| As one of the most important greenhouse gases,the change of N20 concentration in the atmosphere will have a significant impact on the global climate and atmospheric ozone layer.Farmland soil N2O is the main source of anthropogenic N2O emission.Due to the influence of many factors,its emission shows high temporal and spatial variability,which leads to greater difficulty in estimation.How to accurately and efficiently estimate farmland soil N2O emission is still an important topic in front of researchers at home and abroad.Inorganic nitrogen is the most direct substrate for N2O production in farmland soil.The main sources of inorganic nitrogen in soil are exogenous nitrogen input based on fertilization and inorganic nitrogen generated by decomposition of organic matter.At present,there are a variety of mature estimation methods for N2O emission caused by fertilizer input,but the inorganic nitrogen generated by decomposition of organic matter has rarely been mentioned in the past studies.There is no widely accepted method for estimating N2O emission from these nitrogen sources.This paper focuses on the N2O produced by these nitrogen sources.Based on the average monthly air temperature,rainfall and soil C/N ratio in 2000 and 2018 in Chongqing,the N2O estimation model driven by organic matter decomposition is the core,and supplemented by other existing models.An estimation framework of"N2O emission driven by temperature,rainfall--soil respiration--heterotrophic respiration--nitrogen mineralization--organic matter decomposition"was constructed to estimate the N2O emission driven by soil organic matter decomposition in farmland（dry farmland）in Chongqing area,in order to supplement the existing N2O estimation methods and further improve the accuracy of farmland soil N2O estimation.The main research results are as follows:（1）The monthly mean temperature of Chongqing in 2000 ranged from 4.18℃to26.03℃,with an average of 15.49℃.Compared with 2000,the mean monthly temperature in 2018 increased by 3.7%（16.06℃）.The rainfall in 2018 was 1,109.18mm,down 2.3 percent from 1,135.63 mm in 2000.Both temperature and rainfall show a trend of"low-high-low"throughout the year.The low values appear in January-February and December in winter,while the average temperature is high and rainfall is relatively large from June to August in summer.The hot months coincide with the rainy months,which is typical of the rainy and hot months.Although there are some differences between different years,from the perspective of space,rainfall and high temperature centers in 2000 and 2018 basically coincide,and they are located in the middle and west of Chongqing,southwest and southeast of Chongqing.（2）The monthly CO2 emissions of farmland soil respiration of Chongqing in 2000ranged from 38.20-120.38g C/m2.mon,with an average of 81.88g C/m2.mon;The average monthly respiration rate in 2018 was 84.98g C/m2.mon,an increase of 3.8%compared with 2000.The peak value of monthly farmland respiration intensity in 2000and 2018 basically occurred in the hot and rainy June-August,while the respiratory intensity in other months was relatively low,especially in January-February and December,and the intensity of soil respiration was much lower than the annual average.The annual respiration of farmland soil in 2000 ranged from 882 to1024g C/m2.yr,with an average value of 984±21.17g C/m2.yr.Compared with 2000,the annual respiration of farmland soil in 2008 increased by 3.7%（1020±24.53g/m2.yr）.The interannual variation of soil respiration is consistent with that of air temperature,so it is not difficult to see that temperature is the main factor affecting soil respiration intensity.（3）Soil respiration includes autotrophic respiration and heterotrophic respiration.Generally speaking,the amount of heterotrophic respiration can be equal to the decomposition amount of soil organic matter.In this paper,soil heterotrophic respiration（i.e.organic matter decomposition）data were obtained by using soil respiration data and Bond-Lamberty empirical model.From the perspective of space,the spatial distribution of farmland soil organic matter decomposition in 2000 and 2018was basically the same,and the regions with higher intensity were concentrated in central and western Chongqing,southwest Chongqing and southeast Chongqing where there was higher temperature and more precipitation.The decomposition of these regions in 2000 and 2018 were about 500g/m2.yr and 520g/m2.yr,respectively.In terms of district and county level,Xiushan County（518g/m2.yr）and Dadukou District（533g/m2.yr）had the highest amount of soil organic matter decomposition in 2000 and2018,respectively,while Chengkou County had the lowest amount of soil organic matter decomposition（470 and 483g/m2.yr）.In 2018,the available carbon input of farmland soil of Chongqing was 16,046,400 tons.Assuming that all of this carbon input could be returned to the field,it could theoretically compensate for the carbon loss caused by the decomposition of soil organic matter.（4）The N2O emission flux from nitrogen mineralization caused by soil organic matter decomposition of Chongqing area in 2000 ranged from 0.74 to 15.87g/m2.yr,with an average value of 4.19±0.44g N/m2.yr;Compared with 2000,the mean emission flux in 2018 increased by 5.5%（4.42±0.44g N/m2.yr）.The total emissions in 2018 were113,500 tons,up 1.5 percent from 11,800 tons in 2000.The increase of total emission is smaller than the emission flux,which is caused by the reduction of farmland area.From the perspective of space,the center of gravity of N2O emission generated by nitrogen mineralization caused by the decomposition of farmland soil organic matter is also located in central and western Chongqing,southwest Chongqing and southeast Chongqing,and the emission flux of these regions in 2000 and 2018 are about 4.3 and4.6g/m2.yr respectively.In a high temperature and rainy environment,soil respiration is stronger,organic matter decomposition leads to more nitrogen mineralization,and N2O produced by soil nitrification and denitrification also increases.From the perspective of district and county level,the N2O emission fluxes induced by organic matter decomposition in farmland soils of Chongqing district and county in 2000 and2018 had little difference,and the coefficient of variation was all less than 10%.However,due to the different farmland area,the total emission of each district and county had a big difference.In conclusion,there are some differences in the estimation results of N2O emissions in different years,which proves that the framework model has a certain sensitivity to input parameters. |
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