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.