4. DISCUSSIONS
From the element distribution in different soil samples, it can be
concluded that i) the distribution trends of Mg and Ca were similar in
each column, which was consistent with the highly significant
correlation between Mg and Ca (r = 0.90***).
The distribution of Na was partly similar to that of K, with a
correlation coefficient of 0.77***. This means that Mg
and Ca might be associated with the minerals in the samples. ii) The
color bar on the right in Fig. 6 indicates the uniform intensity for the
distribution of each element. It can be seen that K distributions in the
C1 sample were much higher (intensity value larger than 5500 a.u.) than
those of the other samples. Meanwhile, for the samples with the same
salt content, the K distribution was higher in the samples with a lower
SOM content. For instance, the K distributions in C1 (SOM 3.53 g/kg), C4
(SOM 5.60 g/kg), and C6 (SOM 7.52 g/kg) were higher. The SOM was
negatively correlated with the salt content (r = -0.07) and
K+ + Na+ (r = -0.14). iii)
The distribution of elements in samples with lower SOM contents (columns
C1, C4, and C6) was mainly in the top and middle areas, while the
samples with higher SOM contents showed greater distributions of
elements in the middle and bottom areas, especially for samples with low
and medium salt contents (columns C2 and C3). According to FTIR–PAS
spectra, the SOM was a function of depth (Du et al., 2014). The surface
layer of SOM in minerals or the exterior of aggregates is the interface
between the soil and atmosphere. Alternatively, soil pores and SOM form
heterogeneous coatings on the mineral surface via the sorption process
(Du et al., 2014; Kleber & Johnson, 2010). Thus, it could be deduced
that the SOM layer outside the mineral aggregate was thinner in soil
with a low SOM content, and the laser beam could ablate the mineral
after one or two shots. Therefore, Mg, Ca, Na, and K can be detected and
described at the top of the microscale (C1, C4, and C6). However, when
the SOM layer was thicker, the first laser shot was potentially focused
on organic materials. Therefore, the mineral aggregates would be ablated
and monitored from the second shot (C2 and C3).
As a result of FTIR-ATR Heterospectral 2DCOS maps, microscale
differences in the binding of minerals and organic matter could be
illustrated, and the intra-hydrogen bonds in silicate groups were
stronger than those of organic functional groups, such as C=O and C=C,
but weaker than N–H and NH2 when combined with
salt-related compounds (J. Xiao et al., 2018). Furthermore, from the
FTIR-ATR–LIBS Heterospectral 2DCOS maps, it was revealed that the
intensities of Mg, Ca, Na, and K were not closely associated with the
Si–O and O–H in clay minerals. Therefore, the elements did not
originate from clay mineral compounds (Duarte et al., 2015;
Forouzangohar et al., 2013; Yu et al., 2012). Additional positive peak
correlations between 850–680 (C–H bending vibration in aromatics) and
2600–1100 cm-1 (organic functional groups) suggested
that Mg, Ca, Na, and K partially originated from organic compounds
decomposition (Forouzangohar et al., 2013; Ruan & Zhou, 2008; Yu et
al., 2012) in saline soil in Hetao Area in China.