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.