| Greenhouse gas emission is conducive to climate change. Agricultural soil is an important source of greenhouse gas emission. Annual nitrous oxide (N2O) emission in agricultural land accounted for about 10-27% of the total emission of the world. Soil carbon (C), the largest C pool in terrestrial ecosystems, contributes to an annual atmospheric CO2 flux 10 times greater than that from fossil fuel combustion. It is therefore very important to assess the soil organic carbon (SOC) estimation methodology, and to simulate the processes of N2O and CO2 emissions, the mechanisms and factors controlling the emissions and their variation. The information generated is important for the mitigation of agricultural greenhouse gas emission, resource management, and sustainable socioeconomic development.In this thesis, the field experimental data, Australian climate data, Australian soil database and the Denitrification-Decomposition (DNDC) model were used to study the possibility of simulating the N2O and CO2 emission under different straw retention methods and to analyze the relationship of each influence factors. The results were as follows:(1)The climate data, soil properties data and control experimental field observation data were inputted into the DNDC model to adjust the model. The correlation coefficients between the simulated and observed values for water-filled pore space (WFPS) and temperature were 0.9532 (n=126, P＜0.01) and 0.9781(n=365, P＜0.01), respectively. It means that the DNDC model could be used to simulate the soil and climate condition of Griffith region. The correlation coefficients between the simulated and observed values of N2O emission flux were r=0.8523 (n=116, P＜0.01). The results of this study showed that DNDC model could simulate the N2O emission flux which was discharged by the mineral nitrogen changing.(2) The straw incorporate method decreased N2O emission and increased mineral nitrogen, when compared with straw burn methods. The correlations between the observation data and simulation data of daily N2O gas emission, annual N2O emission and emission factors were highly significant. The observed amount of annual N2O of straw burn and straw incorporate were 9.8 t C·ha-1·a-1 and 6.2 t C·ha-1·a-1, respectively, and the simulation amount were 12.96 t C·ha-1·a-1 and 8.08 t C·ha-1·a-1, respectively. Therefore, the DNDC model could be used to simulate the N2O emission under various straw retention scenarios. The mineral nitrogen content under the straw incorporation method was lower than that under the straw burn method during the straw retention period. However, the opposite trend was observed during the crop growth period.(3) The rate and frequency of nitrogen fertilizer application, irrigation frequency, and soil texture could affect N2O emission. Soil pH and soil bulk density did not significantly affect N2O emission.(4) Data on a maize cropping system from the Griffith region, Australia, was used to test whether the DNDC model could simulate the rate of CO2 emissions. The results showed that the DNDC model was able to simulate CO2 emission under alternative straw retention scenarios and to estimate CO2 emission. The correlation co efficients between simulated and observed daily values for treatments of straw burn and straw incorporation under high-nitrogen fertility scenarios were 0.7412 and 0.8233, respectively, in the straw retention period. The corresponding values for the crop growth period were 0.7254 and 0.8227. The observed of annual CO2 emission amount of straw burn and straw incorporate were 4.7 t C·ha-1·a-1 and 3.5 t C·ha-1·a-1, respectively, and the simulation were 3.45 t C·ha-1·a-1 and 2.13 t C·ha-1·a-1, respectively. The DNDC model was found to be more suitable in simulating CO2 emission fluxes under straw incorporation than when the straw was burnt.The DNDC model could also be used to simulate the soil organic carbon amount under different straw retention methods. But the soil organic carbon simulation values were higher than the observation values.(5) This studies have proved that DNDC model could be used to simulate the N2O and CO2 emission flux under different straw retention methods for a long time. The result showed that DNDC model could simulate the N2O and CO2 emission flux under different straw retention methods for a long time. The accuracy of the DNDC model was very high. DNDC model was more suitable in simulating N2O and CO2 emission fluxes under straw incorporation than when the straw was burnt. The stability of DNDC model will not be changed when the environment condition changed.(6) N2O emission was positively correlated with Tmax, Tmin, Tmean, WFPS, Mineral N and CO2, negatively correlated with Rain. Principal component analysis revealed that the major factors influencing N2O emission were WFPS, Mineral N, CO2 and Tmean. Path analysis and determination coefficient analysis indicated that the standard multiple regression equation for N2O emission and each main factor was Y=-37.162+0.5267 X1+ 0.4331 X2+0.3014X3+0.2392X4 (r＝0.924,p＜0.01, n=151). WFPS, CO2, Mineral N, Tmean and the interaction between them were the main factors affecting soil N2O emission. While WFPS was affected by irrigation and rainfall, CO2 emission reflected microbial activities which influenced the decomposition of soil organic matter. Mineral N was affected by the decomposition of retained straw, and fertilizer application. Therefore, these results suggested that irrigation, rainfall, microbial activities, straw retention and fertilizer application likely play an important role in N2O emission, and that management of these practices will help mitigate N2O emission in cropping systems.The standard multiple regression describing the relationship between CO2 emissions and its major controlling factors indicated that soil mean temperature (SMT), daily mean temperature (Tmean) and water-filled pore space (WFPS) were significant. Path analysis and determination coefficient analysis indicated that the standard multiple regression equation for CO2 emission and each main factor was Y=-34.113+0.8067 X1+0.6392 X2 +0.4014X3 (r=0.964, p<0.01, n=260). The SMT, Tmean and WFPS play an important role in CO2 emissions. |
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