Microwave treatment is a green and pollution-free soil disinfection method. The application of microwaves to disinfect soil before cultivation is highly important to increase crop yields and protect the ecological environment. The electromagnetic field is an important parameter influencing the soil temperature field in the process of microwave soil treatment, and the change in soil temperature directly affects soil disinfection. Therefore, this article carried out research on the heating pattern in North China loess due to microwave treatment. First, COMSOL software was employed to simulate the microwave soil treatment process to analyze microwave penetration into soil. Second, with the application of microwaves at the designed frequency produced with a 2.45-GHz tunable microwave generating microdevice, soil with water contents of 0%, 10%, 20%, and 30% was treated for 10~60 s (at 10-s time intervals), and experiments on the influence of the microwave output power, treatment time, and soil moisture content on the soil temperature were performed via the controlled variable method. The simulation results indicate that with increasing soil moisture content, the microwave frequency inside the soil model increases, and the electric field intensity value decreases in the model at the same depth. After microwaves traverse through the 20-cm soil model, the incident field strength is three orders of magnitude lower than the outgoing field strength. The results of the microwave soil treatment experiment reveal that: (1) Compared to microwave output power levels of 1.8 and 1.6 kW, a level of 2 kW is more suitable for microwave soil disinfection. (2) After treatment, the highest temperature occurs on the soil surface, not within the soil. (3) The location of the highest soil internal temperature after microwave treatment increasingly approaches the soil surface with increasing soil moisture content, and the microwave output power does not affect the location of the highest soil internal temperature. Combining the electromagnetic field simulation and microwave soil treatment experiment results, it was found that the higher the field strength is, the higher the temperature value, and the highest soil internal temperature after microwave treatment often occurs at the first electromagnetic wave peak.

Microwave irradiation is a new means of non-toxic, residue-free, and green soil disinfection that prevents and controls soil diseases, insects, and weeds and helps to improve crop quality and yield. Soil microorganisms, as an important part of the ecosystem, are closely related to crop growth and health. To investigate the changes of soil physicochemical properties and microbial communities during microwave soil disinfection for different time periods, phaeozems from northeastern China were selected for microwave treatment at 3, 6, 9, and 12 min, and their physicochemical properties were measured after 30 days of incubation. The test soils (0–20 cm) after 30 days of incubation were used, and high-throughput sequencing was performed to detect changes in their soil microbial structure under different microwave time treatments. Microwave treatment had significant effects on soil pH, nitrate (NO₃^--N), ammonium (NH₄⁺-N), and available phosphorus (AP) content. As shown by the Shannon, Chao, and Ace indices, microwave treatment at 3 min had the lowest effect on bacterial diversity compared to the control treatment (CK). Shannon index decreased by 9.92%, 24.56%, 34.37%, and 38.43% after 3, 6, 9, and 12 min microwave treatments, respectively; Chao index decreased by 7.69%, 18.13%, 32.21%, and 57.91%, respectively; Ace index decreased by 6.40%, 6.98%, 20.89%, and 52.07%, respectively. The relative abundance of beneficial soil microorganisms Micromonospora, Fictibacillus, Paenibacillus, and Bacillus (Firmicutes) increased significantly compared to CK. The results indicated that although the microwave treatment altered the soil microbial community, beneficial soil microorganisms showed faster recovery. In addition, pH, soil organic carbon (SOC), total nitrogen ratio (C/N), soil-available phosphorus (AP), and NO₃^--N were important factors affecting bacterial community diversity and composition following microwave treatment, and bacterial community composition was driven by soil chemical properties such as soil pH, SOC, C/N, and NO₃^--N. Microwave treatments at different time periods affected soil microbial community structure to different degrees, and soil bacteria of copiotrophic taxa (e.g., Firmicutes) were relatively higher than the control. Overall, microwave treatment from 3–6 min may be more suitable for soil disinfection. The study of the effect of microwave on soil physicochemical properties and bacterial microbial community not only provides some scientific reference for the rational application of microwave soil disinfection, but also has positive significance for soil-borne disease control and crop quality improvement.

Soil disinfection is an important agronomic measure to prevent soil-borne diseases, insects, weeds and other hazards. Based on the premise of being environmentally friendly, microwave soil disinfection can improve crop yield and quality in a pollution-free, residue-free and green way. The law of microwave soil heating is the theoretical basis of microwave soil disinfection. Therefore, in this paper, loess soil of North China and black soil of Northeast China are used as test materials to explore the law of soil heating under the action of microwaves. First, COMSOL Multiphysics software was used to simulate the temperature field change in the microwave-irradiated soil, and a simulation model of the temperature field of the microwave-irradiated soil was constructed to analyze the effective working range and temperature distribution characteristics of the microwave-irradiated soil. Second, using the 2.45 GHz microwave treatment, the following conditions were tested: soil moisture conditions of 10%, 15%, 20% and 23% (within the natural moisture content range). The loess and black soil were treated by microwave irradiation for 1~12 min, respectively (1 min/time increment). A single-factor experiment was designed to explore the influence of these factors on the soil heating law. The results show that the two soil surface temperatures are positively correlated with the soil moisture content, both of which satisfy T_surface23% > T_surface20% > T_surface15% > T_surface10%, and the surface temperature of black soil is higher than that of loess. According to the experimental results of the internal temperature distribution of loess and black soil irradiated by microwaves, the surface equations of “irradiation time–soil depth-soil temperature” and “irradiation time–soil moisture content–soil temperature” were constructed by surface fitting. When the irradiation time and moisture conditions are the same, the average temperature inside the irradiation area satisfies $\overline{T}$_{α black soil} > $\overline{T}$_{α loess}. The results of long-term microwave soil heating preliminarily confirmed the feasibility of microwave soil disinfection and the optimal conditions of microwave irradiation in loess of North China and Northeast black soil, which provides a certain reference for the study of soil-borne diseases inactivation at high temperature.