Ecosystem and soil respiration of a poplar plantation on a sandy floodplain in Northern China

Poplar is one of the most widely planted tree species through China's ambitious reforestation effort especially in the northern China since the 1970s. Among various pressing issues related to this plantation, the magnitude and dynamics as well as the biophysical regulation of carbon loss through respiration are critical information needed in the context of wood productivity and carbon sequestration. Ecosystem respiration(Re), soil respiration(Rs), and meteorological conditions of a Populus x euramericana cv. "74/76" plantation located on the floodplain of the Yongding River in Daxing District of Beijing, were measured through an eddy-covariance system, a four-channel soil respiration measurement system(Li-8150, Li-Cor, NE), and a microclimate measurement system from 2007 through 2009. The variation of Re and Rs in the different time scales and their responses to environmental factors including soil temperature and soil water content were analyzed and discussed in this paper. Energy balance closure was evaluated by statistical regression between turbulent energy fluxes of sensible (Hs) and latent heat (LE) against the difference etween available energy of net radiation (Rn) and the soil heat flux (G), which excluded the heat storage from the ground to the height of measurement. The ratio of turbulent energy fluxes to available energy was 0.78, with an average intercept of 18.95 Wm super(-2)s super(-1) at half-hourly scale in all three years indicating that the flux data were reliable in general. The total annual Re was 946.68, 863.58, and 817.30gCm super(-2)a super(-1) for 2007, 2008, and 2009, respectively; and the total annual Rs was 642.11, 629.29, and 609.05gCm super(-2)a super(-1). Rs was the dominant component of Re, accounting for about 70% of Re on an annual basis. The annual Rscan be used to estimate the annual Re in the future studies. The monthly Re and Rs reached the maximum in July or August, and Rs showed a similar seasonal variation in all three years. The similar temporal variation was observed for the monthly Rs and the averaged soil temperature at 5cm depth (Ts5) from 2007 to 2009. The maximum daily Rs and Re were 5.285 and 7.781 gCm super(-2)d super(-1), compared with the minimum value of 0.330 and 0.504 gCm super(-2)d super(-1) for 2007, 2008, and 2009, respectively. Re and Rs were significantly correlated with Ts5, however, we found that Re was mostcontrolled by air relative humidity (RH) among all the climate and soil moisture conditio