Base saturation percentage (BSP) is an important soil chemical index which has implication not only in soil fertility    but also in soil taxonomy  . 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 , but these studied factors mainly are external or indirect factors. As defined as the sum of exchangeable Ca2+, Mg2+, K+, and Na+ relative to total soil cation exchange capacity (CEC) at pH 7.0 or 8.2  , BSP is conceptually affected simultaneously by Ca2+, Mg2+, K+, Na+, Al3+ and H+. Some studies found that BSP was dominated by Ca2+ for the calcareous soils on a small orchard scale  , 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  . 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 . 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.
2. Methods and Materials
2.1. Data Sources
The data of BSPs and exchangeable cation concentrations (H+, Al3+, Ca2+, Mg2+, 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 Figure1), here, the spatial location of the selected soil species in Figurewere roughly determined according to the information of site description of the typical soil profiles in the available literatures . 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.
2.2. Sampling and Measurement of Soil Samples
According to the available historical records, the sampling and measurement of soil samples were as follows: 1) the location of each typical soil profile was mainly decided by the comprehensive consideration of the representation of topography, parent material, land use type, and etc.; 2) a standard soil profile
Figure 1. Spatial distribution of adopted soil profiles of red soil and yellow soil groups.
(1.0 m in width and 1.2 - 1.5 m in depth) was excavated; 3) the horizons were decided according to morphological characteristics of profile; 4) soil sample was collected uniformly within each horizon, and 2 kg of soil was kept left by using quartering and kept in the cloth bag. In the lab, soil samples were air dried, and the impurities of roots and gravels were removed. Then they were grinded to pass 1.7 mm, 0.25 mm and 0.149 mm sieves respectively.
Exchangeable K+ and Na+ were determined by flame photometer method, Ca2+ and Mg2+ determined by the method of EDTA volumetric method, H+ and Al3+ were determined by titration method after 1 mol/L KCl extraction, total amount of base exchangeable cations determined by the method of evaporation and neutralization titration after 1 mol/L NH4OAc (pH 7.0) extraction, cation exchange capacity (CEC) determined by the method of 1 mol/L NH4OAc exchange   .
2.3. 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  . 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 . Here, principle component analysis was not used to do so is because we want to quantitatively know the contribution of each cation to BSP.
3. Results and Discussions
3.1. Statistical Information of Exchangeable Cations and BSPs
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 Al3+ > Ca2+ > H+ > K+ > Mg2+ > Na+, in which Al3+ was significantly higher than all other exchangeable cations. For red soil group, Ca2+ and H+ were significantly higher than K+, Mg2+ and Na+, but no significant difference between Mg2+, K+ and Na+. For yellow soil group, Ca2+ was significantly higher than Mg2+, K+ and Na+, but no significant difference between H+, Mg2+, 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)  .
3.2. 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 Ca2+, Mg2+ and Al3+ 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 Ca2+ and Al3+ while low correlation with Na+, Mg2+ and K+; 3) low correlation were found between H+ with Al3+, Ca2+ with Mg2+ and Na+ in both soil groups and between Mg2+ with K+ and Na+ in yellow soil group.
Table 1. Statistical information of exchangeable cations and BSPs of red soil and yellow soil groups.
*. Different lowercase letters in the same columns show mean difference of exchangeable cations is significant at the 0.05 level, and different capital letters in BSP line show mean difference of BSPs is significant at the 0.05 level.
Table 2. Pearson correlation coefficients (R) between exchangeable cations and BSPs.
** and * Correlation are significant at the 0.01 and 0.05 levels (2-tailed).
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.
Table 4 shows the calculated importance of exchangeable cations to BSPs with the Random Forest Package, it can be seen from Table 4 that, 1) the importance of H+ and K+ of red soil were not significant to BSP, while only that of H+ not significant for yellow soil; 2) the orders of BSPs and the importance of the top three exchangeable cations to BSPs were different between red soil and yellow soil groups, which was Ca2+ (33.81%) > Al3+ (29.87%) > Mg2+ (22.28%) for red soil, while Al3+ (24.91%) > Ca2+ (21.55%) > Mg2+ (19.91%) for yellow soil; 3) although generally the importance of Ca2+, Al3+ and Mg2+ to soil BSPs are greater due to their high concentrations, no significant correlation was found between cation concentrations and their importance to BSPs if all exchangeable cations are considered.
Some studies already proved and explained that Al3+ was the main form in exchangeable acid exchangeable cations in soils of south China    . Our data also showed Al3+ were the highest in all exchangeable cations both in red soil and yellow soil groups, which is common in soils of south China due to intensive desilicification but fersialitization caused by high temperature and precipitation   , so it is normal for Al3+ as one of the important exchangeable cations controlling BSPs of red soil and yellow soil groups in south China.
Table 3. Optimal correlation models between BSPs with exchangeable cations of red soil and yellow soil groups.
Table 4. Importance of exchangeable cations to BSPs in red soil and yellow soil groups.
* Importance is significant at 0.05 level; Norm. means normalized.
The lower concentrations of base exchangeable cations compared with Al3+ could be attributed to the heavy leaching losses of base exchangeable cations, which were higher than 80%  , 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 cmol(+)/kg). Generally speaking, red soil group was formed and evolved in the environment with higher temperature and precipitation (15˚C - 25˚C in annual mean temperature and 1200 - 2500 mm in annual precipitation) compared to yellow soil group (14˚C - 19˚C in annual mean temperature and 1000 - 2000 mm in annual precipitation), which resulted in higher acid exchangeable cations and lower base exchangeable cations of red soil group compared with yellow soil group   .
Some studies found that BSP was dominated by Ca2+ in calcareous soil species on a small orchard scale due to the highest concentration of Ca2+ derived from the calcareous parent material  , but didn’t give information on the importance of exchangeable cations to BSPs. In contrast, our study not only found further that cation concentrations were in order of Al3+ > Ca2+ > H+ > K+ > Mg2+ > Na+ both in red soil and yellow soil groups (see Table 1), but also disclosed the importance of exchangeable cations to BSPs, for examples, the importance order of exchangeable cations to BSPs were Ca2+ (33.81%) > Al3+ (26.58%) > Mg2+ (19.82%) > Na+ (14.49%) > K+ (2.70%) > H+ (2.61%) for red soil group while Al3+ (24.91%) > Ca2+ (21.55%) > Mg2+ (19.91%) > K+ (14.24%) > Na+ (14.21%) > H+ (5.18%) for yellow soil group. However, our study shows that exchangeable cations controlling BSPs are different between red soil and yellow soil groups, we think such a complexity is normal because our study includes 59 and 28 soil species of red soil and yellow groups, respectively, which are widely distributed in south China with various soil forming environments and evolution degrees   . For examples, the spans of annual temperature and precipitation were 15˚C - 25˚C and 1200 - 2500 mm for red soil while 14˚C - 19˚C and 1000 - 2000 for yellow soil, respectively  , 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),
Table 5. Statistical information of exchangeable cations concentrations and BSPs of soils derived from different parent materials.
*. 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.
Table 6. Statistical information of exchangeable cations concentrations and BSPs of soils under different land use types.
*. Different lowercase letters in the same lines show mean difference is significant at the 0.05 level. Soil species with unclear information in land use types were omitted in this study.
and it can be seen from Table 5 and Table 6 that Al3+, Ca2+ and H+ are the top three exchangeable cations in concentrations not only in different parent materials but also in different land use types, and there are certain differences in cation concentrations but no significant difference in BSPs between different parent materials or different land use types.
However, there are still some problems left to be solved in our study, for examples, why H+ is not significantly important to BSP with a higher concentration than K+, Mg2+ and Na+? Why K+ is significantly important to BSP of yellow soil group but not to red soil group?
By using the data of BSPs and cation concentrations of soil species of red soil and yellow soil groups in south China, our study disclosed that the concentrations of exchangeable cations in both red soil and yellow soil groups were in an order of Al3+ > Ca2+ > H+ > K+ > Mg2+ > Na+. BSP of red soil group was codetermined by Ca2+, Al3+, Mg2+ and Na+, while BSP of yellow soil group by Al3+, Ca2+, Mg2+, K+ and Na+. But no significant correlation was found between cation concentration and importance of cation to soil BSP if all exchangeable cations are considered.
This study was supported by projects of the National Natural Science Foundation of China (Grant No. 41877008) and the National S&T Basic Special Foundation Project (No. 2014FY110200). We would like to express thanks to the contribution of all the participants in the data preparation and processing.