| Soil respiration of plantation and its environmental impact is an important research subject which is closely related to plantation carbon fixation. Poplar plantations in Daxing, Beijing (the carbon flux monitoring site of Beijing Forestry University) were studied with the field investigation, trial, monitoring and laboratory analysis methods. We systematically studied soil respiration temporal dynamics and its influencing environmental factors. The dynamic and the contribution of the non-root respiration and root respiration were studied by using trench method and root biomass extrapolation method. Main results are as follows:(1) Soil respiration temporal dynamics of poplar plantation.The diurnal curve of poplar plantation soil respiration (Rs) showed a single peak. Soil CO2 efflux reached the maximum at noon or afternoon and reduced to minimum with the decline of soil temperature (T) at night. Rs is high in summer (late July to early August) and low in winter at the seasonal time scale, which showed as a single-peak curve. The maximum value of Rs in the year of 2007-2009 are 5.83,5.07 and 5.81μmolCO2·m-2·s-1, and the annual total amount of soil CO2 efflux are 54.43,49.06 and 48.79 molCO2·m-2·a-1 respectively. Rs was close to zero in winter. T and soil moisture (W) were the main variables accounting for the diurnal and seasonal dynamic of Rs. Annual dynamic is the combined result of the influence of plant physiology, solar radiation, T, W, leaf area index (LAI) and precipitation and so on.(2) Rs and soil moisture characteristics are associated and analyzed in an innovative way during typical precipitation process, thus finding out the optimum soil water content of Rs is between wilting point and fracture pore water content (80% of field capacity).The relationship between Rs and W during a typical rainfall can be described by a cubic model Rs= a +bW+cW2+dW3), R2 ranges from 70% to 81%. By fitting the soil water characteristic curve (W=0.2636ψ-0.22, R2=0.963) and cutting ring method, we found that field capacity (10KPa) was 15.65%±0.31%, wilting point (WP) (150KPa) was 5.49%±0.52% and the water content of rupture of capillary (WRC) (40KPa) was 11.12±0.47%. The soil moisture threshold 6% and 11%, which were observed from the relationship between Rs and W, met the WP (15.65%±0.31%) and WRC (11.12±0.47%). Therefore, the optimum soil water content of Rs was between WP and WRC (80% of field capacity); when W was below the WP, Rs increased with W increased; when W was higher than WRC (80% of field capacity), the Rs decreased with W increased, W, instead of T, became the main factor of Rs variation.(3) Revealed the soil respiration response to soil temperatureT was the primary factor accounting for the Rs variation during a year. The relationship between Rs and T can be described by exponential equations, three fitted equations were:Rs=0.758e0.044T, Rs= 0.550e0.0537T, and Rs= 0.615e0.0497T; Tcan explain 67%,78% and 72% of the seasonal variation of Rs. The Q10 values of 2007,2008 and 2009 were 1.55,1.70 and 1.63 respectively. Q10 in spring and autumn are higher than in winter and summer. And Q10 in autumn (1.57-1.86) were higher than in spring (1.42-1.79), also higher than the Q10of whole years (1.55-1.70).When W between WP to WRC, Q10 (1.87-1.97) were higher than other W condition and the Q10of whole years.(4) Based on the verification of the classical models, Rs has been segmented modeled with T and W according to soil water characteristic values.The fitting results of 9 T-W double-factor empirical models indicate that exponential model (Ln Rs=a+bT+cW+dTW2) was the best model for Rs of whole years, R2 ranged 0.75-0.81. R2 have been improved after the data was segmented by WP and WRC. When W was lower than WP, exponential model Ln Rs=a+bT+cW+d T Wfitted better than others, R2>0.86; when W between WP and WRC, all the models which have a exponential form fitted better than others, R2>0.80; When W waas higher than WRC, Asymptote model Rs=aehTW/(W+c) was the best model, R2>0.80.Compared with measured soil respiration (RSM), estimated soil respiration (RSE) calculated by models after segmentation were more accurate than models before segmentation. Among segmented models, the RSE of combination of different models in each segment were closer to RSM/than the combination of same models in each segment.(5) Quantified dynamic and contribution of root respiration (RRoot) and heterotrophic respiration (RH), compared the T-W models of RRoot and RH.RRoot, measured with root biomass extrapolation method and trenching method were slightly different. RRoot and RH are higher in summer and lower in winter. The contribution of RRoot, to total soil respiration reached the maximum (59.32%) in August, reduced to the minimum (21.47%) in February. RRoot, and RH both had a exponential relationship with T, Q10 of RRoot, (2.07) was higher than total soil respiration (1.63), and RH (1.45). Among all the double-factor empirical models, exponential models, exponential-power model and Asymptote model fitted better than others for RRoot, R2>0.83; Asymptote model fitted RH better than others for RH, R2=0.90.
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