The Investigation Of Fluorescence Lidar And Its Quantitative Monitoring Of Crops Nitrogen Stress

Nitrogen is an essential element of chlorophyll,protein,genetic material and other organic molecules in plants.It is closely related to crop yield and product quality.Thus,the increase of nitrogen fertilizer can effectively increase the crop yield.However,excessive use of nitrogen fertilizer will not only cause a serious waste of resources,but also cause serious environmental pollution.Therefore,effective monitoring of crop nutrition stress is particularly important.Remote sensing technology is widely used in monitoring agricultural production,crop nutrition stress and other related aspects,for its observation is fast,large-scale,and highly precise.With the continuous development of technology,higher and higher requirements have been put forward for vegetation,especially crops monitoring,along with the higher requirements for the development of technology.In order to achieve the monitoring of the growth status and physiological characteristics of vegetation,especially crops,it's necessary to obtain the crop canopy's spatial and spectral information.Currently,passive remote sensing images can obtain the crop's spectral information,and provide guidance for its growth state from the qualitative point of view.As a kind of active remote sensing detection method,Lidar can obtain fine three-dimensional spatial information of canopy.Compared with the reflectance spectrum,laser-induced fluorescence(LIF)technology is widely used as a new remote sensing technique in crop growth status and early monitoring of nutrient stress,displaying great application potential.At present,the LIF technique mainly uses the fluorescence parameters related to the fluorescence kinetic curve in the detection of crop stress.However,this technique requires dark processing of samples,thus limiting its application in remote sensing.Compared with the fluorescence kinetic curve,the fluorescence spectrum of the vegetation is directly extracted by laser excitation,which can realize large-scale detection of remote sensing application.Since the fluorescence signal is based on the release process within the vegetation,it has a higher sensitivity and monitoring accuracy.In this paper,by combining the advantages of LIF technology and Lidar detection,the fluorescence Lidar is proposed and applied to the quantitative monitoring of nutrient stress,especially nitrogen of rice.Therefore,this paper mainly carries out the following related theoretical methods and technical research:1)The relationship between the fluorescence characteristic parameters and nutrient stress was analyzed by the FluorMODleaf model,based on the physical mechanism of LIF.The feasibility of using LIF parameters to monitor vegetative stress was studied,providing a theoretical basis for applying fluorescence on monitoring crop nutrient stress.In order to study the nitrogen content based on fluorescence characteristics,the relative algorithms were selected for fluorescence spectral information processing and quantitative inversion of nitrogen content.2)The effect of different nitrogen application levels on the fluorescence characteristics was studied.The correlation between nitrogen application levels and fluorescence parameters was analyzed to determine the feasibility of fluorescence parameters for monitoring the nitrogen nutrition stress.Comparative experiments of fluorescence and reflectance spectra were carried out to demonstrate that the performance of LIF technique is superior to that of the reflectance spectra for nutrient stress prediction.This provides experimental support for the further application of LIF technology.The correlation between the fluorescence parameters obtained at different excitation wavelengths and the nitrogen content of the leaves is studied,providing a reference for the excitation wavelength selection in applying fluorescence Lidar for crop nutrient stress monitoring.3)The key technologies of fluorescence Lidar were discussed in this part,including the transmitting unit,the receiving system,the weak signal detection technology,and the design of simultaneous acquisition of fluorescence information and spatial information.These investigations established a technical foundation for studying the prototype system.Then,for the fluorescence Lidar system that can simultaneously acquire the target's fluorescence signal and spatial information,the effect of system parameters such as detection angle and distance between the targets and system on the fluorescence intensity was analyzed,and the corresponding correction models were also established.In addition,scanning experiments were carried out using the built system to detect vegetation scenes,which verified the performance of the system to measure the fluorescent signal and spatial information at the same time,providing experimental support for the further development of the fluorescence Lidar system.4)Based on the proposed fluorescence Lidar system,studies on the qualitative and quantitative inversion of nitrogen nutrition stress for rice was conducted based on consecutive experiments for years.The performance of fluorescence parameters on nitrogen levels and nitrogen content monitoring was analyzed.The results showed the potential of fluorescence Lidar in quantitatively monitoring crop nitrogen nutrition stress.In addition,the influence of fluorescence parameters and algorithms on the quantitative monitoring of nitrogen nutrition stress in rice was analyzed,and the statistical model of quantitatively inversion of nitrogen content was established.The monitoring of nitrogen stress in crop nitrogen was studied by fluorescence Lidar,providing reference and experimental support for the system's further improvement.