基于同位素特征的华北平原菜地N_2O排放监测中取样时间的探讨

Preliminary research on N_2O sampling time based on isotopomer signature measurement of vegetable fields in the North China Plain

【目的】为了长期监测土壤释放N_2O的通量和同位素变化规律,了解产生N_2O的微生物过程,提高对N_2O排放量和排放系数估计的准确性,需要对N_2O的日变化规律做深入研究,以便获得具有代表性的取样时间点及密闭时间。【方法】采用田间原位试验对华北平原的莴苣菜地进行了N_2O排放监测,选取N_2O排放高峰期即施肥灌溉后5 6 d为监测时间段,采用静态气体箱收集土壤释放的N_2O气体,结合气相色谱和质谱技术测定N_2O的含量及其同位素值(δ^(15)N-N_2O,δ^(18)O-N_2O和SP)。试验设2种取样间隔,即2 h和10 min,分别对N_2O日变化规律和密闭时间进行研究。【结果】1)在莴苣菜地N_2O排放高峰期内,N_2O通量日变化范围为34.65 131.45μg/(m^2·h),最大和最小的通量分别发生在13:00和次日5:00,9:00的N_2O通量为83.66μg/(m^2·h),与日通量平均值82.81μg/(m^2·h)相接近。N_2O通量产生日变化的原因与土壤温度有关,相关分析表明,N_2O通量与地下5 cm处土壤温度呈显著正相关(R2=0.82,P<0.01),而与土壤充水孔隙度(WFPS)无显著相关性。2)24 h内,δ^(15)Nbulk-N_2O和δ^(18)O-N_2O随着时间呈现先降低后增加的变化趋势,变化范围分别为-31.22‰-11.09‰和-7.45‰-0.68‰;SP值随时间呈现先增加后降低的变化趋势,变化范围为16.13‰26.41‰。N_2O各个同位素值随时间的变化表明产生N_2O的微生物过程随之变化,但SP值在9:00 17:00较稳定,变化范围为23.26‰26.21‰,极显著高于其他时刻(P<0.01),表明硝化作用在这一时间段内对N_2O的产生起主导作用。3)扣箱40min后,N_2O含量、δ15N-N_2O和SP值都达到稳定状态,因此选取40 min作为单次观测N_2O含量和同位素变化的密闭时间。4)24 h内N_2O通量加权SP值为22.54‰。根据前人总结的规律,本研究中N_2O主要由细菌硝化作用产生,且估计60.92%的N_2O来自于细菌硝化作用,39.08%的N_2O来自于反硝化作用。【结论】华北平原莴苣菜地的N_2O通量和同位素值具有较大的日变化,综合N_2O通量和同位素值,建议选取9:00作为观测莴苣菜地N_2O排放通量和同位素特征值变化规律的时刻,建议静态气体箱密闭时间为40 min。

【Objectives】Emissions of N_2O from vegetable soils are extremely variable in time and therefore there are considerable uncertainties in calculating emission of N_2O flux and emission coefficients. It is thus important to investigate the optional sampling time and characteristic of diurnal pattern for N_2O. More insights into N_2O diurnal variation would also be helpful for long term monitoring of N_2O release and for understanding the associated microbiological mechanisms.【Methods】Field experiment in situ was conducted to study N_2O emission after urineamendment and subsequent irrigation for a lettuce production system in North China Plain in 2012. During periods of high N_2O fluxes in 5- 6 days,N_2O emission was collected manually by a closed static flux chamber at two intervals,two hours for diurnal variation and 10 minutes for appropriate enclosure period. N_2O concentrations were measured by a gas chromatograph equipped with an electron capture detector（ GC- ECD）,and isotopomers such as δ-（15）N- N_2O,δ-（18）O- N_2O and site preference（ SP）（ difference between the center and the end N atom of the N_2O asymmetric molecule） were analyzed and obtained by an elemental analyzer-isotope ratio mass spectrometer（EA- IRMS） system.【Results】1） During the period of high N_2O emission,there was a strong diurnal variation in N_2O fluxes ranging from 20. 41 to 130. 45 μg /（ m-2·h）,with the maximum and minimum values being observed at1 pm in Apr. 27 thand 5 am in Apr. 28-（th）,respectively. Flux measured at 9 am could be used to represent the daily average of the whole day. The fluxes of N_2O emitted from the soil were correlated significantly（ R2= 0. 82,P 0. 01） with the soil temperature at 5 cm,varied markedly across time,but insignificantly with water filled-pore space（ WFPS）. 2） Diurnally,the values of δ-（15）Nbulk- N_2O and δ-（18）O- N_2O decreased first and then increased gradually with time while the values of SP had an opposite trend,suggesting that production processes of N_2O were associated with different microbial pathways. Conversely,the stable values of SP of N_2O from 9 am through 5 pm indicated that nitrification was the dominant pathway for N_2O production. 3） In measuring N_2O emission,appropriate enclosure period in the static chamber should be 40 minutes,which was long enough to allow detection of N_2O concentration,δ-（15）N- N_2O and SP and also short enough to avoid feedback of N_2O concentration and microbial pathway change. 4） The calculated emission-weighted site preference was 22. 54‰. The N_2O emission in this study was dominantly produced from nitrification,contributing 60. 9% of the daily totals of N_2O emission.【Conclusions】There were large diurnal variations of N_2O concentration and isotopomers from lettuce production system in North China Plain. Considering both values of N_2O flux and isotopomers,it was suggested that 9 am was an appropriate time to measure the flux and isotopomer signature of N_2O and that enclosure period for one sample event should be 40 minutes.