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.
Keywords: Mineral–organic association, infrared attenuated total reflectance spectroscopy, laser induced breakdown spectroscopy; two-dimensional correlation spectroscopy, spatial distribution, microscale
INTRODUCTION
Soil salinization adversely affects the physical, chemical, and biological processes of soil, which leads to land degradation and productivity loss (Liu et al., 2018; Sidike et al., 2014; Zovko et al., 2018). The identification of salt-affected soil processes and assessment of the degree of salinization are essential for sustainable agricultural management (Daliakopoulos et al., 2016; Farifteh et al., 2008). The surfaces of minerals and their incorporation into aggregates could physically prevent soil organic matter from extensive decomposition (Lehmann & Kleber, 2015; J. Xiao et al., 2018). On one side, saline soils have been found to have variable effects on carbon and nitrogen mineralization, which are crucial for the decomposition of organic matter and release of nutrients required to sustain productivity and promote a higher fraction of plant intake in the accumulated organic matter (Daliakopoulos et al., 2016; Pathak & Rao, 1998; Xiao et al., 2019; Zhang et al., 2019). On the other side, the addition of salts in soil get negative results of soil organic carbon loss as decreased fertility and adversely affects stability of soil aggregates (Singh, 2016; Six et al., 2000). The processes increase dispersion of clay particles as well as soil erosion rates (de la Paix et al., 2013). Therefore, land degradation has recently become a particular focus on the study of soil carbon turnover process (Su et al., 2010) in the context of salt contents soils (Su et al., 2010; J. Xiao et al., 2018), which would be benefit for understanding the formation and stability of organic-mineral complexes in soils.
The Hetao Irrigation District (HID) is a plain located in an arid/semi-arid region that was formed by the Yellow River along its north bank. This area is fertile and densely populated and has been irrigated with water from the Yellow River since ancient times. Irrigated agriculture uses approximately 90% of the total surface water resources (Wu et al., 2017; Xue & Ren, 2017). For a considerable time, soil salinization in the area has been aggravated by flood irrigation, increased soil erosion, and water deterioration induced by excessive fertilization (Wu et al., 2017). Meanwhile, soil salinity management relies upon the identification of proper methods and techniques for monitoring and accessing salt-affected soils. Salts are contained in minerals in the form of carbonates, halides, sulfates, and borates (Klein & Hurlbut Jr, 1999). However, in situ records of the associations of organic and inorganic groups in soils at the microscale have been obstructed due to methodological and analytical limitations.
Recently, spectral-based analytical technologies have been widely used in soil research because they provide in situ , microscale soil information rapidly (Xing et al., 2019). Mid-infrared attenuated total reflectance spectroscopy (ATR) reflects information concerning molecular bonds, such as the modes of molecular functional groups (e.g. stretching/wagging vibration) and clay in minerals, and has been used for soil identification as well as nitrate, clay, sand, and soil carbon measurements (Kira et al., 2014; Linker et al., 2005; Ma et al., 2019). Laser-induced breakdown spectroscopy (LIBS) is an atomic emission spectroscopy technique. As the plasma cools, continuum, ionic, and atomic emissions occur, revealing the elemental composition of the samples. LIBS could obtain microscale information along the vertical scale from each shot (Ilhardt et al., 2019). Therefore, it can be applied to characterize the distribution of soil organic matter, clay minerals, and salt–related elements at the microscale. Additionally, the sensitivity and spatial resolution of LIBS are high (de Oliveira et al., 2019; Kim et al., 2013; Suyanto et al., 2017; Zaytsev et al., 2018).
The two techniques can be used simultaneously to expand upon the current information concerning minerals–organic association at microscale. Considering the possible overlapping spectral features and the heterogeneity of soil samples, two-dimensional correlation spectroscopy (2DCOS) was employed to present the spectral intensity trends in relation to a perturbation sequence (such as time, temperature, concentration, or spatial distance) over a second dimension (Noda, 2018; Sun et al., 2017; Sun et al., 2019). 2DCOS is typically displayed as contour maps of correlation intensities, as functions of two independent wave numbers/wavelengths. 2DCOS can probe the specific sequence of any subtle spectral changes in response to external perturbations (Noda, 2018; Ruan & Zhou, 2008; Xu et al., 2018), which can be employed to detect changes in organic and inorganic functional groups and structural relationships in mineral-organic processes along spatial and microscales.
Thus, we operated the experiments aimed to investigate the minerals–organic complex employing spectral based techniques at microscale, which would provide another sight to understand the relation of mineral and soil organic matter association process in saline soil. The objectives of this study were to: i) investigate the molecular functional groups feature in saline soil using FTIR-ATR spectroscopy; ii) describe the salinization-related Mg, Ca, Na and K element distribution of soil samples using LIBS spectra; and iii) explore the mineral-organic association in saline soils using 2DCOS analysis.