Effect Of Nitrogen Fertilizer And Straw Returning On CO₂/CH₄Emission From Farmland In Guanzhong Irrigation District Of Shaanxi

With the concentrations of atmospheric greenhouse gases increasing, global warmingattracts more and more attention. Farmland ecosystem which is a main carbon pool on earthand which is most frequently disturbed by human beings has become one of the main sourcesof GHG. Nitrogen fertilization and straw returning work, in some degree, in increasing thecrop yield and in fertilizing soil. To discuss the effect of factors like nitrogen application rate,straw returning, temperature and moister etc to CO₂and CH₄emission, winter wheat andsummer corn rotation ecosystem in Guanzhong irrigation district in Shaanxi province is usedas the research object, and static chamber-gas chromatographic technique is utilized as the insite monitoring to CO₂and CH₄. There are five treatments as below: No N （CK）, Low N,Optimum N, Farmers practice N, Optimum N+straw returning. The main results showed asfollowing:1. CO₂emission changes apparently in different seasons. The average emission fluxes ofeach treatments on winter wheat （N75，N150，N190，N150+S） in growing season are:114.1,123.5,123.6,145.6mg CO₂-C·m^-2·h-1. And the average emission fluxes of each treatments onsummer corn （N0, N90, N180, N280、N180+S） are:138.6,145.4,167.5,163.2,210.6mgCO₂-C·m^-2·h-1. CO₂emission flux is related to atmospheric temperature and soiltemperature. They are very significantly positively related to CO₂emission fluxes in alltreatments on winter wheat. And atmospheric temperature and soil temperature aresignificantly positively related to CO₂emission fluxes on summer corn after the plants werebroken. However, the effect of soil moisture on CO₂emission flux is is more complicated,there is no significant correlation between them.2. Farmland in Guanzhong irrigation district in Shaanxi province is a sink of CH₄. Thetotal absorbing capacity in a year is around426-490mg CH₄-C·m^-2. And the absorbingcapacity in wheat season is higher than that in corn season. Moisture is the key to theemission of CH₄. CH₄emission flux is very significantly positively related to soil moisture when the soil moisture is relatively high. If soil moisture increase to some degree （21%approximately）, farmland will be an emission source rather than an absorption sink, as it usedto be. No significant relationship between temperature and CH₄emission flux has been foundin this experiment.3. Application of nitrogen fertilizer leads to more CO₂emission in the farmland. Thetotal emission of each treatments on winter wheat （N75, N150, N190, N150+S） are:563,604,608,715gCO₂-C·m^-2，and of each treatments on summer corn （N0, N90, N180, N280,N180+S） are:379,391,445,433,557gCO₂-C·m^-2. In some extent, more N application leadsto more CO₂emission in the farmland, but when N application increases to some certaindegree, the growing of CO₂emission ceases. Compared with no N, nitrogen application of165kg·hm^-2,330kg·hm^-2,470kg·hm^-2can increase carbon sequestration in soils by46%,21%,96%respectively. No significant difference has been found between CH₄absorption amountsin different N rates.4. Straw returning increases CO₂emission in the farmland significantly. To winter wheatand to summer corn, the increase amplitude is separately18%and25%, and21%as the totalincrease amplitude in one year. But at the same time straw returning increased carbonsequestration by133%; No significant effect between straw returning and CH₄absorption hasbeen found.