Advances in oxygen detection technologies and methods for the root zone soil

Continuous and stable oxygen supply is very necessary for plant tissues to maintain their normal metabolism. The evaluation of oxygen environment in the root zone can also serve to assess whether or not trees and plants are experiencing hypoxic stress. Research on detection and analysis technology is of great significance for the development of agroforestry and the environmental conservation. This study aims to summary the detection and analysis strategies of soil oxygen distribution. The current technologies were also reviewed to obtain the spatial and temporal distribution of soil oxygen in the root zone, such as the fluorescence imaging, the oxygen sensor point detection, and the soil oxygen diffusion analysis. The future directions were clarified on the root zone soil oxygen detection. The buried oxygen sensor was utilized to monitor the oxygen environment in the soil. Three categories of detection were divided: the zirconia, the electrochemical approach, and the fiber optic sensing. It is difficult to capture the oxygen content of the entire root zone, leading to disrupt the structure of the soil and then cause harm to the root system of the plant. However, the high level of accuracy was obtained for the detection of oxygen concentrations and time resolution. It is possible to acquire the soil oxygen concentration gradient in a short amount of time by modelling soil oxygen diffusion. According to the physical and chemical features of the soil, the gradient can also be obtained to estimate the soil oxygen environment where the plant roots are located. The structure and parameters of the model were closely connected to the chemical and physical characteristics of the soil. It is very necessary to modify the model suitable for the specific situation using the reliable input data from the soil sampling spots. The fluorescence in-situ imaging approach can be used to acquire the real-time distribution of the oxygen concentration in the soil rhizosphere. The non-invasive monitoring can also be employed to uncover the interactions that occur between organisms and the environment in the similar natural conditions. The procedure can be carried out in a laboratory setting to predict the field. The latest advances were compared under the different applicable scenarios in the field of oxygen sensors combined with various types of oxygen transport models. The hot topics and development directions were finally summarized on the detection techniques and models of soil oxygen distribution in the root zone: 1) Oxygen sensors can be the promising trend of miniaturization and integration, and the new types of materials can also be introduced to improve the sensitivity and durability of the oxygen sensors; 2) The accuracy of the models can be continuously improved to consider the multi-scale and multi-factor interactions under the complex soil oxygen environment in the root zone; 3) Sustainable fluorescent dyes can be developed to improve the resolution and depth of fluorescence imaging for the clear imaging of smaller and deeper structures; 4) The water-oxygen relationship can be further analyzed in the plant transpiration. The physiological adaptive capacity of plants can be used to reflect the oxygen environment in the root zone using the oxygen concentration in the xylem. The finding can provide a strong reference for the soil oxygen environment in the root zone of plants.