Ecosystem CO₂ fluxes and abiotic and biotic environmental factors were measured in an alpine grassland at four altitudes from 3600 to 4200 m along a slope of the Qilian Mountains on the northwestern Qinghai–Tibetan Plateau during the growing season of 2007. We used a closed-chamber method combined with shade screens and an opaque cloth to measure several carbon fluxes, GPP, Re_daytime and NEP, and factors, light-response curve for GPP and temperature sensitivity of Re_daytime. Above- and below-ground biomasses and soil C and N contents at each measurement point were also measured.

• (i)Altitudinal pattern of ecosystem CO₂ fluxes: The maximum net ecosystem CO₂ flux (NEP_max), i.e. the potential ecosystem CO₂ sink strength, was markedly different among the four altitudes. NEP_max was higher at the highest and lowest sites, approximately -7.4 ± 0.9 and -6.7 ± 0.6 μmol CO₂ m^-2 s^-1 (mean ± standard error), respectively, but smaller at the intermediate altitude sites (3800 and 4000 m). The altitudinal pattern of maximum gross primary production was similar to that of NEP_max. The Re_daytime, however, was significantly higher at the lowest altitude (3.4 ± 0.3 μmol CO₂ m^-2 s^-1) than at the other three altitudes.

• (ii)Altitudinal variation of vegetation biomass: The above-ground biomass was higher at the highest altitude (154 ± 27 g DW m^-2) than at the other altitudes, which we attribute mainly to the large biomass in cushion plants at the highest altitude. The small above-ground biomass at the lower altitudes was probably due to heavy grazing during the growing season.

• (iii)Features of ecosystem CO₂ fluxes: Re_daytime and GPP were positively correlated with above-ground biomass. The low ratio of Re_daytime to GPP at either the measurement point or the site level suggests that CO₂ uptake efficiency tends to be higher at higher altitudes, which indicates a high potential sink strength for atmospheric CO₂ despite the low temperature at high altitudes. The results suggest that the effect of grazing intensity on ecosystem carbon dynamics, partly by decreasing vegetation biomass, should be clarified further.

  Key words: ecosystem respiration, grazing, gross primary production, net ecosystem production, Qinghai-Tibetan Plateau

  摘要：
  Aims

  Recent studies have recognized the alpine grasslands on the Qinghai–Tibetan plateau as a significant sink for atmospheric CO₂. The carbon-sink strength may differ among grassland ecosystems at various altitudes because of contrasting biotic and physical environments. This study aims (i) to clarify the altitudinal pattern of ecosystem CO₂ fluxes, including gross primary production (GPP), daytime ecosystem respiration (Re_daytime) and net ecosystem production (NEP), during the period with peak above-ground biomass; and (ii) to elucidate the effects of biotic and abiotic factors on the altitudinal variation of ecosystem CO₂ fluxes.

Ecosystem CO₂ fluxes and abiotic and biotic environmental factors were measured in an alpine grassland at four altitudes from 3600 to 4200 m along a slope of the Qilian Mountains on the northwestern Qinghai–Tibetan Plateau during the growing season of 2007. We used a closed-chamber method combined with shade screens and an opaque cloth to measure several carbon fluxes, GPP, Re_daytime and NEP, and factors, light-response curve for GPP and temperature sensitivity of Re_daytime. Above- and below-ground biomasses and soil C and N contents at each measurement point were also measured.

• (i)Altitudinal pattern of ecosystem CO₂ fluxes: The maximum net ecosystem CO₂ flux (NEP_max), i.e. the potential ecosystem CO₂ sink strength, was markedly different among the four altitudes. NEP_max was higher at the highest and lowest sites, approximately -7.4 ± 0.9 and -6.7 ± 0.6 μmol CO₂ m^-2 s^-1 (mean ± standard error), respectively, but smaller at the intermediate altitude sites (3800 and 4000 m). The altitudinal pattern of maximum gross primary production was similar to that of NEP_max. The Re_daytime, however, was significantly higher at the lowest altitude (3.4 ± 0.3 μmol CO₂ m^-2 s^-1) than at the other three altitudes.

• (ii)Altitudinal variation of vegetation biomass: The above-ground biomass was higher at the highest altitude (154 ± 27 g DW m^-2) than at the other altitudes, which we attribute mainly to the large biomass in cushion plants at the highest altitude. The small above-ground biomass at the lower altitudes was probably due to heavy grazing during the growing season.

• (iii)Features of ecosystem CO₂ fluxes: Re_daytime and GPP were positively correlated with above-ground biomass. The low ratio of Re_daytime to GPP at either the measurement point or the site level suggests that CO₂ uptake efficiency tends to be higher at higher altitudes, which indicates a high potential sink strength for atmospheric CO₂ despite the low temperature at high altitudes. The results suggest that the effect of grazing intensity on ecosystem carbon dynamics, partly by decreasing vegetation biomass, should be clarified further.