| Climate changes, characterized by increasing in atmospheric carbon dioxide (CO2)concentration and air temperature, have significant impacts on transformation processes of soil carbon (C) and nitrogen (N) and greenhouse gas emissions. Study on the C and N transformation processes in response to elevated CO2 and temperature and the feedback effect of greenhouse gas emissions to climate changes in paddy soil, will help to improve the management of carbon and nitrogen in farmland, and deal with climate change scientifically. The study was performed at a simulation of elevated atmospheric CO2 concentration and air temperature system (T-FACE), which was established at Changshu County, Jiangsu Province, China, an important crop region for summer rice and winter wheat. Our primary objective of this study was to experimentally research, using the T-FACE system, the effects of elevated CO2 and temperature on soil profile methane (CH4)and nitrous oxide (N2O) concentration and diffusion fluxes, soil gross nitrogen transformation rates with 15N labelling technique, and the contents, turnover rates and temperature sensitivities of soil labile and recalcitrant organic carbon pools. The T-FACE system included four treatments, i.e., an ambient control (CK) and three simulating global climate change scenarios: (1) elevated atmospheric CO2 concentration of 500 ?mol/mol(EC), (2) canopy air temperature elevated by 2? (ET) and (3) elevated CO2 combined with elevated temperature (ECT). The main results were presented as follows:1. We collected soils from T-FACE system after wheat and rice harvested and measured the gross N transformation rates. Results showed that the effects of simulating elevated CO2 and temperature treatments on soil gross N mineralization and immobilization rates have obvious seasonal variation. The effects of simulating elevated CO2 and temperature treatments on gross N transformation rates of soils after wheat harvest were larger than those soils after rice harvested. But there are some rates did not show a seasonal variation. For example, the EC and ECT treatments increased the mineralization of labile organic-N, while the ET treatment increased the mineralization of recalcitrant organic-N.Compared to the CK treatment, the ET treatment enhanced autotrophic nitrification rates by 25.8% and 57.3% of soils after wheat and rice harvested, respectively. Results demonstrated that the effects of the combined treatment on gross N transformation rates were weaker than that of single elevated CO2 or elevated temperature treatment. Besides,elevated CO2 dominated the effects of the combined treatment on gross N transformation rates as responses to the combined treatment were more similar to those in the single elevated CO2 treatment than to those in the single elevated temperature treatment.2. We conducted a 15N tracing analysis with the soils after 2 years and 4 years of exposure to T-FACE system and found that 17 of the total 40 gross N transformation rates were significantly different between the time points. Total gross N mineralization rates at 2 years were significantly increased by 104.2%, 78.9% and 49.0% under the EC, ET and ECT treatments, respectively, compared with CK, but were not altered at 4 years. In both years, both the EC and ECT treatments stimulated the mineralization of labile organic-N,whereas the ET treatment accelerated the mineralization of recalcitrant organic-N. Gross NH4+ oxidation rates at 2 years were significantly reduced by 5.9% and 29.2% in the EC and ECT treatments, respectively, compared with CK but not changed at 4 years. In contrast, the ET treatment significantly enhanced gross NH4+ oxidation rates by 13.8% and 57.3% at 2 and 4 years, respectively. The results suggest that CO2 dominated the counteractive interactions between elevated CO2 and temperature and that the simulating elevated CO2 and temperature treatments at the 4-year time point had minor effects on gross N transformation rates relative to that at the 2-year time point.3. Elevated CO2 and warming had different effects on decomposition of SOC.Compared to the CK treatment, EC, ET and ECT treatments increased the ratio of labile organic C to total organic C, but decreased the decomposition rate for both labile and recalcitrant organic C. We also found that Q10 values of recalcitrant organic C pools(2.42-3.36) were greatly larger than that of labile organic C pools (2.02-2.38). Elevated CO2 and warming showed contrasting effects on temperature sensitivity of SOC decomposition. Elevated CO2 stimulated Q10 of recalcitrant organic C pool, while warming increased Q10 of labile organic C pool. The greatly enhanced temperature sensitivity of SOC decomposition by elevated CO2 and temperature indicates that more CO2 will release to the atmosphere and the losses of soil C may be even greater than previously expected in paddy field.4. Soil profile CH4 and N2O concentrations and diffusion fluxes under rice-wheat rotation system showed a greater seasonal variability. On one hand,CH4 concentrations were higher during the rice season and decreased with depth, while lower during the wheat season and increased with depth. Large variation was observed at the 7 cm depth among all treatments, indicating that most CH4 production, oxidation and transport occurred in the shallow soil layer. On the other hand, N2O concentration decreased with soil depth during the rice season, and the N2O concentration at the 30 and 50 cm depths were significantly greater than the concentration at the 7 and 15 cm depths during the wheat season. The 7 cm in the rice season and 0-30 cm in the wheat season were main production sites in this paddy field. At the 7 cm depth during the rice season, climate change treatments greatly increased soil profile CH4 concentrations and diffusive fluxes. Besides, climate change treatments didn't change profile N2O concentrations during rice season but greatly increased profile N2O concentrations during wheat season.In conclusion, the effects of the combined treatment on gross N transformation rates were weaker than that of single elevated CO2 or elevated temperature treatment, and elevated CO2 dominated the responses of gross N transformation rate to the combined treatment. Elevated CO2 increased the mineralization of labile organic-N, and elevated temperature increased the mineralization of recalcitrant organic-N rates and nitrification rates. Elevated CO2 and temperature increased the contents of labile and recalcitrant organic C pools and decreased the turnover rates of soil organic C. However, the combined treatment increased the temperature sensitivity of decomposition of SOC, indicating that more CO2 would be released from paddy field under the future climate change scenarios.Besides, the increased CH4 and N2O emissions of topsoil under elevated CO2 and temperature might produce positive effects on climate changes. |
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