| Global warming accelerates the retreat of glaciers in alpine regions,resulting in large areas of bare land and endangering ecosystem function and safety.Therefore,vegetation succession on bare lands is crucial to mitigate the negative impacts of climate change.Phosphorus(P),an essential nutrient for biological metabolism,is a critical component of nucleic acid,phospholipid,and ATP,which is very important for organism growth and energy supply.Phosphorus deficiency in the ecosystem can reduce primary productivity,which is detrimental to terrestrial carbon sequestration.In contrast to carbon(C)and nitrogen(N),P is mainly derived from slow rock weathering in undisturbed ecosystems,and thus P becomes a key limiting nutrient for ecological succession,especially at early pedogenesis when plants have not colonized.Soil microbes play a key role in the biogeochemical cycling of soil P and are the engine of soil P cycling during primary succession.Soil microbes can not only promote the solubilization and mineralization of P,which is difficult to use in soil,but also fix P in their biomass through direct absorption and transformation,thus promoting the turnover of P in plant-soil system.However,the mechanism of microbial community structure and function on P cycling in P-rich soils during vegetation succession remains unclear and deserves in-depth research.Based on the natural platform of Hailuogou glacier chronosequence in Gongga Mountain,this study revealed the succession patterns of soil P pools,microbial community structure and function,and further elucidated the underlying microbial mechanism of soil P cycling in P-rich soils at early natural ecosystem by using molecular biology technologies such as amplicon sequencing and functional geochip.The main results were following:(1)During primary succession,the soil inorganic P pool was gradually decreased,while the organic P pool continuously accumulated(accounting for over 80%at the later stages of succession)in both rhizosphere and bulk soils,and the concentration of P in plant leaves continuously decreased.Vegetation succession and soil development significantly increased the concentration of available P in the soils,especially in the rhizosphere soils,and the concentrations of organic P,inorganic P,and available P in the soils were relatively stable at the later stages of succession and mature forests.The concentration of P in senescent leaves was significantly lower than in green leaves,and the C:P and N:P ratios were higher than in green leaves,and these differences were more significant at the later stages of succession,indicating that plants may improve P utilization efficiency by increasing the recovery of P in leaves.Soil P pool in rhizosphere soils was significantly correlated with microbial characteristics,suggesting that microbial biomass turnover and enzyme secretion significantly affected P transformation.And the significant relationship between soil P pools and soil biochemical characteristics in bulk soils indicated that P turnover in bulk soil might be regulated by both biotic and abiotic factors.(2)Microbial community structure(including diversity,composition,and network complexity)and function(P-cycling functional genes and fungal trophic modes)changed significantly with succession.The dominant bacterial taxa change from Proteobacteria to Acidobacteria,and the community diversity(Chao 1 and Shannon index)and network complexity(number of nodes and links)decreased in rhizosphere and bulk soils.The dominant fungal taxa shifted from Ascomycota to Basidiomycota(with relative abundance reaching up to 92%).Fungal community diversity and network complexity were relatively high in both rhizosphere and bulk soils at early and later succession stages.Additionally,the abundance and proportion of genes involved in P_O mineralization,as well as Symbiotrophic fungi,exhibited the highest values along the chronosequence.The succession patterns of P-cycling genes along with bacterial diversity,suggesting that bacterial communities with higher diversity may possess greater functional potential.The Symbiotrophic fungi only showed significant positive and negative correlations with Basidiomycota and Glomeromycota,indicating that changes in vegetation type and litter composition during vegetation succession altered the structure and function of fungal communities.(3)Bacterial community composition and diversity,but not P genes,determine soil P turnover,and Symbiotrophic fungi have a more direct effect on soil P turnover than fungal community composition.Bacterial organic P mineralization in P-rich geological background is biological mineralization,which is driven by the demand of C rather than P.At the early stages of succession,there are numerous bacterial taxa significantly correlated with soil P pools in both rhizosphere and bulk soils.However,the bacterial taxa significantly correlated with soil P pools decrease with succession,while the relative proportion of Symbiotrophic fungi increased.This suggests that fungi may mediate soil P cycling at the later stages of succession by influencing the forms of P in the soil through symbiosis with plants.Therefore,bacteria initiate soil P cycling by altering community structure rather than regulating the functional potential of P cycling at the early stages of succession,while the role of bacterial communities in soil P cycling weakened at the intermediate and later stages of succession and Symbiotrophic fungi play a crucial role in improving P availability.Based on the above results,this study revealed the succession patterns of soil P pools and other soil physical and chemical properties,and microbial community structure and function in Hailuogou glacial retreat area,and further elucidated the microbial mechanism of soil P cycling in early natural ecosystem.These findings of the study contribute to our knowledge of the processes and mechanisms of soil P biological cycling and nutrient dynamics during the succession of natural ecosystems and underscore the importance of microbial communities in the biogeochemical cycling of elements in early ecosystems,which provide a scientific basis for maintaining the services and functions of alpine ecosystems and achieving sustainable development. |
