Study On Dynamic Monitoring Of Soil Biological Activities Based On Acoustic Emission

As a kind of ecological treatment technology,sewage land treatment system with low cost,easy operation,good sewage treatment effect,and other advantages is of great significance in the large towns and rural areas.Soil is the cornerstone of land treatment system,and good soil structure plays a fundamental role in improving the sewage purification capacity.Soil structure is highly dynamic,mainly caused by the continuous biological activities within the soil,and the monitoring of soil biological activities is the key to understanding soil structure changes.However,biological activities within the soil are hard to observe directly,making it difficult to study the soil structure.Thus,studying new techniques for monitoring soil biological activities and mastering the changes in soil structure has become an urgent need in the development of sewage land treatment technology.This study focused on the difficulty in monitoring biological activities within the soil and developed the acoustic emission(AE)monitoring method based on the large number of AE signals generated by these biological activities,allowing continuous monitoring of soil biological activities without damaging the soil structure.The study firstly analyzed the AE signals generated during the earthworm movement and root growth and their acoustic characteristics through small pilot experiments.The AE method was demonstrated to be effective for monitoring biological activities in the soil with the aid of time-lapse photography and particle image velocimetry.A soil system that simulated the soil environment under natural conditions was further constructed,with roots as the object of study,for long-term continuous monitoring of soil temperature,humidity,and AE signals,to master the acoustic characteristics in the complex soil system.By means of multivariate statistical analysis such as principal component analysis,AE signals originated from different biological activities were identified,and the in-situ,non-destructive,and instantaneous monitoring of soil biological activities in the complex soil system was achieved.The main conclusions of this study are as follows:(1)There are significant differences in acoustic characteristics of earthworm movement and root growth in the soil.Experimental results showed that the average daily AE rate for earthworm movement recorded by the AE sensor was 11940 AE/day.In comparison,the average daily AE rate for root growth reached a maximum of 2720 AE/day,implying that the daily AE rate for soil animal's movement was an order of magnitude higher than that of root growth.(2)The AE method can realize dynamic monitoring of soil biological activities by capturing and analyzing AE signals.The earthworm movement mainly occurred in the deeper soil;AE events generated by earthworm movement were predominantly associated with the construction of new burrows in the early stage,while later were associated with the earthworm displacement.The root growth rate was fast to slow process that gradually reached the platform,correlated with the trend of daily AE rate.Water movement in the soil also produced AE but with low strength that was negligible compared to those generated by the root growth process.(3)The monitoring of AE signals in the soil is independent of changes in soil temperature but is significantly related to soil humidity changes.The increasing cumulative number of AE events usually occurred after the rainfall,when the roots absorbed water and grew,causing the mechanical movement of soil particles.In this process,soil humidity decreased linearly due to water absorption in plant roots,and the decrease in soil humidity reflected the activity of plant roots.Soil temperature was mainly influenced by the external environment,and it was not related to the generation of AE events in the soil.(4)AE signals in complex soil system come from a combination of processes such as soil biological activities and mechanical movement of soil particles.Multivariate statistical analysis can realize the identification of AE signals from different sources in the signal set.AE signals with characteristics of low frequency,high amplitude,high energy and strength,and long duration represented the roots activity such as roots extension;AE signals with high frequency,low energy and strength,and short duration represented water movement in the soil and the subsequent mechanical movement of soil particles.