Soil Nutrient Density And Its Characteristics Of Spatial Variablity On The Southern Slopeof Qinling Mountains In Shaanxi Province

The difference characteristics of soil nutrient density at the spatial level were analyzed to reveal the relationship between the soil nutrient density and the site factors in the southern slope in Qingling Mountains.Based on the soil nutrient density under different site factors,the main factors affecting the soil nutrient density on the southern slope of the Qinling Mountains were determined.The data of forest stands,topography and vegetation was obtained by the standard ground investigation method.The results were as follows:（1）The average soil organic carbon（SOC）density in the southern slope of the Qinling Mountains was 125.41 t·hm^-2.The coefficient of variation was 0.42,indicating that there was a large spatial variation in SOC density in this forest region.The SOC density of in Longcaoping Forest Region(143.55 t·hm^-2)was the highest and the SOC density of Changqing Forest Region(90.22 t·hm^-2)was the lowest among the different regions.The SOC density increased firstly and then decreased withthe elevation increasing,and it gradually decreased with the slope increasing.Slope location and elevation were the main factors affecting the SOC density in forest land.Compared with the main forest areas in China,the SOC density on the southern slope of the Qinling Mountains was at a higher level in China.（2）The mean total nitrogen density of the forest land on the southern slope of the Qinling Mountains was 17.58 t·hm^-2,and the average available nitrogen density was 1.15t·hm^-2.The density of total nitrogen and available nitrogen in Longcaoping Forest Region were the highest,which was 24.74 t·hm^-2and 1.50 t·hm^-2,respectively.And the density of total nitrogen and available nitrogen in Changqing Forest Region were the lowest,which was11.28 t·hm^-2and 0.84 t·hm^-2,respectively.There was a bimodal curve between soil total nitrogen and available nitrogen density and elevation.And it gradually decreased with increasing slope.There were extremely significant differences of soil total nitrogen and available nitrogen density between different slope positions.And there was a significant difference in soil total nitrogen density between shady slope and sunny slope,but no significant difference in available nitrogen density.The slope position and slope were the main site factors affecting the soil nitrogen density on the southern slope of the Qinling Mountains.There were significant positive correlation between total nitrogen and available nitrogen and soil organic matter.（3）The mean soil total phosphorus density on the southern slope of the Qinling Mountains was 3.28 t·hm^-2,and the average available phosphorus density was 0.25 kg·m^-2.There was large spatial variation in soil phosphorus density on the southern slope of the Qinling Mountains.The total phosphorus density in the Ningxi Forest Regionand the available phosphorus density in Ningdong Forest Regionwere the highest,which were 3.80 t·hm^-2and0.31 kg·m^-2,respectively.The total phosphorus density and the available phosphorus density in Changqing Forest Regionwere the lowest,which were2.23 t·hm^-2and 0.15 kg·m^-2,respectively.The soil total phosphorus and available phosphorus density increased firstly,then decreased,and then increased again with the increase of altitude,and gradually decreased with the increase of slope.The density of total phosphorus and available phosphorus in the downhill position was the highest,followed by the mid-slope position,and the uphill position was the lowest.There were significant differences between the total phosphorus and available phosphorus concentrations between the uphill position and the mid-slope position and down-slope position.There was no significant difference between the mid-slope position and the down-slope position.There was no significant difference in soil total phosphorus and available phosphorus density between sunny slope and shady slope.The slope position was the main factor affecting the soil phosphorus density on the southern slope of the Qinling Mountains.There was no significant positive correlation between soil total phosphorus,available phosphorus content and organic matter content.（4）The average soil total potassiumdensity in the southern slope of the Qinling Mountains was 239.67 t·hm^-2,and the average available potassium density was 1.16 t·hm^-2.The density of total potassium and available potassium in the forest of Longcaoping Forest Region is the highest,which were 266.53 t·hm^-2and 1.25 t·hm^-2,respectively.The total potassium and available potassium density in the Taibaishan Forest Region were the lowest,which were 196.22 t·hm^-2and 0.89 t·hm^-2,respectively.The relationship between soil potassium density and altitude showed a trend of rising firstly and then decreasing.The relationship between soil available potassium density and altitude showed a trend of decreasing firstly,then rising and then falling.And both of them showed a decreasing trend with the increase of slope.There was a significant difference between the density of total potassium and available potassium in the uphill position and the mid-slope position and down-slope position,while there was no significant difference between the mid-slope position and the down-slope position.The density of total potassium and available potassium in sunny slopes was lower than that in shady slopes,and there was no significant difference.Slope position was the main factor affecting on soil potassium density.There was no significant positive correlation between soil total potassium,available potassium content and organic matter content.（5）The soil nutrient density varies with different forest types.The comprehensive fertility of the Abies fargesii forests and Picea asperata Mast forests were the highest.The Betula albo-senesis forest,Pinus armandii forest,Quercus alienavar.acuteserrata forest and Pinus tabuliformis forest were lower,and the Larix chinensis forest was the lowest.