How to link soil C pools with CO₂ fluxes?

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.