| Functional microbial communities are an important component of ecosystem nutrient cycling and are key to maintaining soil ecosystem function.The high-salt environment has direct and indirect effects on soil enzyme function,carbon,nitrogen and phosphorus functional microbial communities,and then has a long-term and profound impact on the structure and function of soil ecosystems.Revealing soil carbon,nitrogen and phosphorus metabolic processes and their microbial characteristics under high-salinity environmental gradients is of great significance for understanding the characteristics and driving mechanisms of soil saline-alkali ecosystem function changes.However,how soil C,N,and P functional communities change along salinity gradients,thereby affecting soil ecosystem function,remains unclear.Based on the soil of the Chaka Salt Lake area on the Qinghai-Tibet Plateau and the Dingbian Salt Lake area on the Loess Plateau,this study focused on the characteristics of soil functional microbial community changes under high-salinity gradients.The characteristics of carbon,nitrogen,and phosphorus functional microbial communities clarified the microbial formation mechanism and influencing factors of soil ecosystem function changes in high-salt environments.The main conclusions are as follows:（1）Soil salinity weakened microbial carbon limitation,enhanced microbial nitrogen and phosphorus limitation,and the threshold was 0.075g/g soil salinity.Soil p H,electrical conductivity and salinity were important factors affecting soil microbial nutrient limitation.Soil salinity significantly inhibited the activities of leucine aminopeptidase（LAP）,N-acetyl-β-glucosidase（NAG）,and alkaline phosphatase（AP）when the salinity was increased to 0.075 g/g soil,but had no significant effect onβ-glucosidase activity.Soil salinity exhibited the same trend in enzyme stoichiometry characteristics,and the results showed that when soil salinity increased to 0.075 g/g soil,salt significantly inhibited enzyme C/N and C/P,but had no significant effect on enzyme N/P.When the salt content was greater than 0.075 g/g soil,salinity significantly reduced the vector length,indicating that soil microbial nutrient limitation shifted from carbon limitation to nitrogen and phosphorus limitation.Soil salt content,electrical conductivity,and p H directly or indirectly affect soil microbial nutrient limitation by affecting soil nutrients and chemical stoichiometry ratios.（2）Increasing soil salinity inhibited soil carbon fixation and carbon degradation potential,which was jointly driven by soil chemical stoichiometry and enzyme stoichiometry characteristics.After analyzing the results of eight conventional soil carbon fixation pathways,it was shown that soil salinity（greater than 0.075g/g soil）significantly inhibited the Calvin cycle and reverse TCA cycle carbon fixation pathways,but promoted the soil Wood-Jondahl pathway.Increasing soil salinity inhibited starch degradation,includingα-amylase,glucoamylase,and isoamylase,cellulose degradation cellulase,and chitin degradation chitinase,but promoted the degradation of semi-cellulose arabinosidase and pectinα-L-rhamnosidase.Soil carbon fixation is regulated by soil nutrients and soil chemical stoichiometry,while soil carbon degradation is regulated by changes in soil enzyme activity and enzyme stoichiometry.Overall,soil salinity directly and indirectly affects soil carbon cycling by affecting soil chemical stoichiometry and enzyme stoichiometry characteristics.（3）Soil salinity significantly inhibits soil nitrification potential,but has no significant effect on soil denitrification potential.The heterotrophic nitrate reduction pathway is a key pathway for maintaining soil nitrogen function in extremely saline-alkali habitats.Increasing soil salinity significantly inhibits the relative abundance of amo A genes responsible for nitrification,but has no significant effect on the relative abundance of amo B genes.With increasing salinity,the relative abundance of denitrification genes,including nir K,nir S,nor B,and nor C,has no significant effect.Soil salinity greater than 0.05 g/g soil)significantly inhibits the relative abundance of the nir B gene responsible for heterotrophic nitrate reduction,but significantly increases the relative abundance of the nir D salt content>0.075 g/g soil)and nrf H（salt content>0.025 g/g soil）genes.Soil p H and total phosphorus content are important driving factors for soil nitrification,while soil NH4+-N content and p H are important driving factors for soil denitrification.Soil salinization indirectly affects soil nitrogen cycling by affecting soil nutrients,enzyme activity,and enzyme stoichiometry characteristics.（4）Soil salinity significantly inhibits soil organic phosphorus mineralization and inorganic phosphorus transfer potential,but promotes soil phosphorus cycling by increasing the degradation potential of phosphodiesterase（pho D gene）and pyrophosphatase（ppx gene）.Soil salinity significantly reduces the relative abundance of organic phosphorus mineralization genes（opd,phn K,and phn W）,but increases the relative abundance of the pho D gene.Salt content significantly reduces the relative abundance of the ppk1 gene（salt content>0.05 g/g soil）,gcd gene（salt content>0.075 g/g soil）,and ppx gene（salt content>0.075 g/g soil）responsible for inorganic phosphorus transfer,but increases the relative abundance of the ppa gene（salt content>0.05 g/g soil）.Soil salinity,conductivity,p H,and available phosphorus are important driving factors for soil organic phosphorus mineralization,while soil salinity,conductivity,p H,chemical stoichiometry ratio,and SC/P are significant driving factors for soil inorganic phosphorus transfer.Overall,increasing soil salinity affects soil phosphorus cycling directly and indirectly by affecting soil nutrients,phosphatase activity,and enzyme stoichiometry.（5）Soil salinity significantly reduces soil ecosystem function,with carbon functional groups,nitrogen functional groups,and available nutrient ratios being the core influencing factors of soil ecosystem function.Soil salinity significantly reduces soil carbon functional diversity（salt content>0.1 g/g soil）,phosphorus functional diversity（salt content>0.075g/g soil）,and functional diversity（salt content>0.05 g/g soil）.With increasing soil salinity,soil ecosystem multifunctionality significantly decreases（salt content>0.075 g/g soil）.There was no significant correlation between soil carbon functional diversity,nitrogen functional diversity,and functional diversity with soil multifunctionality,while phosphorus functional diversity showed a significant correlation.Using the multiple threshold method to analyze the relationship between soil nitrogen function and multifunctionality,the results showed that nitrogen function is more redundant than other functions,with a maximum threshold of Rmed=0.72.Soil phosphorus is more important than other elements in maintaining soil ecosystem function in saline-alkali habitats.The above results reveal the characteristics and factors of changes in soil enzyme function,carbon,nitrogen and phosphorus function in extremely saline-alkali environments,clarify the impact and factors of soil salinity on ecosystem function,and establish the core role of soil phosphorus in maintaining soil ecosystem function under saline-alkali habitat conditions.The research results can provide a theoretical basis for the restoration of ecosystem function in saline-alkali soil,as well as for integrated management and regulation of saline-alkali soil restoration measures. |
