Introduction: The measurement of soil magnetic susceptibility is rapid, nondestructive, and highly sensitive and has, therefore, been widely applied in soil research. In soil systems, the relationship between environmental factors and magnetic susceptibility is complex and interferes with the interpretation of magnetic susceptibility data. Therefore, clarifying the effects of soil factors on magnetic susceptibility and their mechanisms is necessary to explain changes in magnetic susceptibility. Magnetic characterization of tropical forest soils, which is primarily indicative of the climate, has been relatively poorly studied. Therefore, the magnetic characteristics of tropical forest soils and their correlations with soil physical and chemical properties must be systematically studied. Here, we describe a tropical rainforest soil profile that has not been disturbed by human activities and its patterns of low-frequency magnetic susceptibility (χ_lf), frequency-dependent magnetic susceptibility (χ_fd%), and susceptibility of anhysteretic remanent magnetization (χ_ARM).Methods: Soil sampling was conducted in six plots in Xishuangbanna, China. Soil profiles were explored in 11 layers, and various soil properties were measured. Magnetic susceptibility was assessed using susceptometry, and structural equation modeling was used to analyze the relationships between soil factors and magnetic susceptibility.Results: In all profiles, the values of χ_lf and χ_ARM increased with depth from 0 to 30 cm, decreased with depth from 30 to 80 cm, and tended to stabilize below 80 cm. χ_fd% values increased with depth from 0 to 80 cm and decreased with depth below 80 cm, with particularly rapid attenuation at the bottom of the F1 and F6 profiles. Soil properties were determined, including bulk density, water content, electrical conductivity (EC), soil particle size, soil organic carbon (SOC), pH, free iron (Fe_d), poorly crystalline iron (Fe_o), total iron (Fe_t), total nitrogen (TN), and the primary chemical elements P, S, K, Si, Al, Mn, Mg, and Zr. Linear regression and structural equation modeling were used to explore the relationships between soil factors and χ_lf, to identify the main factors influencing the vertical distribution of soil χ_lf, and to analyze the processes and mechanisms by which various factors affect χ_lf. The results showed that the positive influence path followed EC → SOC → TN → Fe_o → χ_lf, and the negative influence path followed pH → N → Fe_o → χ_lf. Fe_o was the most important factor that directly affects χ_lf (β = 0.952). The ferrihydrite in Fe_o forms fine-grained magnetite and maghemite via the aging process, which is the main mechanism for increasing χ_lf in the 0–80-cm layer in this area. TN had an important effect on χ_lf because it affects Fe_o (β = 0.85).Discussion: SOC, pH, and EC indirectly affect χ_lf by promoting the conversion of ferrihydrite and silicate-bound iron to Fe_o. This study highlights the role of forest soils in storing SOC and nutrients, contributing to the ecological management of forest soils.

Magnetic studies of loess-paleosol sequences have shown that the magnetic properties of loess and its parent material is sensitive to weathering degree. However, the relationship between magnetic properties and weathering processes in red soils of southern China remains unclear due to the diversity of parent rocks and the influence of climate. This study focused on red soils developed on weakly magnetic sandstone, in tropical and subtropical regions of Yunnan, China. The principal element content, magnetic susceptibility, temperature-dependent susceptibility curve(ꭓ-T curves), hysteresis loop and Isothermal Remanent Magnetization (IRM) of the samples were determined, the weathering characteristics and magnetic properties of profile samples were analyzed, and their relationship was examined, as well as the impact of climate change on fine-grained ferrimagnetic minerals and magnetic properties. The results revealed that red soils developed on weakly magnetic sandstone exhibited significant correlations between three magnetic indices, low-frequency magnetic susceptibility (χ_lf), frequency-dependent susceptibility (χ_fd),(χ_post−pre + χ_d)/χ_para, and the silica-sesquioxide ratio (SAF), indicating that these indices can be used to assess soil weathering. Additionally, this study proposed a new index based on the 1 < Log10B_h < 1.5 mT component obtained from decomposition of the IRM curve, which can also be used to assess soil weathering. The enhancement of magnetic properties in red soils along a single profile was found to be related to the increase in the content of fine-grained ferrimagnetic com-ponents controlled by weathering. Comparing the two climatic regions, it was found that in Yunnan, China, as the climate changes from subtropical humid to tropical humid, the weathering of the red soil profiles becomes stronger, and the fine-grained ferrimagnetic components increase, indirectly enhancing the magnetic properties of red soils.