| Microbial organic phosphorus(Po)mineralization ensures continuous phosphorus(P)availability in subtropical forest ecosystems.Pure planting of Chinese fir often leads to a decrease in soil P availability compared with mixed-species planting of Chinese fir and broadleaf trees.However,the underlying microbial mechanism that mixed planting affects soil P availability remains unclear.In this study,we collected soil samples from the mixed-species plantations of Chinese fir(MCP),pure Chinese fir plantations(PCP),and secondary natural forest(SNF)in southeastern China.We determined soil physicochemical properties,P fractions,and microbial community,and characterized soil Po-mineralizing ability and Po-mineralizing fungi(Pmin-F)and bacteria(Pmin-B)taxa through targeted laboratory incubations.According,the microbial mechanism of mixed planting governing soil P availability was analyzed by using correlation analysis,network analysis,random forest model and structural equation model.We found that:(1)The results of the study on the effects of mixed planting on soil fertility show that,mixed planting significantly enhanced the soil multi-nutrient cycling(SMC)along the soil vertical spatial scale.However,the processes and mechanisms of soil SMC elevation by mixed planting in different soil layers were different.Specifically,fungal communities mainly contributed to SMC in the surface soil,and bacterial communities were the primary drivers of SMC in the subsurface soil.Further,fungal preferentially mediates the transformation of available phosphorus and soil organic carbon,and contribute 77.0% of SMC variation in the surface soil,while bacterial mainly drive the transformation of ammonium nitrogen and soil organic carbon,and account for 60.0% of SMC variation in the subsurface soil.Our results indicate that the improvement of surface soil fertility by mixed planting was mainly related to the improvement of soil P availability.(2)The results of the study on the effects of mixed planting on soil P forms and microbial community show that,Po is the main component of soil P pool in Chinese fir plantations,accounting for 46.9%～73.1% of soil total phosphorus.The effect of mixed planting on soil Po was greater than that of inorganic P.Mixed planting promoted the conversion of P to more labile forms.In young and mature stand ages,mixed planting could improve the Po-mineralization ability and P availability,but not in middle stand ages.Basidiomycota and Ascomycota were the dominant fungi in the soil of Chinese fir plantation,accounting for more than 90% of the total fungal abundance.Acidobacteria,Proteobacteria,and Chloroflexi were the dominant bacteria,which accounted for 79.5%-91.1% of the total bacterial abundance.Mixed planting affected the β diversity of fungal and bacterial communities.Mixed planting significantly increased the α diversity of the fungal community in young and mature stand ages,while having no significant effect on the α diversity of the bacterial community.Moreover,mixed planting increased the abundance of Ascomycetes and decreased the abundance of Basidiomycetes,while having no significant effect on the composition of the bacterial community.In addition,compared with symbiotic fungi and pathogenic fungi,mixed planting has significantly affected the abundance of saprophytic fungi.Furthermore,the bacterial functions related to the soil carbon and nitrogen cycle in mature stand age were significantly improved by mixed planting.(3)The results of the study on the effects of mixed planting on soil Po-mineralizing microorganisms show that,Penicillium,Apiotrichum,Saitozyma,and Trichoderma were the dominant Pmin-F in Chinese fir plantations,accounting for 94.6% of the total abundance of Pmin-F.The most abundant Pmin-B was Burkholderia-Caballeronia-Paraburkholderia,accounting for 94.4% of the abundance of all Pmin-B.Pmin-F showed higher phylogenetic diversity than Pmin-B.Mixed planting mainly affected the composition rather than the category of Pmin-F,but had a marginal effect on the composition and category of Pmin-B.(4)The results of the study on the influence of Po-mineralizing microorganisms on Po-mineralization show that,the difference in Po-mineralization capacity in PCP,MCP,and SNF was mainly determined by Pmin-F rather than Pmin-B.Although the Pmin-F community differed among the forest types,interactions between Pmin-F and the remaining soil microorganisms were beneficial for improving the Po-mineralizing capacity in all forest types.Weakening of the interactions between microbial communities led to a decline in the Po-mineralization capacity of PCP and MCP,and led to an irreversible decline in the Po-mineralization capacity of SNF.Our results demonstrated that the cooperation of soil microorganisms is critical for the enhancement of soil P availability,and the intensity of such cooperation is positively correlated with above ground vegetation diversity.(5)The results of the study on the microbial mechanism of mixed planting governing soil P availability show that,the effect of mixed planting on soil P availability could be attributed to the co-regulation of soil Labile-P and Moderate labile-P,which controlled changes in soil P availability.In this process,soil Po-mineralization is the key determinant.The Pmin-F and Pmin-B have different ecological functions in the regulation of P availability.The Pmin-F community preferentially explained 85% of the variation of soil labile P through the secretion of acid phosphatase,whereas the Pmin-B community mainly explained 71% of the variation of moderately labile P.Mixed planting can affect the composition and diversity of soil Po-mineralizing microbial community through the regulation of soil p H,thereby regulating soil P availability.In conclusion,soil fertility can be significantly improved by mixed planting.In the surface soil,the improvement of soil P availability by mixed planting is one of the key processes to improve soil fertility.In this process,microbial-involved soil Po-mineralizing is the key factor governing soil P availability.Mixed planting can enhance the corporation between soil microbial communities by improving soil p H,promoting the mineralization of Po,and thus achieving the improvement of soil P availability.Specifically,mixed planting increased soil Labile-P and Moderate labile-P contents by enhancing the cooperation of Pmin-F and Pmin-B with the remaining soil microorganisms,respectively.Our study is of great significance for the study of the restoration of degraded forests and provides a theoretical basis for the sustainable development of the plantation industry. |
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