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<p>Microbial release of CO<span class="inline-formula">₂</span> from soils to the atmosphere reflects how environmental conditions affect the stability of soil organic matter (SOM), especially in massive organic-rich ecosystems like the peatlands and grasslands of the Qinghai–Tibetan Plateau (QTP). Radiocarbon (<span class="inline-formula">¹⁴</span>C) is an important tracer of the global carbon cycle and can be used to understand SOM dynamics through the estimation of time lags between carbon fixation and respiration, often assessed with metrics such as age and transit time. In this study, we incubated peatland and grassland soils at four temperature (5, 10, 15 and 20 °C) and two water-filled pore space (WFPS) levels (60 % and 95 %) and measured the <span class="inline-formula">¹⁴</span>C signature of bulk soil and heterotrophic respired CO<span class="inline-formula">₂</span>. We compared the relation between the <span class="inline-formula">Δ¹⁴</span>C of the bulk soil and the <span class="inline-formula">Δ¹⁴</span>CO<span class="inline-formula">₂</span> of respired carbon as a function of temperature and WFPS for the two soils. To better interpret our results, we used a mathematical model to analyse how the calculated number of pools, decomposition rates of carbon (<span class="inline-formula"><i>k</i></span>), transfer (<span class="inline-formula"><i>α</i></span>) and partitioning (<span class="inline-formula"><i>γ</i></span>) coefficients affect the <span class="inline-formula">Δ¹⁴</span>C bulk and <span class="inline-formula">Δ¹⁴</span>CO<span class="inline-formula">₂</span> relation, with their respective mean age and mean transit time. From our incubations, we found that <span class="inline-formula">¹⁴</span>C values in bulk and CO<span class="inline-formula">₂</span> from peatland were significantly more depleted (old) than from grassland soil. Our results showed that changes in temperature did not affect the <span class="inline-formula">Δ¹⁴</span>C values of heterotrophic respired CO<span class="inline-formula">₂</span> in either soil. However, changes in WFPS had a small effect on the <span class="inline-formula">¹⁴</span>CO<span class="inline-formula">₂</span> in grassland soils and a significant influence in peatland soils, where higher WFPS levels led to more depleted <span class="inline-formula">Δ¹⁴</span>CO<span class="inline-formula">₂</span>. In our models, the correspondence between <span class="inline-formula">Δ¹⁴</span>C, age and transit time highly depended on the internal dynamics of the soil (<span class="inline-formula"><i>k</i></span>, <span class="inline-formula"><i>α</i></span>, <span class="inline-formula"><i>γ</i></span> and number of pools) as well as on model structure. We observed large differences between slow and fast cycling systems, where low values of decomposition rates modified the <span class="inline-formula">Δ¹⁴</span>C values in a non-linear pattern due to the incorporation of modern carbon (<span class="inline-formula">¹⁴</span>C bomb) in the soil. We concluded that the stability of carbon in the peatland and grassland soils of the QTP depends strongly on the direction of change in moisture and how it affects the rates of SOM decomposition, while temperature regulates the number of fluxes. Current land cover modification (desiccation) in Zoigê peatlands and climate change occurring on the QTP might largely increase CO<span class="inline-formula">₂</span> fluxes along with the release of old carbon to the atmosphere potentially shifting carbon sinks into sources.</p>