Distribution Characteristics Of Soil Organic Nitrogen Fractions And Its Influcing Factors In The Upper Minjiang River

The Upper Minjiang River is located between the Qinghai-Tibet Plateau to the Sichuan Basin,which is a typical region in terms of its sensitivity to global climate change and fragility of environment.Soil organic nitrogen fractions(SONFs)is a critical component of the terrestrial nitrogen cycle with a proportion of 90%for soil nitrogen.Thus,it is very important to study soil organic nitrogen fractions(SONFs)distributions and its influencing factors at the region to estimate their potential response capacities and buffer abilities to global climate changes.In this paper,we main study the geographic distribution of SONFs and its relationship with environmental variation,and explore the relationships between SONFs with influencing factors by combining multivariate statistics such as multiple regression analysis,principal component analysis.The main research conclusions were as follows:(1)In the Upper Minjiang River,the concentrations of SONFs were ranged from 0.82 to 4.05 g kg-1.Acid-hydrolyzable nitrogen(AHN)was the dominant fractions of soil organic nitrogen(SON),its concentration was ranged from 0.56 to 2.61 g kg-1 and proportion of SON was 58.57 to 78.92%.The concentrations of Non-hydrolyzable nitrogen(NHN)were ranged from 0.26-1.44 g kg-1 and proportion of SON was 21.00-41.53%.Amino acid nitrogen(AAN)and Hydrolysable unknown N(HUN)were the dominant fractions in SONFs with concentrations of 0.22-1.03 g kg-1 and 0.19-0.94g kg-1,respectively.Amino sugar nitrogen(ASN)concentrations with 0.03-0.14 g kg-1 were the lowest in study area.(2)SONFs except for ASN demonstrated significant differences among the eight soil types in 0-40 cm(P<0.01)and generally decreased with soil depth.The concentrations of SONFs in alpine meadow soil were significantly higher than in the other soils.Furthermore,the proportions of AHN to SON of all the soil samples generally decreased with soil depth in the profile distribution.In contrast,NHN augmented with the soil depth.However,the proportions of NHN to SON increased follow the sequence from yellow cinnamon soil to alpine meadow soil.The proportion of AAN to SON suggested AAN was the dominant fraction in SON while the proportion of ASN to SON was the lowest.The concentrations of SONFs except for ASN obviously increased from the southeast to the northwest in this study area.Moreover,SONFs showed a linear or exponential augment with increasing latitude(P<0.01).SONFs except for ASN in the two layers and HUN in 20-40cm layer showed positive linear decrease with increasing longitude.(3)SONFs except for ASN showed significant positive correlation with soil organic matter(SOM),total phosphorus(TP)and silt,in which SOM and SONFs had the highest correlation coefficients,while they present obvious negative correlation with pH,total kalium(TK),sand and clay.The correlation coefficients between AAN and soil properties in 0-20 and 20-40 cm soil layers were higher than the other fractions(P<0.05).(4)The SONFs concentration was significantly affected by parent material,topography,plant community,land use and distance.It is noteworthy that SONFs except for ASN under different parent material demonstrated remarkable differences(P<0.01).SONFs showed a linear or exponential augment with increasing latitude(P<0.05).Moreover,the SONFs concentration except for ASN on the north and west aspects were significantly higher than those on the south and east aspects(P<0.05).SONFs except for ASN in two soil layers under different plant communities demonstrated remarkable differences(P<0.05),and showed the similar increase trends with plant communities distributed from arid valley shrub communities to evergreen broadleaf forest communities and deciduous broadleaf forest communities,then dropped at coniferous forest communities,at last increased from subalpine meadow communities to alpine meadow communities.SONFs concentration showed significant difference under different landuses(P<0.05).SONFs concentration under shrub land were significantly lowers than under other landuses(P<0.05),grassland hold the highest SONFs concentration.In 0-20 cm,SONFs except for ASN demonstrated significant differences among different distances(P<0.01)and increased with the rise of distance.(5)AAN showed strong negative linear correlations with mean annual temperature(MAT)while the other fractions strong negative exponential correlations with MAT(P<0.01).The fractions except for ASN in 20-40 cm soil layer showed strong negative exponential correlations with mean annual precipitation(MAP)in the two layers(P<0.05).According to the coefficient of determination(R2)of regression models,SONFs were the more sensitive to MAT than MAP.Change values for NHN and AAN concentrations in 0-20 cm soil layer was decreased by 0.203 g kg-1,0.050 g kg-1 from-3 to-2 ? and from 14 to 15?,respectively.Similarly,NHN reduced by 0.279 g kg-1,0.132 g kg-1 from 400 to 500 mm and from 900 to 1000 mm MAP,separately.NHN and AAN among SONFs had the largest change sizes for per centigrade degree or each 100 mm precipitation in the layer,and were the most sensitive fractions to climate change.Moreover,this result implied that SONFs were more sensitive in the upper mountain(-3--2 ?)than in the valley bottom(14-15?).(6)Principal component analysis showed that parent material,altitude,slope aspect,land use,plant community,land use were positive impact factors for SONFs.Multiple stepwise regression analysis was used to analyze the major and minor effects of these influencing factors on the SONFs.The explained variance(R2)of 0-20 cm model ranged from 0.228 to 0.938.The MAP was a significantly negative impact factor for AHN,but the positive impact on AAN(P<0.05).The explained variance(R2)of 20-40 cm model ranged from 0.246 to 0.877.Parent material,altitude,slope aspect,plant community,were positive impact factors for SONFs.The MAT and land use were significantly negative impact factors for HAN,but MAP was the positive impact factors for AAN(P<0.05).