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Soil acidification has occurred and accelerated in Jiaodong Peninsula, China, causing serious problems, such as manganese (Mn) toxicity and soil organic carbon (SOC) losses. However, the relationships between SOC and soil Mn are still unclear. This study aimed to explore the changes of Mn fractions and SOC under soaking conditions and the acidification‐induced positive coupled relationship between them. A sequential extraction for Mn species after acid soaking of a Cambisol was conducted, and the soil (pre‐ and post‐soaking) was characterised by X‐ray diffraction (XRD) and scanning electron microscope (SEM). The results showed that acidification of a Cambisol (from pH 6.48 to pH 2.79) induced SOC losses (15.8% of initial SOC). Meanwhile, lower mobile or immobile Mn (especially oxide‐Mn) significantly decreased, accompanied by an increase in readily soluble Mn. It was further shown that the amount of decrease in free Mn reached 268.7 mg kg−1, accounting for 93.8% of the total loss of soil Mn, indicating that the reductive dissolution of amorphous Mn (III/IV) oxides is the dominant reaction in acidified Cambisols. This result was also supported by XRD and SEM. Correlation analysis showed that under soil acidification, the reductive dissolution of amorphous Mn (III/IV) oxides drove losses of both SOC and soil Mn. Therefore, a possible reaction mechanism was proposed: at neutral and alkaline pH, immobile complexes of Mn (III/IV) and particulate organic carbon (POC) formed; and, at acidic pH, readily mobile complexes of Mn (II) and dissolved organic carbon (DOC) formed. Our research suggested that Mn‐organic carbon complexes regulated the transport, transformation, and leaching of soil Mn and SOC, and future research should focus on the soil acidification‐driven Mn reduction‐carbon oxidation processes. Highlights: Acidification accelerated soil organic carbon losses.Acidification induced reductive dissolution of amorphous Mn (III/IV) oxides.A positive couple relationship between SOC and soil Mn existed in acidified soils. [ABSTRACT FROM AUTHOR]

* Premise of the study: The potassium cation (K+), one of the most abundant cations in cells, improves plant tolerance to various abiotic stresses. Alligatorweed (Alternanthera philoxeroides) is well known for its strong capacity to accumulate K+. The distinctive K+ accumulation capability of alligatorweed is linked to a high-affinity K+ transport facilitated by K+-uptake transporters (ApKUPs). * Methods: A putative K+ transporter gene, ApKUP4, was isolated from alligatorweed using degenerate primers and rapid amplification of cDNA ends (RACE) techniques. Gene expression profiles were performed by quantitative real time PCR and northern blot analysis. Moreover, we introduced ApKUP4 into Arabidopsis to determine its function in improving crop nutrition and NaCl stress tolerance. * Key results: ApKUP4 was localized throughout the entire alligatorweed plant, and its expression was stimulated in the stems and roots under K+ deficiency, osmotic stress, and salinity stress. Northern blot analysis revealed that ApKUP4 was present in all tested organs of transgenic Arabidopsis plants. Compared with the wild type, Arabidopsis plants overexpressing ApKUP4 showed improved growth and K+ homeostasis. Moreover, ApKUP4 overexpression in Arabidopsis plants enhanced plant tolerance to salinity stress, as evidenced by reduced water loss and ROS generation, associated with enhanced photosynthesis, nutritional status, and enzymatic antioxidants. * Conclusions: The present study provides direct evidence that the alligatorweed K+ transporter gene, ApKUP4, contributes to salinity tolerance in transgenic Arabidopsis seedlings, demonstrating the essentiality of potassium homeostasis for plant salinity tolerance. [ABSTRACT FROM AUTHOR]