Changes in Soil Magnetic Susceptibility and the Environmental Implications under the Conversion of Soda Saline–Alkaline Wastelands into Paddy Fields

Abstract

The magnetic susceptibility (MS, including low-frequency mass-specific magnetic susceptibility χ_lf and percent of frequency-dependent magnetic susceptibility χ_fd%) of soil is crucial for indicating pedogenic processes and indirectly affects soil fertility. Alternating drought and waterlogging significantly decreased the MS (χ_lf and χ_fd%) of soil. In the western Songnen Plain of China, converting soda saline–alkaline wastelands into paddy fields causes continuous drought–waterlogging alternation of the soil. In this study, we aimed to reveal the environmental implications of MS (χ_lf and χ_fd%) throughout this conversion and develop an identification method for soda saline–alkaline soils based on the relationship between the MS (χ_lf and χ_fd%) and saline–alkaline indicators. Topsoil samples (0–20 cm) were collected from a soda saline–alkaline paddy field and wasteland in Niuxintaobao County, Da'an City, Jilin Province, China. Based on the MS (χ_lf and χ_fd%) and saline–alkaline indicators (e.g., total dissolved solids TDS, exchangeable sodium saturation percentage ESP, cation exchange capacity CEC, exchangeable sodium ENa) of soil, correlation analysis and structural equation modeling using maximum likelihood estimation (ML-SEM) were conducted. The results revealed that the χ_lf and χ_fd% of paddy soil were lower than those of soda saline–alkaline wasteland. It's due to long-term waterlogging and human disturbances like irrigation, fertilization, and cultivation. Soda saline–alkaline soils are inherently less magnetic than other soil types. Under waterlogged and reducing conditions, ferrous magnetic minerals are destroyed, causing further magnetic attenuation during Hydragric Anthrosols' evolution. Generally, the MS (χ_lf and χ_fd%) of black soils ranges from 10 < χ_lf < 100 (× 10⁻⁸ m³ kg⁻¹) and 2% < χ_fd% < 10%. In contrast, research shows that χ_lf < 14 ×10⁻⁸ m³ kg⁻¹ (Mean ± SD = 9.26 ± 1.56) and χ_fd% < 4% (Mean ± SD = 2.46 ± 1.05) are suggested to be diagnostic characteristics of the surface layer of soda saline–alkaline soils in Northeast China. The correlation analysis and ML-SEM results indicate that the main factors influencing χ_lf in the paddy soils were the ENa, Cl⁻, TDS, ESP, Na⁺, and SO₄²⁻, with net effects of −0.990, 0.688, −0.3502, −0.3000, 0.2331, and 0.1746. ENa, TDS, and ESP significantly inhibited the χ_lf of paddy soils, while the Cl⁻, Na,⁺ and SO₄²⁻ contents significantly promoted it. In contrast, the χ_fd% of the soils showed no correlation with any of these indicators. The χ_lf of paddy soils variations can indirectly indicate the fluctuations of soil saline–alkaline indicators. In sum, MS (χ_lf and χ_fd%) can serve as an alternative index for describing changes in soil salinization levels and has potential implications for soil salinization assessment.