Modeling Impacts Of Farming Management Practices On Greenhouse Gas Emissions In The Middle Reaches Of The Heihe River

How to control the greenhouse gas (GHG) emissions in different farming management systems has become one of the most important problems for human beings, because the greenhouse gases have been recognized as one of the main factors contributing to global climate change, and agroecosystems are a major source of anthropogenic greenhouse gas emissions. For the complex giant agroecosystems, soil carbon storage and carbon sequestration capacity are the important indicator to assess the potential for the reduction in anthropogenic greenhouse gas emissions and the sustainable development of agriculture. In summary, the proposal of an appropriate farming management practice will benefit from the study on C and N cycle mechanism of agroecosystems, the understanding of vulnerability of agroecosystems, and the exploration of quantitative relationships among the research goals. To make an appropriate farming management practice, we took summer maize fields in the middle reaches of the Heihe River as the study subject, then we collected data by field observation and utilized DNDC model to simulate the greenhouse gas fluxes and soil organic carbon content in the long time series. By sensitivity analysis, we distinguished the sensitivity factors that have important impacts on the research goals from numerous uncertain factors, and then calculated the relative sensitivity of each sensitivity factor; finally we obtained the simulated centennial-scale greenhouse gas fluxes and soil organic carbon content under different simulation scenarios.(1) The experiment included four treatments:â‘ OM (manures at2000kg C ha-1);â‘¡N (nitrogen applied as urea at300kg N ha-1);â‘¢MN (manures at2000kg C ha-1and nitrogen applied as urea at300kg M ha-1);â‘£B (neither fertilizer nor mannure). In this study, we measured the soil temperature and water-filled pore space (WFPS) at the top10cm depth, and we also measured N2O and CO2fluxes from the surface soil and nitrate (NO3-) and ammonium nitrogen (NH4+) at the top10cm depth in a summer maize field in Zhangye City, Gansu Province, northwestern China in2010. Then we used these data to verify the validity of DNDC model by statistical analysis.Between the modeled and observed values, the r2values were0.98for soil temperature and0.67for WFPS, and the relative deviations of soil temperature, WFPS, N2O, CO2, NO3-and NH4+were about3.5%,26.2%,45%,25%,43%,40%, respectively. The model performed very well in temporal match and captured the peak and valley value of the objectives of this study. Furthermore, the results of statistical analysis verified the validity of the model. In the treatment of OM and B, the modeled values were lower than the observed values for NO3-and NH4+. The possible reason for such results is that the local rural irrigation method is flooding, i.e., direct flooding of the crop field with water that mainly comes from underground and river flow. In the Heihe Oasis agricultural areas, groundwater NO3-pollution is very serious, and the DNDC model version9.2did not take into account such load of NO3-.(2) The meteorological data and soil physical and chemical data were utilized to simulate the fluxes of N2O and CO2, crop yields, soil organic carbon content and net global warming potential (GWP) under four different management scenarios. The results indicated that the N2O and CO2fluxes were the highest in the MN treatment and were4.57kg N ha-1y-1,6386.35kg C ha-l y-1, respectively. Crop yields were about2303kg C ha-1y-1for the N treatment and2301kg C ha-1y-1for MN treatment. SOC was about0.0142kg C kg-1for the MN treatment, which was the highest in the four treatments. The highest value of the net GWP was346kg CO2-equivalent ha-1in the B treatment and the lowest value of the net GWP was-6331kg CO2-equivalent ha-1in the MN treatment. The results showed that with OM treatment, N2O emission was low, and the SOC at0-20cm soil depth remained high, but crop yield was low; with N treatment, N2O and CO2emission was high, crop yield was also high, but the SOC at0-20cm depth was low, which is inappropriate for sustainable utilizing of soil; with MN treatment, the N2O and CO2emission was the highest, but crop yield was very high, only surpassed by that of the N treatment, SOC at0-20cm was the highest, and net GWP is negative and lowest-which means substantial carbon fixation-thus this treatment is beneficial to preservation of environment and sustainable development of agriculture; with treatment B, the controlled treatment, N2O and CO2emission, crop yield and SOC at0-20cm was the lowest, but the net GWP was the highest, which means this treatment is unsuitable for reduction of greenhouse gas emission and sustainable development of agriculture.(3) The results of the sensitivity tests showed thatâ‘ the sensitivity factors in N2O emissions were followed the order of the soil pH, nitrogen fertilizer application rate and SOC content. N2O flux decreased with the increase in soil pH, and increased with the increase in nitrogen fertilizer application rate and SOC content.â‘¡the CO2flux was sensitive to SOC content, manure amendment and crop residue incorporation rate, successively. And the CO2flux increased with the increase in these three factors.â‘¢the net GWP was sensitive to SOC content, crop residue incorporation rate and soil pH in turn. And the net GWP decreased with the increase in SOC content and increased with the increase in crop residue incorporation rate and soil pH.(4) The DNDC model simulated long-term changes in SOC, N2O and net GWP in four alternative management scenarios. The results indicated thatâ‘ Increasing the nitrogen content in soil did not solely increase SOC content significantly. Manure amendment and crop residue incorporation rate were the key factors in SOC content, and they were positively correlated.â‘¡N2O flux increased with the increase in nitrogen fertilizer application rate. Improvement in the crop residue incorporation rate and manure amendment can increase N2O flux greatly, and manure amendment was the most significant effects on N2O flux.â‘¢Net GWP increased with the increase in nitrogen fertilizer application rate, and had the same interannual variation tendency at different levels of nitrogen fertilizer application rate. Compared with the basic scenarios, improvement in the crop residue incorporation rate and manure amendment can lead to the increase in simulation results in late stage. However, the simulation results in100yr showed that increasing the crop residue incorporation rate was an effective method for decreasing net GWP.In further farming management, it will be an effective method to mitigate the net GWP by increasing crop residue incorporation rate, decreasing nitrogen fertilizer application rate and increasing and manure amendment However, the most appropriate farming management plan will be decided according to the actual situations in the specific implementation process.