3.3. Spatial distribution of metal element contents
The Mg, Ca, Na, and K lines intensities of six selected samples were obtained from the LIBS spectra (depth in micrometers). The analyzed soil sample properties and calculated correlation coefficients are shown in Fig. 5. Fig. 5a indicates that the SOM was almost uncorrelated with the pH (r = -0.001) and was not significantly correlated with the pH, salt, Ca2+, Mg2+, K++ Na+, or CEC contents. Additionally, the pH was not significantly correlated with the salt content or CEC. The salt content was significantly correlated with the CEC (r = 0.98***), K+ + Na+(r = 0.93***), Mg2+ (r= 0.73***), and Ca2+ (r = 0.42***). The correlation coefficients of Ca2+ with Mg2+ (r = 0.43***) and CEC (r = 0.30***) were significant. The Ca2+contents were neither uncorrelated nor significantly correlated with the K+ + Na+ contents (r = 0.09), but the K+ + Na+ contents were highly significantly correlated with the CEC contents (r = 0.96***). The element contents at each ablation spot could not be obtained using the traditional method, owing to technical limitations. However, the LIBS spectral line intensity was positively related with the element contents. Thus, the elemental spectra of six selected samples were analyzed, the relationships for which are shown in Fig. 5b. The Mg line intensity showed the highest significant correlation with Ca (r = 0.90***), while the Na line intensity showed a higher correlation coefficient with K (r= 0.77***). The correlation coefficient between Mg and K was 0.73***. The spatial distribution maps of Mg, Ca, Na, and K were plotted based on the intensity of the element in Fig. 6.
For the low-salt content samples (columns C1 and C2), the contents of the four elements differed (C1: salt 1.08 g/kg and SOM 3.53 g/kg; and C2: salt 1.09 g/kg and SOM 11.38 g/kg). It was found that Mg, Ca, Na, and K were mainly distributed in the upper area of each sample in column C1. However, the distributions of the elements were more even in column C2. It was noted that the distribution of Mg was similar to that of Ca, while the distribution of Na was similar to that of K. The elemental distribution of the medium-salt content samples (column C3: salt 9.49 g/kg and SOM 13.80 g/kg; and C4: 9.55 g/kg and SOM 5.60 g/kg) differed from that of the low-salt content samples. When the SOM content of the medium-salt samples was higher (13.80 g/kg), Mg, Ca, and K were mainly distributed in the middle and bottom areas of the sample. However, the four elements showed greater distribution trends with the lower SOM contents (5.60 g/kg). For high salt-content samples (column C5: salt 10.17 g/kg and SOM 13.20 g/kg; and C6: salt 16.10 g/kg and SOM 7.52 g/kg), the four elements were mainly distributed from the top to the middle areas.