Effects Of Intensive Management On The Community Structure And Diversity Of CO₂-assimilating Bacteria And Diazotroph In Phyllostachy Pubescens Stand

Bamboo forests are widely distributed in many regions,including Asia,Pacific Islands,Latin America and Europe.Phyllostachys pubescens(moso bamboo)covers 74% of the total area of bamboo forests in China,primarily in the south,and is a significant component of forest ecosystems.Long-time intensive management has been confirmed to lead to increased soil CO2 emissions,decrease soil organic C storage,and especially impact the community structure of soil bacteria and fungi,including ammonia oxidizing archaea(AOA)and ammonia oxidizing bacteria(AOB).In most of the studies,CO2-assimilating bacteria and diazotrophs play key roles in the biogeochemical cycles of carbon and nitrogen.However,their roles in bamboo forests have not been studied earlier.We examined the effects of long-term intensive management on the abundance and diversity of soil CO2-assimilating bacteria and diazotrophs in a Phyllostachys pubescens bamboo forest.Topsoil(0-20 cm)and subsoil(20-40 cm)samples were collected in moso bamboo stands and analyzed using quantitative PCR(q-PCR),terminal restriction fragment length polymorphism analysis(T-RFLP),and cbb L and nif H clone libraries.Differences in community structures of CO2-assimilating bacteria and diazotrophs over time and their relationships with soil properties were assessed using redundancy analysis(RDA).We found that long-term intensive management significantly increased soil nutrient contents but decreased soil p H.Among the soil parameters measured,N content correlated with cbb L abundance,but SOC did not.In Lishui(LS),the abundance of bacterial cbb L in the topsoil was highest in plots managed intensively for ten years,correlating positively with N content(P<0.05).In the subsoil,cbb L abundance decreased linearly with intensive management time and correlated positively with C:N ratio(P<0.05).In Changxing(CX),the cbb L abundance in the topsoil was higher than in LS,and the abundance increased after long-term management in both the topsoil and subsoil.In the CX subsoil,cbb L abundance correlated positively with AN(P<0.05).RDA showed that NH4+-N and NO3--N were the most significant soil parameters influencing the community composition(P<0.05).Permutational multivariate analysis showed that geographical position,rather than the soil depth or intensive management time,was the most important factor affecting the diversity and abundance of CO2-assimilating bacteria and diazotrophs.The dominant 177 bp T-RF represented facultative autotrophic bacteria,such as Bradyrhizobium and Mesorhizobium.It was found that the abundance,community composition and diversity of diazotrophs recovered after long-term management,driven by nutrient accumulation.In LS,the nif H abundance correlated positively with p H in the subsoil(P=0.040),but not in the topsoil.The nif H abundance correlated positively with NH4+-N(P=0.008)in the topsoil,while in the subsoil the nif H abundance correlated negatively and positively with NH4+-N(P=0.008)and C:N ratio(P<0.001),respectively.According to the permutational multivariate analysis,intensive management time was the major factor influencing the nif H gene abundance,followed by the site,and the interaction between site and IMT.Soil depth and intensive management time had significant effects on the diazotroph diversity.In this study,less diazotroph genera were detected than in previous studies.The diazotrophic communities were dominated by rhizobial genera,such as Bradyrhizobium,Mesorhizobium and Azorhizobium.