Soil fertility under different forest types in the Helan and Liupan Mountain ranges of Ningxia Province

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.