Preliminary Study Of Processes Of Nitrous Oxide Emission From Soil & Plant In The Northeast Farmland

N₂O is a kind of very important greenhouse gas.It has damaging effects on the ozone layer and increases UV-B radiation of global surface.However,the estimating of sources and sinks of atmospheric N₂O is still a lot of uncertainty,because the estimating of the strength of certain source is insufficient or certain important source was not found.Soil is the most important source of the N₂O.Nitrification and denitrification are the two uppermost micrological processes for the emission of nitrous oxide in soil,but they have tremendous difference in the contributions to the emission of nitrous oxide.The major factors influencing the emission process of nitrous oxide in soil include soil water content,soil temperature,soil NH₄⁺-N and NC₃^--N content, soil organic matter content and soil pH,etc.It is of great importance to estimate and predict the nitrous oxide flux that distinguishing the source of the N₂O via nitrification and denitrification and understanding how the change of environmental factors affects the emission of nitrous oxide.CHEN guan-xiong firstly reported that the plants in its normal physiological conditions can release N₂O.This important discovery provided a new research orientation to estimate the biological N₂O source.This paper mainly studies the emission proceses of nitrous oxide via nitrification and denitrification in the soil of farmland in Northeast China,and the effect of environmental factors on the emission process of nitrous oxide.Here the dry farmland soil is taken as the research object and the acetylene inhibition technique and the oxygen inhibition technique as experimental methods,the acetylene to inhibit the autotrophic nitrification,the pure oxygen to inhibit the denitrification.This experiment also measures the N₂O emission from soybean and soil under field condition.It studied preliminarily the contribution of soybean to N₂O emission from the soil-plant system in field by applying the gas chromatography and isotope ratio mass spectrometry.The experiments include two parts:one is the emission proceses of nitrous oxide emission via nitrification and denitrification with the environmental factors controlled.The factors include the soil temperature,the soil water content,the avaible nitrogen(NO₃^-,NH₄⁺)content,the phosphorus-to-nitrogen ratio(P/N)in the additional matter,the additional organic matter content.In the first part,the incubation experiments were conducted with soil collected from the Shenyang station.Then the N₂O emission rate,the contribution ratio of nitrification and denitrification to nitrous oxide emission,the N₂O conversion ratios and the functions of the influencing factors on the N₂O conversion ratios are elicited.The other one is measuring the N₂O emissions from soybean and soil under field condition.It preliminarily studied the contribution of soybean to N₂O emission from the soil-plant system in field by applying the gas chromatography and isotope ratio mass spectrometry. The results and the conclusions of the experiment are shown as follow:The temperature influences the activities of the nitrifiers and denitrifiers.The N₂O emission rate and the N₂O conversion ratio via nitrification and denitrification increase when temperature rised from 4℃to 33℃.The N₂O emission rate and the N₂O conversion ratio via denitrification also increase within the temperature range from 4℃to 33℃.According to the statistics,it shows that the N₂O emission rate via denitrification correlates positively with the temperature;from 4℃to 33℃,the N₂O contribution ratio of nitrification does not correlate significantly with the temperature.In the scope of 10%～140%WFPS,the N₂O conversion ratio via denitrification correlates positively with the soil water content.When the soil water content exceeds 80%WFPS,the contribution ratio of denitrification to the N₂O emission is higher than nitrification.In the scope of 100%～140%WFPS,the soil pore is full of water and the soil is in the complete anaerobic environment,so the N₂O emission via nitrification can be neglected.When NH₄⁺-N is added,it promotes the N₂O emission via nitrification and denitrification.When the addition rate is 25mg/kg,the N₂O emission rate and the N₂O conversion ratio via nitrification are the largest, respectively.In the range of 0～25mg/kg NH₄⁺-N,it promotes the N₂O emission rate and the N₂O conversion ratio via nitrification and denitrification.According to the statistics,the N₂O conversion ratio via denitrification correlates positively with the addition NH₄⁺-N;the N₂O conversion ratio via nitrification does not correlate with the addition of NH₄⁺-N,the N₂O conversion ratio via nitrification correlates apparently with the addition of NO₃-N;The N₂O conversion ratio via denitrification does not correlate with the addition of NO₃^--N.When the phosphorus-to-nitrogen ratio of the added matter is enlarged,it will promote the nitrification activity.According to the statistics,the N₂O conversion ratio via nitrification and correlates significantly with the addition of the phosphorus-to-nitrogen ratio,but the N₂O conversion ratio via denitrification does not correlate with the addition of the phosphorus-to-nitrogen ratio.When the phosphorus-to-nitrogen ratio of the added matter is enlarged,it will promote the N₂O emission of the farmland soil.According to the statistics,it shows that the N₂O conversion ratio via nitrification correlates negatively with the addition of the stalk,the N₂O conversion ratio via denitrification dose not correlate with the addition of the stalk.When the stalk is added,it promotes the total N₂O emission rate.It shows that it can promote the N₂O emission of the farmland soil by increasing the addition of the stalk.The N₂O emission from soybean was studied in the paper.The results show that:there are two emission peaks which occurred at flowering and autumn stages of plants,and then the N₂O flux from soybean and soil are declining.During the soybean's growing period,the contribution of soybean to N₂O emission from the soil-plant system is analysed using GC and IRMS.The results shows that:the contribution of soybean to N₂O emission from the soil-plant system accounts for 20-49%using GC,but the contribution of soybean to N₂O emission from the soil-plant system accounts for 3-70%using IRMS.The results of two methods are different,but the trends that the contribution of soybean to N₂O emission from the soil-plant system are similar.It proves that plant is an important role in the contribution of N₂O emission from the soil-plant system.