Characteristics of soil respiration and net ecosystem carbon budget from tobacco-planted soil with different organic amendments

Organic material inputs are recommended to increase soil fertility and crop growth, but organic amendments may also increase carbon (C) emissions. A field experiment was conducted to comprehensively evaluate the effects of different organic amendments on soil respiration and the net ecosystem carbon budget (NECB) in a successive tobacco cultivation system. The experiment included five treatments: no fertilizer (NF), only chemical fertilizer with nitrogen, phosphorus and potassium (NPK), NPK plus ryegrass as green manure (NPKG), NPK plus wheat straw (NPKS) and NPK plus biochar (NPKB). Compared with the NPK treatment, organic amendments significantly improved the leaf yield and quality. The NPKG, NPKS and NPKB treatments significantly increased the cumulative emissions of carbon dioxide (CO2) from microbial respiration (Rh) and total respiration (Rs) during the growing season. Furthermore, the NPKG and NPKS treatments significantly increased the cumulative CO2 efflux from root respiration (Ra) by 25.2 and 21.4 % in 2020 and by 35.2 and 37.7 % in 2021, respectively. The NPKG treatment significantly decreased the mean temperature sensitivity (Q10) of Rh during the growing season in 2021, whereas the NPKB treatment increased the Q10 values of Ra and Rh during the fallow season. The Ra and Rh were positively correlated with root biomass at 0-90 days after transplanting and soil labile C concentrations, respectively. The annual NECB values of the NF and NPK treatments were both negative. However, the NPKG, NPKS and NPKB treatments significantly increased the annual NECB, and the values were 0.37, 0.11 and 0.73 t C ha-1, respectively. Our results indicate that a combination of chemical fertilization, ryegrass incorporation and application of wheat straw or biochar is an effective practice to promote plant growth and soil C sequestration in successively planted tobacco farmland.