Spatial Distribution Characteristic Of Soil Organic Nitrogen Fraction And Its Impact Factors In The Yalong River Valley

The Yalong River Valley,which located in southeastern of Qinghai-Tibet Plateau,spans large gradients in topography and climate with rich biodiversity and fragile ecosystems.The fragile ecosystems sensitively respond to natural environmental conditions change.Soil organic nitrogen fractions occupied a proportion of 85%of soil nitrogen,and can respond and regulate environmental conditions.It is essential to investigate the relationships between soil organic nitrogen fractions(SONFs)and environmental conditions,and provide a scientific basic data for study area restoration,utilization and protection.The study explored SONF special distribution in the Yalong River Valley by combining field investigation,laboratory analysis and data compilation.Additionally,the effect of elevation,aspect,mean annual temperature(MAT),and precipitation(MAP)on soil organic nitrogen fractions and the responses among SONFs,plant community and plant species richness were explored.The main research results were as follows:(1)Soil organic nitrogen concentrations ranged from 842.85-4018.47 mg kg-1,its mean value was 2240.64 mg kg-1 and proportion of soil total nitrogen was 93%.Non-hydrolysable nitrogen(NHN)concentrations ranged from 339.60 to 1530.00 mg kg-1.The ranges of hydrolysable ammonium nitrogen(HAN)and amino acid nitrogen(AAN)concentrations were 117.38-685.88 mg kg-1 and 154.16-996.51 mg kg-1,respectively.And they made up the main proportion of total hydrolysable nitrogen(THN)with proportion of 19%and 26%for soil organic nitrogen,respectively.Secondly,hydrolysable unknown nitrogen(HUN)concentrations and amino sugar nitrogen concentrations(ASN)were 86.94-569.03 mg kg-1 and 38.35-279.57 mg kg-1(2)SONFs showed significantly different among various soil types(P<0.05).HAN and AAN concentrations in Molli-Gelic Cambosols and Mat-Cryic Cambisols were higher than in the other soil types,and all of them were no difference among Ferri-Udic Argosols,Hapli-Udic Argosols and Molli-Cryic Argosols,and Calci-Ustic Argosols and Gelic-Orthic Primosols hold the lowest concentrations;ASN exhibited no difference among all soil types;NHN concentrations with 1221.52 mg kg-1 were the highest in study area.(3)All SONFs except for ASN logistically decreased with longitude and logistically increased with latitude(P<0.05).The study demonstrated that SONF concentrations increased with increasing elevation.When elevation varied from 4000 m to 5000 m,SONF concentrations dramatically decreased to be lowest.SONF concentrations were higher in north and west than in south and east(P<0.05).(4)SONF concentrations were exponentially decreased with MAT increasing(P<0.05).When MAT gradients varied from 0-5? to 5-10?,the decrease values of THN,NHN,HAN and HUN were the highest.The variation of AAN and ASN reached at summit when MAT gradients varied from-5-0? to 0-5?.In addition,when MAT rise 10C,the decline values of SONFs in alpine were 0.9-4.5 times greater than the values in valley floor.SONF concentrations logistically decreased with MAP increasing,when MAP varied form 750 mm to 850 mm,SONFs varied with different values.(5)SONF concentrations showed significant difference among various plant communities(P<0.05).Each SONF expressed significant elevation trends.SONF concentrations under valley xerophytic shrub communities were lower,then increased from deciduous broadleaf forest communities to evergreen broadleaf forest communities,and to shrub communities,grass communities hold the highest SONF concentrations,and finally sharply decreased to alpine scree communities.Plant species richness showed logistically increased with SONF concentration increase.(6)SONF concentrations showed significant difference under various landuses(P<0.05).SONF concentrations under shrub land were significantly lowers than under grassland and higher than under arbor forest lands(P<0.05);valley xerophytic land and desert grass land hold the lower SONF concentrations.(7)Correlation analysis suggested that SONFs exhibited significant positive correlation with plant species richness(P<0.01);except for HAN and AAN,the other SONFs were significantly positively with elevation and negatively correlated with MAT(P<0.05);NHN and HUN hold significant positive correlation with landuse,and the other SONFs showed no correlation with landuse(P>0.05);SONFs were significantly negatively correlated with longitude and positively correlated with latitude(P<0.05).Multiple linear regression analysis demonstrated that all SONFs were affected by plant species richness.Except for ASN,the other organic nitrogen fractions were affected by aspect.Additionally,THN and AAN were influenced by elevation,and NHN was influenced by MAT,and HAN was affected by elevation and plant community,and HUN was influenced by elevation.