Knowledge about the influence of tobacco biochar (BC) on soil carbon dioxide (CO2) emissions and carbon (C) budget in a karst region of southwest China is limited. The karst field experiment was conducted in randomly arranged plots and evaluated five treatments: a plot without BC addition (B-0), with 1.0 t BC ha(-1) (B1C1), 10 t BC ha(-1) (B10C10), 25 t BC ha(-1) (B25C25) and 50 t BC ha(-1) (B50C50). Soil CO2 emissions were assessed using closed chambers at 10-day intervals during a tobacco-growing season. Soil temperature and moisture were simultaneously measured at 5-, 10-, and 20-cm depths. BC amendment increased tobacco productivity, soil organic matter, total nitrogen, available phosphorus, and available potassium contents but decreased total sulfur content. Compared with the control (B-0), daily average soil CO2 fluxes significantly increased by 20.0%, 26.3%, 39.4% and 50.2% in the B-1, B-10, B-25, and B-50 treatments, respectively. The cumulative soil CO2 emissions for the entire tobacco-growing season were significantly higher in BC amendments than in the control. Furthermore, soil CO2 fluxes were observed to be positively correlated with soil temperature but negatively correlated with soil moisture. Soil respiration sensitivity to temperature (Q(10)) significantly increased at greater soil depths, whereas decreased in BC-amended soils. Moreover, the BC amendments significantly increased C gain from -1.17 t h(-1) in the B-0 treatment to 19.48 t ha(-1) in the B-50 treatment. Based on C budget, the increase in CO2 emissions in the BC-treated soil was compensated by the higher tobacco biomass and soil C storages. These results suggest that the application of tobacco BC to karst farmland enhances the soil C sequestration.

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