Dynamics and allocation of recently photo-assimilated carbon in an Inner Mongolia temperate steppe

Information on carbon (C) dynamics and allocation in plant–soil system is essential for understanding the terrestrial C cycle. Using a 13C pulse-labeling chamber (1 m × 1 m) technique, we carried out three separate experiments in an Inner Mongolia temperate steppe (Leymus chinensis–Stipa grandis–Cleistogenes squarrosa). The first experiment determined mainly the temporal variation of δ13C (‰) signatures over the chase period of 6–27 July in a fenced site. The second experiment compared the dynamics and allocation of recently assimilated C over 10–20 August between a fenced site and a grazed site. The third experiment measured the effect of N application on assimilated C fluxes over 26 August–4 September in a fenced site. The above- and below-ground partitionings of labeled 13C were found to vary with site, growth stage and management state. The labeled 13C in shoots was maximal during the first day after labeling and then declined, whereas it roughly increased in roots. There was the absence of significant variation in soil δ13C. In the fenced site, the labeled 13C partitioning to the shoots accounted for 24.4, 16.8 and 11.1% of initial additions by 10 days after the labelings on 6 July, 10 August and 26 August 2003, respectively. However, the percentage of recently assimilated C partitioning to the roots, about 22–23%, was almost unchanged throughout growing stages. In the grazed site, the labeled 13C of about 50% was respired, 13% was remained in the shoots, and 37% was translated to the roots; the corresponding percentages, for the fenced site with N, were approximately 60, 20 and 18%, respectively. This study suggests that carbon was rapidly and substantially cycled in the Inner Mongolia temperate steppe by means of photosynthesis and respirations. It appears that the grazing and the N application had significant effects on the dynamics and allocation of recently photo-assimilated C in the plant–soil system.