynamics by incubating soil without new C input over months or years, and (iii) analyzing turnover rates of C pools based on their 13C and 14C isotopic signature. The uncoupled approaches are applicable for non-steady state conditions only and have limited explanatory power. The more advantageous coupled approaches partition simultaneously pools and fluxes based on one of three types of changes in the isotopic signature of input C compared to soil C: (i) abrupt permanent, (ii) gradual permanent, and (iii) abrupt temporary impacts. I show how the maximal sensitivity of the approaches depends on the differences in the isotopic signature of pools with fast and slow turnover rates. The promising coupled approaches include: (a) δ13C of C pools and CO2 efflux from soil after C3/C4 vegetation changes or in FACE experiments (both corresponding to continuous labeling), (b) addition of 13C or 14C labeled organics (corresponding to pulse labeling), and (c) bomb-14C. I show that physical separation of soil C pools is not a prerequisite to estimate pool size or to link pools with fluxes. Based on simple simulation of C aging in soil after the input, the discordance of MRT of C in pools and of C released in CO2 was demonstrated. This discordance of MRT between pools and fluxes shows that the use of MRT of pools alone underestimates the fluxes at least for two times. The future challenges include combining two or more promising approaches to elucidate more than two C sources for CO2 fluxes, and linking scientific communities investigating the pools with those investigating the fluxes.

Highlights

  • PreambleAt the first conference the results of various approaches to separate pools of soil organic matter (SOM), and carbon (C) pools in soil, were presented and discussed

  • The future challenges include combining two or more promising approaches to elucidate more than two C sources for CO2 fluxes, and linking scientific communities investigating the pools with those investigating the fluxes

  • Some approaches linking soil C pools with CO2 fluxes are suitable for non-steady state conditions, whereas other approaches using isotopic disequilibrium between C input and soil organic matter (SOM) pools can be applied for soils under steady state (Table 1)

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Summary

Preamble

At the first conference the results of various approaches to separate pools of soil organic matter (SOM), and carbon (C) pools in soil, were presented and discussed. These approaches are based on chemical and physical fractionations (extractability, particle and aggregate size, density, etc.) as well as their combinations (von Lutzow et al, 2007; Bruun et al, 2010). Based on the turnover time, possible contributions to the CO2 fluxes from soil to the atmosphere were discussed, but not measured. There was no overlap of the colleagues participating in both conferences!

Why it is crucial to link C pools with CO2 fluxes?
Steady state of pools and isotopic steady state
Approaches to link pools and fluxes
Uncoupled approaches
Kinetic approach in incubation studies
Concluding remarks on uncoupled approaches
Gradual permanent impact
Concluding remarks on coupled approaches
Challenges
Findings
Conclusions
Full Text
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