Abstract: Characterizing the carbon turnover in terrestrial ecosystems is critical for understanding and predicting carbon dynamics in ecosystems. We used in situ ¹³C pulse labeling to track photosynthetic carbon fluxes of Nitraria tangutorum from shoot to root and soil in the Ulanbuh Desert. The objectives of this study were: 1) To determine the transfer dynamics of newly photosynthesized carbon to different carbon pools including leaves, stems, roots, soil, and respiration. 2) To quantify the allocation rate of newly fixed carbon among different carbon pools. 3) to estimate the carbon budget of a N. tangutorum desert ecosystem. It was found that the distribution of ¹³C-newly fixed carbon in leaves, stems, and roots of N. tangutorum and in soil was significantly different in different labeling periods. The ¹³C abundance value in leaves and stems rose rapidly to the highest value after labeling for 1 h, at which point the ¹³C abundance value was 520.1‰ and 592.5‰, 14 and 20 times higher than the control, respectively. Thereafter, the ¹³C abundance value gradually decreased with time until it stabilized. The labeled ¹³C was found in the roots and soil after labeling 18 h, and the ¹³C abundance values reached 9.5‰ and -23.8‰, respectively. The amount of newly fixed carbon lost through shoot respiration and soil respiration reached a maximum after labeling for 1 h and 18 h, respectively. At the end of the labeling period, about 35.59% of labeled carbon was transferred to the shoots, 16.67% was retained in root, 31.92% was lost as respiration (shoot respiration+soil respiration) and 15.82% remained in the soil. In the three carbon pools, i.e., shoot, root, and soil pools, shoots consistently had the highest proportion of ¹³C in the plant-soil system during the 32 days. Based on the ¹³C partitioning pattern and biomass production, we estimate a total of 2895.6 kg C·ha^-1·yr^-1 was fixed by these desert plants during the vegetation growth season. This study suggests that N. tangutorum plays an important role in carbon sequestration in the carbon cycle of desert ecosystems.

Key words: desert plant, newly fixed carbon, distribution dynamics, distribution pattern, stable isotope