Study on Exchangeable Cation Determining Base Saturation Percentage of Soil in South China

Base saturation percentage (BSP) is an important soil chemical index in soil fertility and soil taxonomy. However, it is still unclear what exchangeable cation dominates BSP of soil in south China. Therefore, in this study, the data of BSPs and exchangeable H, Al, Ca, Mg, K and Na of 109 and 45 horizon samples of 50 and 28 soil species in red soil and yellow soil groups in the Database of Chinese Soil Species were used to explore further the characteristics of BSPs and exchangeable cations as well as the correlation between BSPs and exchangeable cations. The results showed that the concentrations of exchangeable cations in both red soil and yellow soil groups were in an order of Al (4.55 ± 1.47 and 4.22 ± 1.2 cmol(+)/kg) > Ca (0.32 ± 0.21 and 0.36 ± 0.24 cmol(+)/kg) > H (0.23 ± 0.13 and 0.19 ± 0.10 cmol(+)/kg) > K (0.16 ± 0.09 and 0.16 ± 0.11 cmol(+)/kg) > Mg (0.13 ± 0.09 and 0.11 ± 0.08 cmol(+)/kg) > Na (0.08 ± 0.06 and 0.11 ± 0.06 cmol(+)/kg). For red soil group, Al concentration was significantly higher than those of other exchangeable cations, Ca and H concentrations were significantly higher than those of K, Mg and Na; while for yellow soil group, Ca, H and K concentrations were significantly higher than those of Mg and K. BSP of red soil group was codetermined by Ca, Al, Mg and Na, with the contributions of 33.81%, 19.82% and 14.49%, respectively; while BSP of yellow soil group was codetermined by Al, Ca, Mg, K and Na, with the contributions of 24.91%, 21.55%, 19.91% and 14.21%, respectively. A higher concentration of exchangeable cation does not mean the higher importance of the cation to soil BSP.


Introduction
Base saturation percentage (BSP) is an important soil chemical index which has implication not only in soil fertility [1] [2] [3] but also in soil taxonomy [4] [5].
Existing studies have pointed out that BSP could be affected by climatic, geochemical, and environmental conditions, such as acid rain and dust deposition, waterlogging condition, pH, nitrogen application, organic matter and clay contents, cation exchange capacity and so on [6], but these studied factors mainly are external or indirect factors. As defined as the sum of exchangeable Ca 2+ , Mg 2+ , K + , and Na + relative to total soil cation exchange capacity (CEC) at pH 7.0 or 8.2 [7] [8], BSP is conceptually affected simultaneously by Ca 2+ , Mg 2+ , K + , Na + , Al 3+ and H + . Some studies found that BSP was dominated by Ca 2+ for the calcareous soils on a small orchard scale [9] [10], but no consideration was given to the importance of different exchangeable cations to BSPs.
Red soil and yellow soil groups (in Chinese Genetic Classification) are two important zonal soil groups widely distributed in the hilly area of tropical and subtropical south China which are characterized by high desilicification but fersialitization [11] [12]. There are 133 and 76 soil species in red soil and yellow soil groups, respectively, which were derived under diverse climate conditions, parent materials and land use types [13]. However, it is still unclear that what exchangeable cations dominate BSPs of red soil and yellow soil groups; therefore, the objective of our study is to understand further the characteristics of BSPs and exchangeable cations, and to quantitatively analyze the importance of different exchangeable cations to BSPs of red soil and yellow soil groups.

Data Sources
The data of BSPs and exchangeable cation concentrations (H + , Al 3+ , Ca 2+ , Mg 2+ , K + and Na + ) are from Chinese Soil Database (http://vdb3.soil.csdb.cn/). After the comparison of data completeness and the elimination of abnormal data by the method of μ ± 3σ, 136 and 58 horizon samples from 59 and 28 soil species in red soil and yellow soil groups in South China were adopted, respectively (see Figure 1), here, the spatial location of the selected soil species in Figure were roughly determined according to the information of site description of the typical soil profiles in the available literatures [13]. Other main soil groups such as latosol and latosolic red soil groups were not considered due to the insufficient typical profiles (less than 8 soil profiles) after the elimination of abnormal data of BSPs and exchangeable cations.

Data Processing, Modeling and Mapping
Microsoft Excel 2016 and IBM Statistics SPSS 20.0 were used for data processing, correlation modeling and mapping, difference significance of exchangeable cations and BSPs were tested by LSD method of one-way ANOVA, Pearson correlation (2-tailed) was used to describe the correlation between exchangeable cations and BSP [16] [17]. The contributions of exchangeable cations to BSP is calculated as follows: by using SPSS, the data of exchangeable cations and BSP were normalized, the multiple linear regression model between BSP and exchangeable cations was obtained, and then the percentage of the coefficient of i cation to total sum of the coefficients of all exchangeable cations in the model was calculated as the contribution of i cation to BSP [18]. Here, principle com-  Table 1 gives the statistical information of exchangeable cations and BSPs of red soil and yellow soil groups. It can be seen from Table 1 that except Na + , there were no significant differences in other exchangeable cations between red soil and yellow soil groups (p < 0.05). BSP of yellow soil group was significantly higher than that of red soil group (p < 0.05). cation concentrations of both red soil and yellow soil groups were in an order of Al 3+ > Ca 2+ > H + > K + > Mg 2+ > Na + , in which Al 3+ was significantly higher than all other exchangeable cations. For red soil group, Ca 2+ and H + were significantly higher than K + , Mg 2+ and Na + , but no significant difference between Mg 2+ , K + and Na + . For yellow soil group, Ca 2+ was significantly higher than Mg 2+ , K + and Na + , but no significant difference between H + , Mg 2+ , K + and Na + . 4) All exchangeable cations and BSPs of red soil and yellow soil groups were in moderate variation (coefficient of variation < 100%) and in positive skew distribution (skewness > 0), while their probability density curves of all exchangeable cations and BSPs were near very flat (kurtosis = 0) or flat (0 < kurtosis < 0.67) [16] [17].

Correlations between BSPs and Exchangeable Cations
For the correlation between BSP with exchangeable cations, it can be seen from Table 2 that, 1) for red soil group, BSP had notable correlation (R = 0.5 -0.8, the same below) with Ca 2+ , Mg 2+ and Al 3+ while low correlation (R = 0.3 -0.5, the same below) with Na + for red soil group; 2) for yellow soil group, BSP had notable correlation with Ca 2+ and Al 3+ while low correlation with Na + , Mg 2+ and K + ; 3) low correlation were found between H + with Al 3+ , Ca 2+ with Mg 2+ and Na + in both soil groups and between Mg 2+ with K + and Na + in yellow soil group.   Table 3 lists the optimal correlation models between BSPs with exchangeable cations and the multiple linear regression models between BSPs and exchangeable cations, it shows that most of the optimal models are in a quadratic pattern.

Discussion
Some studies already proved and explained that Al 3+ was the main form in ex-  The lower concentrations of base exchangeable cations compared with Al 3+ could be attributed to the heavy leaching losses of base exchangeable cations, which were higher than 80% [23] [24], due to the high temperature and precipitation in south China (annual temperature and precipitation as 16˚C -25˚C and 800 -2000 mm, dryness less than 1.0). A significant difference was found in BSPs between red soil group and yellow soil groups, which could be attributed to the accumulation effects in the differences of the sums of acid exchangeable cations and base exchangeable cations between the two soil groups. It could be found that although there was significant difference in exchangeable cations, the sums of acid exchangeable cations and base exchangeable cations, the sum of acid exchangeable cations of red soil (4.77 ± 1.50 cmol(+)/kg) was higher than that of yellow soil group (4.41 ± 1.27 cmol(+)/kg), while the sum of base exchangeable cations of red soil group (0.69 ± 0.32 cmol(+)/kg) was lower than that of yellow soil group (0.74 ± 0.35  [12], meanwhile, the parent materials include granite, sandstone, quaternary red clay, pelite, quartzite, tuff, basalt, shallow-sea sediment, andesite and limestone, etc. and the land use types include dryland, forest, orchard, shrub, grassland, etc. (see Table 5 and Table 6, only main parent materials and land use types presented in the tables), *. Different lowercase letters in the same lines show mean difference is significant at the 0.05 level. Not included soil species with unclear information in parent materials or with the less numbers of profiles.