Characteristics Of Soil Phosphorus Fractions Under The Heterogeneous Understory Vegetations In A Chinese Fir Plantation

Phosphorus（P）is considered to be an important limiting factor for net primary productivity in subtropical ecosystems.Vegetation types can regulate input and output of soil phosphorus affecting its availability.At present,however,it is still unclear how understory vegetation affects soil phosphorus fractions and its supply capacity.In this study,the study site was set in a Chinese fir plantation in Xiqin Teaching Forest Farm of Fujian Agriculture and Forestry University.Soil samples at 0-80 cm soil depth and in the rhizosphere were collected from three dominant understory vegetation communities:Indocalamus tessellatus,Diplazium donianum,and Oreocnide frutescens,respectively,in this plantation.Using Hedley fractionation method and molecular biology techniques（i.e.high-throughput sequencing）coupling with soil traditional analysis method,soil P fractions,phosphatase activity,and phosphate-solubilizing microbial community were investigated and their relationships were checked.The main results obtained in this study were as follows:（1）In April,the content of labile phosphorus in 0-40 cm soil layer of Oreocnide frutescens was always higher than that of Indocalamus tessellatus and Diplazium donianum,and it was the most obvious in 20-40 cm soil layer.The content of moderately labile phosphorus in 0-10cm soil layer was significantly higher than that of Indocalamus tessellatus,and it was significantly higher in 60-80 cm soil layer.In November,the content of moderately labile organic phosphorus in 0-80 cm soil layer of Indocalamus tessellatus was significantly higher.In the monthly variation,soil labile phosphorus in November was significantly lower than that in April,the most obvious in Oreocnide frutescens.There was no significant seasonal difference in the content of moderately labile phosphorus in the0-20 cm soil layer of Indocalamus tessellatus,but it was significantly lower in November than that in April in the soil of Diplazium donianum and Oreocnide frutescens.The main factors affecting the change of phosphorus components in 0-10 cm,10-20 cm and 20-40 cm soil layers in April were NO₃^-,soil water content and acid phosphatase activity,respectively.The main factors affecting the change of phosphorus components in 0-10 cm,10-20 cm and 20-40 cm soil layers in November were microbial biomass phosphorus,p H and soil moisture content.In short,the ability of phosphorus supply and retention of labile organic phosphorus in Oreocnide frutescens soil profile was strong in April,but more labile phosphorus was consumed in the growth process.The Indocalamus tessellatus soil profile had high potential for phosphorus supply,and the utilization of moderately labile organic phosphorus in surface soil was low in the growth process.（2）The phosphorus fractions in rhizosphere soil of different understory vegetation were significantly different.The content of labile phosphorus fractions in rhizosphere soil of Oreocnide frutescens was significantly higher than that of Indocalamus tessellatus and Diplazium donianum,which was the most obvious in April.The content of labile inorganic phosphorus and moderately labile inorganic phosphorus in rhizosphere soil of Indocalamus tessellatus was significantly lower.These results indicate that the availability and supply capacity of phosphorus in rhizosphere soil of different understory vegetation were the highest in Oreocnide frutescens and the lowest in Indocalamus tessellatus.In the monthly variation,the content of non-labile inorganic phosphorus in rhizosphere soil of Diplazium donianum and Oreocnide frutescens in July was significantly lower than that in April,indicating that the increased demand for Diplazium donianum and Oreocnide frutescens the soil non-labile phosphorus fractions.（3）The abundance of pho D gene,Chao1 and Shannon index in rhizosphere soil of Indocalamus tessellatus were significantly lower than those of Diplazium donianum and Oreocnide frutescens.The dominant phyla of phosphorus-solubilizing microorganisms in rhizosphere soil of the three understory vegetations was composed of Proteobacteria and Actinobacteria.From the genus level,the Bradyrhizobium was the dominant genus,and the relative abundance of Bradyrhizobium in rhizosphere soil of Indocalamus tessellatus is significantly higher than that of Diplazium donianum and Oreocnide frutescens,which is negatively correlated with HCl-Pi.The RDA results showed that Acidovorax was the main phosphate-solubilizing microorganism affecting the change of phosphorus composition in rhizosphere soil.The relative abundance of Acidovorax was the highest in Diplazium donianum,and it was negatively correlated with Residual-P.This study speculated that understory vegetation limited access to P would stimulate the abundance of different types of phosphorus-solubilizing microorganisms,thereby promoting the activation of non-labile phosphorus and alleviating P limitation.