Influences Of Nitrogen Deposition On Soil Phosphorus Fractions And Their Mechanisms In Subtropical Castanopsis Carlesii Forest Ecosystem

Elevated nitrogen?N?deposition may exacerbate soil P limitation of terrestrial ecosystems,especially in the subtropical forest soil where is characterized as P deficient.However,the responses of soil P fractions to N enrichment and the mechanisms in this region are still unclear.To address these issues,we studied a natural Castanopsis carlesii forest in Fujian,China subjected to N addition from 2012,three treatments were established,including a control treatment without N addition?CT?,a low-N treatment of 40 kg ha^-1 yr^-1?LN?,and a high-N treatment of 80 kg ha^-1 yr^-1?HN?.We determined soil P fraction by modified Hedley-P method and soil microbial community by PLFA and high-throughput sequencing.In addition,soil enzyme activity,microbial biomass carbon,microbial biomass N,microbial biomass P,and soil abiotic factors such as pH,exchangeable cation,and iron-aluminium oxides were also investigated.We found that N addition significantly decreased organic-bound Fe and Al?Fe_p and Al_p?and free Fe and Al?Fe_o and Al_o?,but increased amorphous Fe and Al?Fe_d and Al_d?.Meanwhile,the NaOH-Po decreased significantly in both growth and non-growth seasons after N enrichment,whereas Resin-P increased in non-growth season but decreased in growth season in both LN and HN treatments.These results suggested that N deposition accelerate the decompstion of organic P to enhance the availibilty of P,and the content of soil liable P were influenced by the P demands of microbes and plant in growth and non-growth seasons.We also found N addition enhanced MBP and decreased both MBC:MBP and MBN:MBP,which indicated that N deposition stimulated the P competition between microbes and plant.In addition,N application significantly increased?G and ACP activities.N addition altered the composition of fungi but not bacteria.Compared to CT treatment,the abundances of phosphate solubilizing bacteria decreased while phosphate solubilizing fungi increased in both LN and HN treatments.According to RDA,significant correlation between soil P fractions,Fe_p and Al_p,AMF,and ACP were noted,which implied that organic-bound Fe and Al,AMF,and ACP may predominantly affect the mobilization and transformation of soil P fractions under with and without N application.Pearson correlation analysis revealed that Bradyrhizobium,Mesorhizobium,and Aspergillus may contribute to mineralization of organic P and desorption of inorganic P in soil.Predictive functional profiling of microbial communities by PICRUSt showed that N addition significantly enhanced the activities of acid phosphatase,alkaline phosphatase D,4-phytase,3-phytase,histidinol-phosphatase,and phospholipase C.This finding further support the critical roles of microbes and acid phosphatase in soil P supplement by accelerating the decomposition of organic P.In conclution,enhance minerlization of organic P and increase available P maybe a possible pathway to meet increase P demands in the natural Castanopsis carlesii forest under N deposition.Iron-aluminium oxides and EMF are the key abiotic and biotic factor,respectively.Our research is beneficial to clarify the mechanism of soil P cycling in subtropical forest soil where P-deficiency under elevated N deposition in the future.