Interlayer organic matter within hydroxy-interlayered clay minerals enhances soil organic carbon stability under long-term organic fertilization

The intercalation of organic matter in clay minerals is believed to be essential for soil carbon sequestration because nanoscale interlayer spaces can effectively protect organic matter from degradation. However, the lack of a direct method for the detection of interlayer organic matter has limited the identification of organic matter and the evaluation of the properties of organic matter–clay complexes. In this study, the association between the organic matter and hydroxy-interlayered clay minerals (HIMs) in red earth collected from Qiyang County (Hunan Province) in southeast China was investigated by combining X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM)-electron energy loss spectroscopy (EELS) methods. The presence of organic matter in the interlayer space of HIMs was confirmed by HRTEM-EELS for the first time, providing direct visible evidence of interlayer organic matter in the HIMs of red earth. Hydroxy-Al was identified as the interlayer hydroxy cation in the HIMs. The organic molecules were associated with Al-OH in the interlayered hydroxy-Al polymer and exhibited resistance to oxidation by H2O2. Organic fertilization increased the amount of organic matter on the external surface and in the interlayer space of the HIMs, implying the feasibility of enhanced carbon sequestration in red earth.