significantly decreased by 58.4% only in 60 Mg/ha
FGD-gypsum treatment. In the 20–30 cm soil layer, there was
no significant difference in the concentrations of Na+ among
all treatments, while the molar percentage significantly
decreased by 18.4% in 60 Mg/ha FGD-gypsum treatment.
Similar trends also occurred after 18 months, but the concentration
of Na+ in 30–60 Mg/ha FGD-gypsum treatments
significantly decreased by 26.1%–28.7% in the 20–30 cm soil
layer. This is because the Ca2+ provided by FGD-gypsum
can displace exchangeable Na+ on soil colloids (Richards,
1954), which generated the water-soluble Na+ that concurrently
leached downward and accumulated at lower soil
layers after natural rainfall events occurred (Wang et al.,
2005). When excess Na+ was removed, the concentration of
water-soluble Na+ was reduced, and the concentrations of
total cations, especially due to Ca2+, were greatly increased
leading to the decrease of the molar percentage of Na+
correspondingly.