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The main areas of forest in Ningxia Province lie in the Helan and Liupan mountain ranges.These forests have important ecological and economic significance.The fertility of forest soil in these areas has not been systematically studied.The objective of this study was to evaluate soil fertility under six forest types in the Su-Yukou forest zone of the Helan mountain range and in the Xi-Xia and Die-Diegou forest zones of the Liupan mountain range.The natural forest types were Pinus tabulaeformis × Populus davidiana,Picea crassifolia × Populus davidiana,Potentilla parvifolia,and Larix principis-rupprechtii.The planted forest types were Quercus liaotungensis and Larix principis-rupprechtii.Five separate profiles(replications) were sampled in an 'S' pattern for each soil type.The profiles were divided into A,B,and C horizons and then sampled,making a total of 18 soil samples.Soil physical and chemical properties were measured with conventional methods.The results were analyzed using univariate analysis of variance.Differences among forest types and soil horizons were analyzed with Duncan′s test(SSR).Soil fertility characteristics were evaluated using principal component analysis. The results indicated significant differences in soil fertility under the six forest types.These differences can probably be attributed to differences in the accumulation and decomposition of forest litter,in soil parent material,or in climatic conditions.Soil porosity ranged from 54.5 to 72.2% and decreased as soil depth increased.Soil bulk density ranged from 0.72 to 1.21 g/cm3 and particle density ranged from 2.55 to 2.68 g/cm3.Soil bulk density and particle density both increased as soil depth increased.Soil particle density in the Liupan mountains was higher than that in the Helan mountains.Soil organic C(24.03—65.37 g/kg),total N(1.48—3.49 g/kg),NO-3-N(1.88—10.50 mg/kg),NH+4-N(5.02—11.01 mg/kg),total P(0.37—1.19 g/kg),available P(4.82—13.38 mg/kg),and available K(82.03—244.62 mg/kg) increased as soil depth increased.Total K(18.92—26.14 g/kg) increased as soil depth increased.Total K concentrations were higher in soil from the Liupan mountains than in soil from the Helan mountains.The soil C ∶N ratio(11.74 to 19.88) tended to be highest in the B horizon followed by the C and A horizons.The cation exchange capacity(23.94—40.30 cmol/kg) decreased as soil depth increased.In contrast,soil pH(7.09—8.09),exchangeable sodium percentage(0.59%—2.47%) and base saturation percentage(51.24%—80.57%) increased as soil depth increased.Clay content(5.46%—10.20%),total dissolved salt,(0.33—1.12 g/kg),and CaCO3 content(1.44—14.23 g/kg) did not change significantly with soil depth.The soil C ∶N ratio,pH,exchangeable sodium percentage,total dissolved salt,and CaCO3 content of soil from the Helan mountains was higher than that of soil from the Liupan mountains.Factor analysis was used to describe soil fertility under the six forest types.Three soil fertility components(organic matter factor,environmental factor,and NO-3-N factor) were derived from 14 physico-chemical soil properties.Component scores showed significant differences among the three fertility components under different forest types.The soil under the Picea crassifolia × Populus davidiana forest had the highest component score for the organic matter factor.Soils from the Helan mountain range had the highest component score for the environmental factor,and soil from the natural Potentilla parvifolia and Larix principis-rupprechtii forest had the highest component score for the NO-3-N factor.

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).