High-frequency dynamic observation of N₂O emission flux from cropland in the North China Plain

Nitrous oxide (N₂O) is one of the primary greenhouse gases, which depletes ozone sphere and results in strong greenhouse effects. Nowadays, a series of studies on greenhouse gases emissions of vegetation-soil-atmosphere system has been carried out domestic and overseas with the measurement method of static chamber/gas chromatograph, in which gas sampling time is concentrated in a fixed period during daytime rather than around the clock. Moreover, its' sampling frequency is usually longer than one minute inducing difficulty to implement a real-time, high-frequency and continuous determination of greenhouse gas emissions in situ for a long time. As an important source of N₂O emission, farmland ecosystem is a human-disturbed system with fast N recycle and large N₂O emission. To improve N₂O emission observation methods and provide a reference for N₂O change study in the North China Plain, we chose a typical farmland ecosystem, a maize field at the Yucheng Comprehensive Experiment Station, Chinese Academy of Sciences, and monitored N₂O concentration change with a new monitor instrument TGA200A (Trace Gas Analyzer 200A) to achieve a automatically real-time monitoring of N₂O emission for day and night. The TGA200A was equipped with a laser launch tube simultaneously controlled by current and temperature and some related measuring devices. When sample and reference gases (with a known concentration) entered the analyzer synchronously, the target gas concentration was determined by through scanning and comparing the linear absorption wavelengths of laser energy between sample and reference gases. The monitoring was from the middle of June 2017 to September 2017. The results showed, firstly, the N₂O emission was higher in night from 0:00 to 6:00 and 18:00 to 24:00 (0.618-1.171 mg·m^-3) than in daytime from 6:00 to 18:00 (0.526-1.145 mg·m^-3) in fine weather. N₂O emission was facilitated under higher atmospheric temperature in daytime, but the emission peak appeared in 15:00 to 17:00 indicating a significant hysteresis of temperature effect. Secondly, in rainy day, maize field N₂O emissions were increased by 0.033 mg·m^-3 in 3 hours with a proper rainfall and presented an accumulative effect. But if the rain was too heavy for a long time, the N₂O emission would be reduced. Thirdly, N₂O emission was affected by strong wind, but this conclusion needed more verification because such result was not regular. This study demonstrated the data determined by the TGA200A was useful and reliable for study on dynamic change of N₂O emission in different weathers. TGA200A was available in an automatically and real-time monitor of N₂O emission for day and night with different environment elements (temperature, rain, wind) and made it possible to reduce human costs and errors in greenhouse gas flux observation.