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.