Moisture and temperature effects on the radiocarbon signature of respired carbon dioxide to assess stability of soil carbon in the Tibetan Plateau

Abstract. Microbial release of CO₂ 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 (¹⁴C) is an important tracer of the global carbon cycle and can be used to understand SOM dynamics through the estimation of time lags between C 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 ¹⁴C signature of bulk soil and respired CO₂. We compare the relation between the Δ¹⁴C of the bulk soil and the Δ¹⁴CO₂ 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 (k), transfer (α) and partitioning (γ) coefficients affect the Δ¹⁴C -bulk and Δ¹⁴CO₂ relation, with their respective mean age and mean transit time. From our incubations, we found that ¹⁴C from peatland was significantly more depleted (old) than from grassland soil. Our results showed that changes in temperature did not affect the Δ¹⁴C values of respired CO₂ in either soil. However, changes in WFPS had a small effect on the ¹⁴C CO₂ in grassland soils and a strong influence in peatland soils, where higher WFPS levels led to more depleted Δ¹⁴CO₂. In our models, we observed large differences between slow and fast cycling systems, where low values of k modified Δ¹⁴C patterns due to the incorporation of ¹⁴C-bomb in the soil. Hence, the correspondence between Δ¹⁴C and age and transit time strongly depended on the internal dynamics of the soil (k, α, γ and number of pools) as well as on model structure. We conclude that the stability of carbon in these systems depends strongly on the direction of change in temperature and moisture and how it affects the rates of SOM decomposition. Finally, Δ¹⁴C modelling along with empirical data from SOM dynamics is a useful tool to improve predictions on interactions between terrestrial and atmospheric carbon.