Net ecosystem carbon exchange and its main driving factors in meadow grassland ecosystems in the Horqin Sand Lands

The objectives of this study were to investigate the main drivers of ecosystem carbon dynamics in meadow grasslands in the Horqin Sand Lands,and to understand the relationship between net ecosystem CO2exchange(NEE)and key meteorological factors in this region.We continuously measured(NEE)and environmental factors in meadow grasslands in the Horqin Sand Lands from October 2013 to September 2014 using the eddy covariance method.The results showed that the diurnal variation in NEE differed significantly among seasons,especially among vegetation growing seasons.The highest seasonal diurnal absorption rate of CO2 was in July,followed by August,September,June,and then May.The NEE was controlled by photosynthetically active radiation(PAR)and the leaf area index(LAI)during vegetation growing seasons.The relationship between NEE and PAR was expressed by a rectangular hyperbolic function.The simulated apparent initial average light-use efficiency was 0.0015μmol CO2/μmol PAR,and the maximum photosynthetic rate(Pmax)was0.65μmol CO2/(m2·s).The NEE was influenced by LAI,which was expressed by piecewise functions.When the LAI was greater than 3.08,NEE-PAR showed a gradual saturation-type relationship.The higher the LAI,the stronger the response of net carbon exchange to the amount of PAR.A light saturation response occurred when 1.5VPD2kPa,and NEE was significantly inhibited under high VPD.The day-time net CO2 exchange was inhibited by heavy rainfall(cumulative rainfall40mm/day),while sustained low-intensity rainfall(rainfall length15h)stimulated soil respiration at night.The night-time respiration rate of the ecosystem(Reco)increased exponentially as the soil temperature(Ts)increased,and the temperature-sensitive coefficient Q10 was 2.63.