Soil cation exchange capacity and exchangeable base cation content in the profiles of four typical soils in the Xi-Shui Forest Zone of the Qilian Mountains

Soil cation exchange capacity(CEC) has been used as an indictor of soil fertility and buffering capacity.The exchangeable base cation content and saturation percentage reflects the bioavailability and movement of soil nutrients.This study was conducted to determine the distribution of CEC and exchangeable base cations in the profiles of four typical soils in the Xi-Shui Forest Zone of the Qilian Mountains.The soil types were brown calcic(BC) soil,grey cinnamon(GC) soil,chestnut soil(CH) soil,and alpine meadow(AM) soil.Three separate profiles(replications) were selected for each soil type.The profiles were divided into A,B,and C horizons and then sampled,making a total of 36 soil samples.Soil CEC as well as exchangable K+,Na+,Ca2+,Mg2+ were determined in the laboratory.Differences in the cation exchange capacity among soil types and horizons were analyzed by analysis of variance and multiple comparisons.Correlation analysis was also conducted to determine the relationships between soil cation exchange properties and soil physico-chemical properties such as soil organic carbon,total N,pH,CaCO3,available P,available K,clay(0.002 mm) content,silt(0.002—0.02 mm) content and sand(0.02—2 mm) content.The results indicated that CEC in the profiles ranged from 4.80 to 48.10 cmol/kg and the total exchangeable base(TEB) content ranged from 4.67 to 21.34 cmol/kg.Both the CEC and the TEB content decreased as soil depth increased.The GC soil had the highest CEC and TEB content followed by the AM soil,the CH soil,and then the BC soil.Averaged across the four soil types,exchangeable Ca2+ comprised 71.6% of the TEB content,exchangeable Mg2+ comprised 22.9% of the TEB content,exchangeable K+ comprised 3.3% of the TEB content,and exchangeable Na+ comprised 2.2% of the TEB content.For the BC,GC,and CH soils,the exchangeable Mg2+ and Na+ content tended to be greater in the upper part of the soil profile whereas the exchangeable Ca2+ and K+ content tended to be greater in the lower part of the soil profile.For the AM soil,the exchangeable Ca2+ content tended to be greater in the upper part of the soil profile,the exchangeable Mg2+ and Na+ content tended to be greater in the middle part of the soil profile,and the exchangeable K+ content tended to be greater in the lower part of the profile.The relative order of the exchangeable cations and the saturation percentage in different soil types varied among the horizons.Soil organic matter was the major factor contributing to CEC.Soil CEC increased as the silt content increased,but decreased as the sand and CaCO3 contents increased.The base saturation percentage(BSP) ranged from 44.4 to 97.2% among the four soil types.The BSP increased as soil depth increased because the recovery of base saturation due to biological factors is weaker than the effect of eluviation.Correlation analysis indicated that soil exchangeable Na+ content and saturation percentage were positively correlated with exchangeable Mg2+(P0.01).The saturation percentage of exchangeable Na+ and Mg2+ were positively correlated with the soil CaCO3 content(P0.01).Furthermore,there were highly significant positive correlations between pH and BSP,available P and CEC,and available K and exchangeable K+(P0.01).