Research On Infrared Spectroscopy Sensing Method Of Soil Respiration And Monitoring System Design

Soil respiration, as the world’s biggest carbon library and an important part of carbon cycle process in terrestrial ecosystems, can be always used as the important biological indicator of soil quality, soil fertility and the indicator of soil health. Therefore, the study and real-time monitoring of soil respiration is not only the key to accurately assess the global carbon balance of payments, but also the great significance for the current carbon cycle research and slowing the progress of global greenhouse effect. Existing soil respiration monitoring systems, such as Li-8100 and Li-6400 made in LI-COR company, are expensive and have a poor portability, which largely restrict its popularization and development. Based on the above-mentioned disadvantages, the main objective of this thesis is to design a soil respiration monitoring system with good stability and repeatability, low cost, convenient usage and accurate measurement results.First, this thesis studied the infrared spectroscopy sensing method, the measurement method of peak height provides a theoretical basis for quantification of the gas sensor; long optical path theory provides theoretical basis for innovative design of gas sensor chamber. And the soil respiration experiments proved that the main component of releasing gas of soil respiration is carbon dioxide. Through the comparative analysis between gas infrared spectrum and NIST spectrum library, the characteristics wavelength (4.26μm) for the quantitative analysis of carbon dioxide was selected, which provides technical basis for the subsequent gas sensor design.Second, through the guidance of previous theoretical and technical research, the core component of soil respiration monitor system, a soil respiration gas sensor based on NDIR technology with long optical path was design The sensor adopted the difference measuring method of single light path and double wavelength, which, to a large extent, reduces the instability of infrared light source and the optical path interference. The sensor chamber adopted multiple reflections to improve optical path and coating technology, which improves the detection ability and sensitivity of monitoring system. For hardware design, the sensor combined with peripheral circuit constituted the core circuit board of soil respiration monitoring system. For software design, the method combining hardware filtering with software filtering was adopted to eliminate signal interference and improve the measurement precision of the system. Additionally, the thesis proposed a method of measuring the soil respiration rate and wrote the supporting system software.Finally, the self-designed gas sensor with long optical path based on NDIR technology was applied to soil respiration monitor system and the system was assembled; the field experiment indicted that the system design is correct.