Study On Characteristics Of Carbon Budget In Tobacco Field Ecosystem Of Enshi Area

Study on cropland ecosystem carbon budget has become a hot issue of current concern. Tobacco as an important economic crop, its ecosystem carbon budget is special for the special characteristics of ridge planting, fertilization and periodic harvest. Besides, referring the carbon budget of tobacco field ecosystem was seldom reported. In order to investigate the characteristics of carbon budget of tobacco ecosystem, the closed chamber-infrared method was used to measure the diurnal variation of CO2 fluxes during different growth periods. And variations of soil respiration rate and its influencing factors under different fertilization treatments during the tobacco growth period. Besides, the net ecosystem CO2 daily exchange and the contribution of root respiration to the soil respiration and the carbon budget during the whole growth period were also calculated. The main results were as follows:(1) The CO2 fluxes of tobacco ecosystem had a significant diurnal variation with a “U” pattern at rosette period, vigorous growth period, topping period and harvest period. The tobacco ecosystem absorbed CO2 during the mainly daytime with a peak appeared at about 12:00, while after the harvest period that released CO2 on the reverse. The ecosystem released CO2 at night. The range of diurnal variation CO2 fluxes were varied in different growth periods, and the maximal range of CO2 fluxes was occurred at the topping period with the value of ﹣2419.48~594.82 mg·m-2·h-1.(2) Ridge-soil respiration rate displayed significant seasonal variation with a “M” pattern under different fertilization treatments, which including no fertilizer(CK), inorganic fertilizer(NPK) and inorganic plus organic fertilizer(NPKOM) during the growth period. Two peaks were occured on the 81 th and 107 th after tobacco seedling transplanted, respectively. Soil respiration rate in NPK and NPKOM treatments were both significantly higher than that in CK treatment after 14 days of tobacco seedling transplanted. But there was no significant difference between the NPK and NPKOM treatments.(3) The variations of ridge-soil respiration rate were mainly affected by soil temperature and tobacco root biomass during the tobacco growth period, and significant correlation between respiration rate and the soil moisture content was not found. Soil respiration rate exponentially increased with the increase of soil temperature. The soil temperature at 5 cm depth and 10 cm depth could explain 60.4%~67.6% and 67.9%~76.1% variations of soil respiration rate during the whole growth period, respectively. Furthermore, the index of temperature sensitivity(Q10) of soil respiration rate was increased with fertilizing amount and soil depth increased. And the soil respiration rate increased with the tobacco root biomass increased from tobacco transplanting period to the topping period. But the soil respiration rate was mainly affected by soil temperature at the at harvest period. Furthermore, the variations of ridge-soil microbial respiration rate and inter-row soil respiration rate were mainly affected by soil temperature during the whole growth period, and 41.2%~71.0% and 65.1%~77.1% variations of soil respiration rate were explained by the soil temperature at 5 cm and 10 cm depth, respectively.(4) The net ecosystem CO2 daily exchanges(NEE) were ﹣1.87 g·m-2·d-1,﹣3.19 g·m-2·d-1,﹣15.71 g·m-2·d-1 and ﹣4.98 g·m-2·d-1 at rosette period, vigorous growth period, tippingt period and harvest period, respectively. The highest value was found at topping period, and the lowest was found at rosette period. Furthermore, the tobacco ecosystem showed a carbon sink at the four periods. When after the harvest period, the NEE was 2.75 g·m-2·d-1, which showed a carbon source.(5) The cumulative CO2 emissions of ridge-soil in CK, NPK and NPKOM treatments were 294.17 C g·m-2, 466.40 C g·m-2 and 449.11 C g·m-2 during the whole growth period, respectively. And the cumulative CO2 emissions in NPK and NPKOM treatments, with no significant difference between each other, were both significantly higher than that in CK treatment. The contribution of root respiration to the soil respiration in CK, NPK and NPKOM treatments were 45.1%, 47.8% and 42.82%, respectively, in the whole growth period. The Net Ecosystem Productivity(NEP) of tobacco field under CK, NPK and NPKOM treatments were 1131.83 C kg·hm-2, 1736.97 C kg·hm-2�'� 1535.74 C kg·hm-2, respectively, and NPK and NPKOM treatments were significantly higher than that in CK treatment, while all treatments showed a carbon sink.