| Soil organic carbon accumulation and soil magnetic enhancement are important research elements in global change ecology and soil magnetism respectively.Many studies have shown that soil organic carbon and magnetic susceptibility have a correlated relationship.Soil magnetic susceptibility is a physical property of iron oxides in soil,and different soil magnetic susceptibility characterize the content of different iron forms in the soil.At the same time,different Fe-form affect the storage of soil organic carbon.Therefore,Fe oxides are the "link" between soil organic carbon and soil magnetic susceptibility.The relationship between soil organic carbon and soil magnetization is mainly between the ratio of soil organic carbon and the content of different iron forms.The magnetic variation symbolizes the transformation between Fe forms of different magnetic strengths,and it is not clear whether the process is accompanied by the gain or loss of soil organic carbon.In this study,six sample plots were selected from forest and cornfield in the tropical acidic red soil area.We analyzed in situ magnetization,low frequency magnetization,soft magnetic ratio,soil organic carbon,soil iron oxides and soil physicochemical properties in the 0-140 cm soil layer by stratified sampling.The purpose is to clarify the mechanism between soil organic carbon accumulation and soil magnetism change.The results showed that（1）In the vertical profile,low-frequency soil magnetization and soil organic carbon were significantly higher（P < 0.05）in the 0-40 cm soil layer in both the forest and the cornfield than in the soil layer below 40 cm.Although human cultivation in cornfield usually lead to a decrease in soil organic carbon in the topsoil layer,the change in soil organic carbon in all layers of cornfield was not significant（P > 0.05）compared to the same layer of the forest,and the magnetization of maize fields was significantly higher（P < 0.05）from 0-40 cm.This indicated that the iron form was transformed and the magnetic strong minerals were enhanced to protect soil organic carbon,resulting in no significant decrease in soil organic carbon.（2）Soil reactive iron and soil organic carbon showed a significant positive correlation（P < 0.05）.Structural equation modeling showed that soil nitrogen,water content,reactive iron and p H were the main factors promoting soil organic carbon storage in the red soil.Reactive iron can act on soil organic carbon storage in a direct or indirect way.Compared to the forest,reactive iron was significantly higher（P < 0.05）in the 0-40 cm soil layer of cornfield and the weight of the effect on organic carbon increased by11.65%.This was due to the fact that tillage disrupts the lattice structure of the native minerals,activating iron and promoting organic carbon storage.（3）There was a significant positive correlation between soil organic carbon and magnetization rate and reactive iron（P < 0.05）.The soft magnetic ratio was significantly higher（P < 0.05）in the 0-40 cm soil layer of the cornfield.It showed that the percentage of ferrous magnetic minerals increased and the transformation process of ferrous magnetic minerals can lead to a change in soil magnetization.Specifically,the elevation of active iron promotes the storage of organic carbon and the production of more stable iron-bound organic carbon,during which the iron oxides become ferrous magnetic minerals,resulting in increased soil magnetism.The combination of soil organic carbon and iron into a strongly magnetic iron-carbon complex prevents further oxidation of iron into weakly magnetic iron minerals,so that the complex maintained a high magnetization rate and a more stable soil organic carbon.The mechanism of action between soil organic carbon and magnetism is explored to provide a reference for the next prevalent magnetic enhancement phenomenon and the study of mineral transformation in soil.It is also worth trying to further clarify the mechanism,feasibility and conditions of using magnetometry to determine soil organic carbon. |
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