Effects Of Chicken Farming In A Lei Bamboo(Phyllostachys Praecox) Forest On Soil Fertility And Microbial Community Structure Characteristics

China is one of the countries with the most abundant bamboo resources,accounting for about one-third of the world’s total bamboo forest area,and has a long history of bamboo forest management.However,excessive intensive management will lead to the decline of bamboo biodiversity,soil nutrient loss and bamboo productivity,and the ecological environment of bamboo forest is gradually becoming unbalanced.Under-forest Economy,also known as agroforestry,is a“nature-based”solution to the abovementioned problems.Bamboo-chicken farming（BCF）,as a model of under-forest Economy,exhibits the potential to improve soil ecosystem function in bamboo forests by modifying biogeochemical processes and promoting multiple ecosystem services involving carbon（C）and phosphorus（P）cycling and carbon use efficiencies（CUEs）.However,the effect of BCF on soil C and nutrient cycling and its mechanism under different density conditions remain poorly understood.Coupling the changes of soil C,P composition and microbial community to study the differences of soil quality under different densities has a very important reference value for clarifying the appropriate culture density of bamboo forests.In this study,we compared the soil properties,C fractions,P fractions,and microbial community compositions in the surface soil（0-20 cm）of a typical bamboo（Phyllostachys praecox）-chicken farming system under different grazing densities（represented as distances of 5,15,25,and 35 m from the henhouse,respectively）.The variables were also compared with the soil measurements from an adjacent pure bamboo forest without chicken framing（control site）.Furthermore,soil microbial biomass（C,N,and P）,and extracellular enzymatic activities（EEAs）were determined,and metabolic limitation of microbial community and microbial CUEs were quantified.Taken together,this study will help to further investigate the intrinsic linkage between soil microorganisms and carbon and phosphorus fractions under BCF conditions and clarify the metabolic patterns of microorganisms under different density conditions.The main results are as follows:（1）The results showed that BCF significantly increased（p<0.05）soil p H,soil organic carbon（SOC）,ammonium nitrogen（NH₄⁺-N）,Nitrate nitrogen（NO₃^--N）,available K（AK）,and available P（AP）.However,there was no significant difference in soil organic carbon among treatments.The changes in the proportions of soil active organic carbon pools easily oxidized organic carbon（EOC）,dissolved organic carbon（DOC）,and microbial biomass carbon（MBC）accounted for 7.62-11.85%,0.23-0.33%,and 0.31-0.56%of the total soil organic carbon,respectively.The EOC content varied regularly with the grazing density,so it may be a sensitive indicator of changes in soil organic matter caused by different grazing densities.Passive organic carbon Ca-SOC and Fe/Al-SOC accounted for 1.24-1.98%and33.91-37.52%of the total soil organic carbon,respectively.High Fe/Al-SOC values were associated with high grazing density,thereby implying that such condition could improve carbon sequestration in the long term.（2）The contents of labile P（Resin-Pi and Na HCO₃-P）,moderately labile P（Na OH-P and1 M HCl-Pi）and sparingly labile P（conc.HCl-P）in BCF soils increased by 69.08-158.40%,103.14-245.19%and 13.41-41.90%,respectively,compared with those in pure bamboo forests,which indicated that the level and potential of phosphorus supply to the soil of Lei bamboo forests were increased under BCF conditions.We observed that labile and moderately labile P dominated soil P accumulation under BCF.Resin-Pi,Na HCO₃-Pi,and 1 M HCl-Pi increased by 100-233%,83-183%and 414-1314%,respectively,compared with the control.The ratio of labile and moderately labile organic P to the Pt content decreased significantly with increasing grazing density from 38.54%（control）to 17.65%（5-m）.（3）The degree of variation in microbial community structure and abundance in BCF soils largely depends on the grazing density.Higher grazing density reduced fungal community diversity,and the number of species（Chao1 index）and species richness（Observed species index）were significantly reduced in the 5-m treatment.However,bacterial community diversity was not significantly influenced by grazing density.Principal coordinate analysis（PCo A）showed that the soil microbial community in BCF soil was significantly separated from that in the pure bamboo forest,and the sites of 15-,25-,35-m treatment were closely concentrated,while 5-m treatment was significantly deviated from other treatments and control.Redundancy analysis（RDA）indicated that p H may be the most dominant factor affecting microbial community structure in BCF systems,followed by 1 M HCl-Pi.（4）Mortierellomycota and Rozellomycota may be the major players in soil carbon cycling in BCF soil fungal communities,and both abundances are closely related to soil passive organic carbon（Fe/Al-SOC and Ca-SOC）as well as active organic carbon（EOC and DOC）.The phosphobacteria Flavobacterium,Arthobacter,Pseudomonas,Streptomyces,and Burkholderia were found in studied soils and their relative abundances increased with increasing graze density（except Burkholderia）.Inorganic P（Resin-Pi,Na HCO₃-Pi,and 1 M HCl-Pi）was positively correlated with Flavobacterium,Pseudomonas,and Arthrobacter but negatively correlated with Burkholderia;this highlights the different functional bacteria involved in P cycling.（5）We found that the EEAs（β-1,4-glucosidase,BG;β-1,4-N-acetylglucosaminidase,NAG;L-leucine aminopeptidase,LAP;and alkaline phosphatase,ALP）were significantly enhanced by increasing the grazing density,and significant differences in microbial investments in C-,N-,and P-acquiring enzymes were obtained for different grazing densities.The high values of degree of microbial elemental homeostasis(H’_C:N=16.95 and H’_C:P=11.49)indicates that soil microorganisms were able to maintain stoichiometric homeostasis under different grazing density conditions.The microbial community was limited by carbon and nitrogen in the control bamboo forest,but these limitations were gradually alleviated as the grazing density increased.In addition,the microbial CUE(both CUE_C:N and CUE_C:P)increased with higher grazing densities.However,no significant difference was observed in SOC at different grazing densities,which may be caused by reductions of organic matter input from litter and roots in the BCF system.The higher CUE values of soil microorganisms in BCF system may not indicate the high soil C sequestration.Overall,Our results showed that the changes of microbial community structure and soil nutrient characteristics caused by BCF changed soil ecological function.From low to high density,soil C sequestration,availability and supply capacity of soil P,and relative abundance of microbial community related to soil C and P cycle showed an increasing trend,while the changers in soil C:N:P stoichiometry and ecological enzyme stoichiometry showed different patterns.But,under high density conditions,the decreasing trend in soil fungal community diversity,higher labile and moderately labile P fractions,lower C:P and C:N ratios and reduced soil microbial biomass（C,N,P）suggest that high density may lead to increased risk of C and nutrient loss and reduced biodiversity in BCF systems,which might pose a threat to the sustainable landuse of bamboo forest soil.