Abstract:

Aims Droughts are common in arid and semiarid regions and affect the capacity of carbon sequestration of grassland ecosystems by influencing the process of ecosystem carbon cycling. We analyzed the continuous measurements of ecosystem CO₂ fluxes during three growing seasons (May-September) over a Leymus chinensis steppe in Inner Mongolia in order to examine the effect of drought stress on carbon accumulation of this grassland ecosystem.

Methods We used the eddy covariance technique to measure CO₂ fluxes during the 2004-2006 growing seasons. Only 126 and 215 mm precipitation fell during the 2005 and 2006 growing seasons, respectively, far less than normal (in 2004, 364 mm); therefore, the steppe was in an extreme drought condition.

Important findings Maxima for gross primary productivity (GPP) and ecosystem respiration (R_e) were 4.89 and 1.99 g C·m^-2·d^-1, respectively, in the 2004 growing season (normal year). However, in drought years, GPP and R_e were 1.53-3.01 and 1.38-1.77 g C·m^-2·d^-1, respectively. GPP and R_e in the drought years decreased by 68% and 11%, respectively, compared with the normal year. Accumulated GPP and R_e were 294 and 180 g C·m^-2, respectively, during the growing season in 2004 and 102-123 and 132-158 g C·m^-2, respectively, in drought years. Accumulated GPP and R_e decreased 58%-65% and 12%-27%, respectively, in drought years compared with those of the normal year. The slope of the curve in the sensitivity for R_e to T_s (Vant’Hoff type) reached its maximum at θ = 0.16-0.17 m³·m^-3; below or above this value of θ, the sensitivity of R_e to T_s decreases. GPP and R_e decline under drought stress conditions, with GPP having a larger decline. Long-term and continuous drought reduced C-accumulation and resulted in the steppe ecosystem switching from a carbon sink in typical years to a carbon source in drought years.

Key words: carbon accumulation, drought, ecosystem respiration, gross primary productivity, Leymus chinensis steppe, net ecosystem exchange