LIBS and FTIR-ATR spectroscopy studies of mineral-organic associations in salinized soil

Abstract

The direct characterization of the spatial distribution of elements and compound binding of salt-organic associations in soil is imperative for understanding the mechanisms of organic matter decomposition and nutrient release in soil degradation and development processes. Modern spectroscopic techniques provide a feasible method for analysis at the microscale. In this study, mid-infrared attenuated total reflectance spectroscopy (FTIR-ATR) was used to obtain molecular functional group information, laser-induced breakdown spectroscopy (LIBS) was apply to obtain micro-level distribution features of elements in soil, and two-dimensional correlation spectroscopy (2DCOS) analysis was conducted to illustrate the binding combination features of mineral-organic associations in salinized from the Hetao Irrigation District in China. The results showed that the distributions of Mg, Ca, Na, and K were heterogeneous at the micro-level; the spatial distributions of Mg and Ca showed a significant correlation (r = 0.90***), while K displayed a negative correlation with the SOM contents. In soil with lower SOM contents, the elements were distributed at the top of the ablation area and enhanced with the increasing SOM content, which reflected the trends of the SOM layer thickness outside the mineral-organic associations at the micro-level. Furthermore, 2DCOS analysis suggested that the hydrogen bonds in silicate groups were stronger than those of organic functional groups, such as C=O/C=C, when combined with salt-related compounds, and Mg, Ca, Na, and K did not originate from clay mineral compounds in salinized soil, but partially originated from deposited organic associations.