Study On Soil Quality And Soil Erosion Characteristics In Middle-Western Qilian Mountains

Qilian Mountain areas were not only an important water conservation land and ecological safety barrier in northwest China,but also an important animal husbandry production base in Gansu and Qinghai provinces.Due to the multiple impacts of climate warming,drought,overgrazing,reclamation and development,the grassland ecosystem in this area has suffered serious land degradation and soil erosion,which has gradually reduced the primary productivity,stocking capacity and soil quality of grassland.In this study,four vegetation type zones?temperate desert,temperate steppe,alpine meadow,and shrub land?of 0-40 cm soil depths were selected in Qilian Mountains as the subjects.The distribution characteristics of soil indicators at different vegetation types,soil depths and different terrain feature were analyzed,the minimum data set?MDS?of soil quality assessment was determined,the characteristics of soil erosion and its influence on soil quality were studied by RUSLE model,which can provide important scientific basis for the monitoring soil quality,studying on the influence mechanism of soil quality,and controlling of grassland degradation in Qilian Mountains.The main results were as follow:?1?In our study,soil organic matter and total nitrogen were significantly affected by vegetation types and soil depths.Among the four vegetation types,the contents of soil organic matter and total nitrogen followed a descending order of temperate desert<temperate steppe<shrub land<alpine meadow at the 0-30 cm depth,and the distribution of total phosphorus was relatively uniform.The soil organic matter and total nitrogen contents gradually decreased with soil depth?0-40cm?,and they tended to be higher in the surface layer than in lower layers,the phenomenon of surface aggregation was common.Terrain feature has also a significant effect on the distribution of soil nutrients,soil organic matter and total nitrogen contents in the temperate steppe zone increased gradually with altitude gradient,reached a peak at 2800-2950 m,and then decreased.At the 0-40 cm depth,organic matter under shady slope was significantly higher than that under other slope directions by 12.84%-54.28%?P<0.05?.Total nitrogen under shady slope was significantly higher than that under sunny slope by 31.93%,46.21%,respectively?P<0.05??at 20-40 cm?.Organic matter and total nitrogen are negatively correlated with annual average temperature,and positively correlated with annual cumulative precipitation and soil moisture content.?2?Vegetation type and soil depth had a highly significant effect on soil aggregate indexes,and their interactions had a significant impact on mean weight diameter?MWD?,and geometric mean diameter?GMD?.The percentage of water-stable aggregates?WSA?,MWD,and GMD all showed the same increased in order:temperate desert<temperate steppe<alpine meadow<shrub land,but PAD and D behaved just opposite to them.With the increase of soil depth?0-40 cm?,aggregation degree,and the stability of soil aggregates and their structures gradually decreased.Soil aggregates were significantly affected by terrain feature,the stability and aggregation degree of soil aggregates first increased and then decreased with increasing altitude gradient,and the maximum point appeared at elevation of2800-2950 m.Among the four slope directions,the sizes of WSA,MWD,and GMD all were highest in shady slope,followed by half sunny slope and half shady slope,and the sunny slope was the smallest,while D and PAD were opposite.The water stability of the aggregate was negatively related to the annual average temperature,but significantly positive correlated with the annual cumulative precipitation.?3?The single-fractal dimension?D_V?ranged from 2.080 to 2.500 with an average value of 2.420,which indicated that the overall soil texture was coarse in Qilian Mountains.D_V was found to be significantly different among these soil textures,and showed as follows:sand<loamy sand<sandy loam<silty loam<silt.In addition,the vegetation type had significant influence on the soil particle composition and fractal parameters,D_V followed the increasing sequence:temperate desert<temperate steppe<alpine meadow<shrub land.The capacity dimension?D₀?in alpine meadow was significantly higher than that of shrub land,indicating that there was a lack of soil particles in shrub land,and the size distribution range was smaller than that of alpine meadow.Furthermore,information dimension?D₁?,D₁/D₀,and correlation dimension?D₂?were shown as follows:temperate desert<shrub land<temperate steppe<alpine meadow,indicating that the inhomogeneity of particle size distribution in temperate desert was the largest,followed by temperate steppe and shrub land,so soil particle size distribution of alpine meadow is the most uniform.?4?Soil erosion amount was 1.86×10⁸ t/a and the erosion areas was 1.62×10⁵km² of the Qilian Mountains in 2017 by RUSLE model,which was dominated by micro-degree erosion.Analyses of the erosion intensity classifications revealed the areas of moderate,strong,and extremely strong erosion.They accounted for 10.46%of the total erosion area,but 76.70%of the total erosion amount,which were mainly distributed in the areas such as Nanshan mountain,Lenglong mountain,and Datong mountain.At the 0-40 cm depth,K value increased with the increase of soil depth,indicating that the deeper the soil depth was,the higher the sensitivity degree of soil to erosion had,and it was much easier to cause soil erosion.The distribution of K value along the slope direction and altitude showed obvious difference because of the effect of terrain feature,K value generally decreased first and then increased with the increasing altitude,and the soil erosion resistance was the strongest at the altitude of2800-2900 m.The distribution of K value varied across slope directions,it under sunny slope was significantly higher than that under shady slope and half shady slope.?5?6 soil quality key indices were chose as the MDS indices for soil quality assessment by correlation analysis and principal component analysis,including soil organic matter,soil moisture content,C/N,sand content,Cu,and GMD.The weight order of indicator in weight from high to low was:soil organic matter>C/N>sand content>soil moisture content>geometric mean diameter GMD>Cu.Soil quality index ranged from 0.175 to 0.679 with middle variance coefficients?CV?of 39.713%,and the mean was 0.380.Soil quality distribution was significantly affected by vegetation type,soil depth and terrain feature,the spatial distribution characteristic of soil quality among the different vegetation type was as follows:temperate desert<temperate steppe<alpine meadow<shrub land.Compared with that under sunny slope,the GMD under half sunny slope and half shady slope significantly increased by 75.03%,and 163.12%,respectively?P<0.05?.Furthermore,it decreases with the increase of soil depth.There was a strong correlation between the soil quality distribution and altitude of grassland,which gradually increased and then decreased with increasing altitude,and reached a peak at 2800-2900 m.Soil erosion intensity was negatively related to soil quality index,which was an important reason for soil quality reduction in the Middle—Western Qilian Mountains.