Haixia Li

Despite extensive research on soil organic carbon in karst regions, the synergistic changes in multiple carbon fractions and their stabilization mechanisms across a complete rocky desertification gradient remain poorly understood. To clarify how soil carbon pools and their drivers change during karst rocky desertification, we selected Kaiyuan City, Yunnan Province, China, as the study area. Total carbon (TC), soil organic carbon (SOC), and their related fractions, including particulate organic carbon (POC), mineral-associated organic carbon (MAOC), iron-bound organic carbon (Fe-OC), calcium-bound organic carbon (Ca-OC), and soil carbon isotopic composition (δ¹³C), were analyzed under different degrees of rocky desertification. SOC and TC followed a nonlinear pattern: increasing from no to potential desertification, decreasing at light and moderate stages, and rising again at the severe stage, indicating a phased response rather than a monotonic decline. POC was lowest under no rocky desertification and increased significantly after desertification occurred, reaching its maximum at the severe stage. MAOC peaked at the potential stage. With increasing rocky desertification severity, POC/SOC increased from no to moderate stages and then slightly decreased, whereas MAOC/SOC generally decreased. Fe-OC and Ca-OC were lowest under no desertification and increased after desertification occurred, pointing to enhanced mineral protection. Soil δ¹³C values under moderate and severe desertification were higher than under no, potential, and light desertification, implying intensified decomposition and a relative increase in C₄ plants. Mean weight diameter (MWD) and geometric mean diameter (GMD) did not differ significantly among rocky desertification stages (p > 0.05). In contrast, fractal dimension (FD) differed significantly only between the light and moderate stages (p < 0.05). Correlation and redundancy analyses showed that soil water content, bulk density, and porosity were the key factors driving variation in SOC and its fractions. These findings provide both a theoretical basis and practical guidance for soil restoration and ecological management in karst faulted basins affected by rocky desertification.

In this study, representative soil profiles developed on clastic rock parent materials in Yunnan Province were investigated to elucidate the formation mechanisms of soil magnetic properties under weakly magnetic parent material conditions and to evaluate the response of magnetic enhancement to chemical weathering and pedogenic differentiation. A combination of environmental magnetic measurements, bulk geochemical analyses, weathering index calculations, and ternary diagram discrimination was applied to characterize soil magnetic behavior, magnetic grain size distribution, and chemical weathering processes. The results show that the clastic rock parent materials exhibit overall low magnetic intensities, with low-frequency magnetic susceptibility (χ_lf) ranging from 2.543 × 10⁻⁸ m³/kg to 595.652 × 10⁻⁸ m³/kg. Under this weakly magnetic background, soils in the study area display pronounced pedogenic magnetic enhancement, with magnetic parameters showing clear and systematic vertical differentiation along soil profiles, indicating that soil magnetic signals are primarily controlled by pedogenesis. The frequency-dependent susceptibility (χ_fd%) generally falls within the range of 5.403%–17.574%, with a mean value of 12.898%, suggesting a substantial contribution from fine-grained magnetic particles. Magnetic grain size diagnostics further indicate that newly formed superparamagnetic (SP) and stable single-domain (SSD) particles generated during pedogenesis dominate the magnetic enhancement signal. The results of the Chemical Index of Alteration (CIA) indicate that approximately 78% of the profiles reach the strong weathering category (CIA > 85), while only 22% fall into the moderate weathering category (CIA: 65–85). Correlation analyses further reveal that grain-size-sensitive magnetic ratios (e.g., χ_fd%, χ_ARM/SIRM) exhibit a strong correspondence with chemical weathering intensity indicators. These findings suggest that, under weakly magnetic parent material conditions, pedogenically induced magnetic enhancement can be more readily identified and quantitatively assessed. The integration of environmental magnetism and geochemical approaches, therefore, provides a robust framework for investigating pedogenic differentiation and supports high-resolution paleoenvironmental reconstruction in regions dominated by weakly magnetic parent materials.

Active iron (Fe _o ) plays a vital role in soil health and sustainable agriculture. However, the impacts of various land‐use practices on Fe _o remain obscure. This study thoroughly investigated these effects, providing a scientific foundation for soil management and agricultural strategies. This study compared soil Fe _o content between a tea plantation and a maize field in Yunnan, China. Additionally, the physicochemical properties, elemental content, and microbial community characteristics were analyzed. Structural equation modeling (SEM) was employed to assess factors influencing Fe _o . Soil acidification in the tea plantation extended to a depth of 140 cm. Fe _o content in the maize field surface layer (0–30 cm) was significantly higher than that in the tea plantation. Primary factors influencing Fe _o content in both soils included soil organic carbon (SOC), soil water content (SWC), pH, and redox potential (Eh), with microbial activity playing a lesser role. In the tea plantation, the contribution of SOC to Fe _o decreased by 3.86%, while the negative effect of soil pH on Fe _o increased by 9.52%, indicating that soil acidification and reduced SOC content are the major reasons for the lower Fe _o content in the tea plantation compared to the maize field. This study demonstrates that, at a localized scale, variations in soil Fe _o are more strongly influenced by SOC and soil pH than by microbial activity. These findings underscore the importance of mitigating soil acidification and maintaining SOC levels to sustain agricultural soil health and promote Fe _o accumulation.

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.

Increasing forest soil organic carbon (SOC) storage is important for reducing carbon dioxide (CO ₂ ) emissions from terrestrial ecosystems and mitigating global climate change. Although the effects of altitude, temperature and rainfall on organic carbon have been studied extensively, it is difficult to increase SOC storage by changing these factors in actual forest management. This study determined the SOC, soil physical and chemical properties, nutrient elements, heavy metal elements, soil minerals and microbial biomass in the 0–140‐cm soil layer of the monsoon broad‐leaved forest in the acid red soil region of southwestern China by stratification. We tried to identify the soil factors affecting the SOC storage of the forest in the acid red soil region and determine the weights of the factors affecting the SOC, with the aim of improving the SOC retention capacity in forest management by changing the main soil factors affecting SOC storage. The results showed that the soil factors affecting the forest SOC storage in this area are total nitrogen (N, 22.7%) > soil water content (19.9%) > active iron (including poorly crystalline iron, Fe _o , 15.5%) > pH (9.5%) > phosphorus (P, 9.4%) > aluminium (Al, 8.9%) > silicon (Si, 7.1%) > sulphur (S, 6.8%). Of these factors, N, the water content, Fe _o , and P are practical factors for forest management, whereas the pH, Al, Si and S are not. SOC was significantly positively correlated with the soil N concentration, water content, active iron content and P concentration ( p < .05). In acidic red soil areas, with active iron as the highlight, N, soil water content, phosphorus and active iron jointly regulate the forest SOC storage capacity. Consequently, in actual forest management, any measures to promote soil N and water content and to activate inactive iron can enhance the storage of SOC, as appropriate input of N and P fertiliser and irrigation in dry years and the dry season. Highlights: The soil environmental factors affecting SOC storage in forest soil are quantified Activation of inactive iron helps SOC storage in forest soil Irrigation and N and P input are effective for helping SOC storage in forest soil N, WC, P and Fe _o jointly regulate SOC in tropical acid red soil forest