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

Abstract

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 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 14C signature of bulk soil and heterotrophic respired CO2. We compared 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 values in bulk and CO2 from peatland were significantly more depleted (old) than from grassland soil. Our results showed that changes in temperature did not affect the Δ14C values of heterotrophic respired CO2 in either soil. However, changes in WFPS had a small effect on the 14CO2 in grassland soils and a significant influence in peatland soils, where higher WFPS levels led to more depleted Δ14CO2. In our models, the correspondence between Δ14C, age and transit time highly depended on the internal dynamics of the soil (k, α, γ 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 Δ14C values in a non-linear pattern due to the incorporation of modern carbon (14C 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 CO2 fluxes along with the release of old carbon to the atmosphere potentially shifting carbon sinks into sources.