Soil Phosphorus Dynamics And Associated Factors Along Elevation Gradients In Yulong Snow Mountain

Elevation has been considered as the natural gradient for assessing the structure and function of ecosystems under global climate change.Because there are vertical differences in environmental climate and vegetation types under different altitude gradients,which are characterized by changes in temperature,moisture,soil properties and microbial activity,and hence can impact the dynamic changes of soil phosphorus(P)fractions in mountain forest ecosystems.However,the effects of vegetation,microbe and environments on soil P dynamics at different altitudes remain unclear.In this study,six typical vegetation ecosystems along the elevation gradients(2600-3900 m)of Yulong Snow Mountain were selected to expore soil P dynamics and associated factors.Based on the modified Hedley extraction method,HCl O4-H2SO4 Molybdenumantimony resistance colorimetric method,hydrochloric acid-ammonium fluoride method,indoor culture method,chloroform fumigation method and fluorescence microversion technology,we tried to examine soil P fractions(total phosphorus,TP;Available phosphorus,AP;Organic phosphorus,OP;Inorganic phosphorus,IP;Labile phosphorus,resin-Pi,Na HCO3-Pi and Na HCO3-Po;Medium labile phosphorus,Na OH-Pi,Na OH-Po and DHCl-Pi;Stable phosphorus,CHCl-Pi,CHCl-Pi and Residual Pt)and P mineralization during dry and wet seasons.Soil microbial attributes and related influencing factors(including leaf C,N,P contents,soil properties,soil microbial C,N and P biomass and phosphatase activity).The main conclusions are as follows.(1)Soil phosphorus components along elevational gradientsSoil P fractions were significantly affected by different altitudes and seasons(P<0.05).All soil P were the highest at 2900 m and 3200 m,indicating P availability and supply capacity were larger at the low middle elevations.Meanwhile,the pattern of P fractions weas higher at the low-middle elevations than that at the middle high elevations,due to acidic soils and high moisture at middle and high elevations whichcould accelerate the weathering rate,thus reducing the content of P.The soil available P content was higher in wet season than that in dry season,suggesting that the suitable hydrothermal conditions could utilize microbial life activities to promote the dissolution of mineral P and the mineralization of organic P in the wet season.In addition,the active P only accounted for 1.7% of the total P ratio,while inactive P(the sum of mid-active P and stable P)accounted for more than 95%,indicating that inactive P was the main component of the ecosystem,and the available P was low for direct biological use.(2)Soil organic P mineralization along elevational gradientsThe soil organic p mineralization rate fluctuates in the range of-224.74 to106.51 mg kg-1 d-1 along the elevational gradients.Our results showed that the release of organic P mineralization and soil P biological fixation occurred simultaneously in different elevations of Yulong Snow Mountain,and the value of P mineralization rate was positive when the mineralization was strong,and it was negative when biological fixation was strong.In the first 25 days of incubation,the rate of soil organic P mineralization fluctuated with the time of incubation,but tended to be stable in the later period,indicating that the adaptability of soil microorganisms to the environment was low in the early period of incubation.Compared with the wet season,the fluctuation range of organic P mineralization rate was relatively small in dry season,indicating that the soil temperature was suitable and moisture was higher in the wet season,which was conducive to promote the metabolic process of soil microorganisms.In the dry season,the Pmineralization rate was the highest at 3200 m altitude,indicating organic P mineralization was released.In other elevations,organic P mineralization was immobilized,indicating that higher soil and microbial C and N were favorable to organic P mineralization in the middle elevations.(3)Soil microbial attributes and influencing factors in P dynamics along elevational gradientsSoil microbial P biomass and phosphatase activity significantly varied with different elevation gradients(P<0.05),due to changes in climatic conditions and soil microenvironments.The soil microbial biomass was higher at 2900 m elevation thanother elevations,indicating that the higher SOC and SON contents at middle and low elevations provided guarantee of survival for microorganisms which was more conducive to the activities of soil microorganisms.Soil P activity was the highest at 3900 m elevation in wet season,while soil phosphatase activity was the highest at 2900 m elevation in dry season,indicating that soil microorganisms under low temperature stress in the wet season could secrete more phosphatase to obtain energy and nutrients at the high elevation.For soil mositure was an important condition affecting enzyme activity in the dry season.The suitable hydrothermal conditions at were more conducive to microbial activity with more phosphatase secretion at the low-middle.Correlation analysis showed that abiotic factors(DOC,SM and NO3--N)and biological factors(Leaf-P and MBP)were the main factors in affecting soil P fractions.There was a significant positive correlation between soil P and soil soluble organic carbon and microbial biomass P(P<0.05),indicating that SOC provided sufficient energy and material for microbial life activities and then enhanced microbial biomass to transform soil P fractions.When soil P supply was insufficient,the turnover of soil microbial biomass P could supply Leaf-P in soil solution,which was conducive to the absorption and utilization of P by plants.There was a significant negative correlation between soil P and nitrate nitrogen(P<0.05),indicating that microbial assimilation and nitrification of inorganic N would further release more protons to promote the release of available P.Soil organic P mineralization rate was significantly correlated with the SOC,SON,MBC and MBN(P<0.05),and had a significant negative correlation with the SOC,indicating that soil organic P mineralization rate depended on soil organic matter,C and N contents and soil microbial activity.Organic C had a protective effect on organic P,and hence the accumulation of SOC was beneficial to the fixation of organic P.Based on the path analysis,we found that the micro-climates indirectly affected soil P fractions transformation,by influencing the soil microbial biomass,and affected microorganism by enhancing the phosphatase activity and reduced the content of microbial biomass energy distribution strategy,promoting the transformation of more stable P to active P fractions,to meet the needs of microbes to P in different elevations.In summary,our results showed that soil P fractions and mineralization were affected by both abiotic and abiotic factors along elevation gradients of Yulong Snow Mountain,primarily because changes in soil properties and microbial biomass along the elevational gradients,thus further affecting soil P availability and cycing.Therefore,our results suggested that,soil environmental factors could influence the availability and transformation process of soil P,which should be considered in the future forest ecosystem protection and management.