Alpine shrubland is one of the most important dominant vegetation types on the Qinghai-Tibetan Plateau, and because of its biological uniqueness, accurately estimating its ecosystem carbon function is very difficult. This is particularly true in the case of shrub encroachment induced by ongoing climate warming. Shrubs sequester much more carbon (C), and their woody branches have a higher carbon to nitrogen (C/N) ratio than do grasses. In addition, soil organic matter and litter are predicted to decompose more rapidly under future warmer scenarios. Consequently, predicting changes in the ecosystem carbon budget caused by more shrubland in the future is crucial but difficult.
In this study, the referee CO₂ fluxes were estimated by eddy covariance method from 2003-2010 in the alpine Potentilla fruticosa shrubland of Haibei Tibetan Autonomous Prefecture, China. The respiration rates of the soil and ecosystem were observed using a static chamber, and biomass was estimated by harvesting plants four to six times per month from May 2009 through June 2010. Finally, the net ecosystem primary production was evaluated to assess whether the ecosystem was a sink or a source of carbon.
Because shrubs were mixed with grasses in the study area, our experimental design included three treatments, each with three replicates, in the shrubland; the treatments were shrub plots, grass plots, and exposed field plots. In each plot, soil temperature was measured daily in each year of the study, while respiration rate was measured at intervals. The correlation between soil temperature and respiration rate on the days when both were measured was used to estimate the daily respiration rate. The results showed that daily respiration rate in the shrub, grass, and exposed field plots was 1.49-48.27, 1.04-39.14, and 1.01-16.99 g CO₂ m^-2 d^-1, respectively. The soil, vegetation and ecosystem respiration rates fluctuated distinctly among seasons, and all three reached their maxima in August. The annual carbon accumulations were 444.93, 441.36, and 886.28 g C/m², respectively. The respiration rates of the shrub, grass, and exposed field plots correlated exponentially and significantly with the soil temperature at a depth of 5 cm (R²=0.95, 0.94, and 0.83, respectively). Respiration entropy, Q₁₀ (the magnitude of the respiration rate change with a 10 ℃ change in temperature), of the treatments was 4.40, 4.13, and 3.16, respectively. The respiration rate in the shrub and grass plots was relatively sensitive to temperature, implying that plant respiration might contribute to ecosystem respiration much more in warming scenarios. The average 8-year net plant primary production was 468.55 g C/m² from 2003 to 2010, ranging from 345.02 g C/m² in 2005 to 633.96 g C/m² in 2009. The below-ground net plant primary production was 3.89 times that of the above-ground net plant primary production. Subtracting soil heterotrophic respiration, the alpine shrubland ecosystem acted as weak carbon sink and on average absorbed 27.19 g C/m² per year from the atmosphere. However, carbon function varied over time, from releasing -108.72 g C/m² in 2005 to assimilating 175.06 g C/m² in 2009. The magnitude and direction of carbon function was consistent with the results of an authentic eddy covariance technique in this alpine shrubland ecosystem, although our value of 27.19 g C/m² was slightly lower than in the previous study. Our results indicated that this comprehensive method integrating data from static chambers and plant harvests provided a credible estimate of the ecosystem carbon budget.