Effects Of Nitrogen And Phosphorus Additions On Soil Phosphorus Fractions And Microbial Characteristics In Chinese Fir Plantation

Phosphorus（P）is in insufficient supply in highly weathered subtropical forests and often difficult to meet plant needs.In recent years,with the increasing deposition of atmospheric nitrogen（N）,soil P limitation has been further aggravated,and the exogenous P fertilizers addition is one of the main measures to alleviate soil P limitation.Chinese fir（Cunninghamia lanceolata）is the most important fast-growing silvicultural species in the subtropics,facing prominent problems such as site decline and difficulty in maintaining long-term productivity,and the changes in soil P supply in Chinese fir forests under the background of N deposition are worthy of attention.At present,the effects of exogenous N and P inputs on soil P fractions in fir plantation forests and their microbiological mechanisms are not clear.In 2011,based on the subtropical Chinese fir plantation,the subtropical Chinese fir plantation was carried out six treatments including-P+N0(0 kg P hm^-2 a^-1+0 kg N hm^-2 a^-1),-P+N1(0 kg P hm^-2 a^-1+50 kg N hm^-2 a^-1),-P+N2(0 kg P hm^-2 a^-1+100 kg N hm^-2 a^-1),+P+N0(50 kg P hm^-2 a^-1+0 kg N hm^-2 a^-1),+P+N1(50 kg P hm^-2 a^-1+50 kg N hm^-2 a^-1)and+P+N2(50 kg P hm^-2 a^-1+100 kg N hm^-2 a^-1),and each treatment comprised three randomized block experiment.Soil samples were collected from 0-5 cm,5-10 cm and10-20 cm soil layers in August 2020,and the soil physicochemical properties,soil P fractions（Hedley method）,soil microbial community structure（high quantitative sequencing）and soil enzyme activities（96 microplate method）were measured.To clarify the distribution characteristics of soil P,soil microbial community structure and function,and to reveal the microbiological mechanism of N and P additions on soil P turnover in Chinese fir plantations.These results can provide theoretical basis for soil nutrient management and sustainable management of subtropical plantations.The main results are as follows:（1）N addition significantly increased the concentrations of mineral nitrogen（MN）,ammonium nitrogen（NH₄⁺-N）and nitrate nitrogen（NO₃^--N）,and significantly decreased the p H value and available P（Olsen-P）concentration under+P treatment in each soil layer（P<0.05）.Compared with N0,the soil moisture concentration（SMC）of the 0-5 cm and5-10 cm layers under-P treatment significantly increased under N2 addition,and the dissolved organic carbon（DOC）of each soil layer under-P treatment significantly increased under N1 addition.N addition effect was not found on soil bulk density（BD）,capillary porosity（CP）and total nutrients.N addition significantly decreased the Resin-P concentration in each soil layer,the Na HCO₃-Pi/Po concentration in the 0-5 cm layer under+P treatment,and the Na OH-Pi concentration in the 0-5 cm and 10-20 cm layers.P addition significantly increased the Olsen-P,total P（TP）,and P fractions concentrations in the 0-5 cm and 5-10 cm layers.The above results showed that N addition decreased soil P availability,while P addition increased soil P availability.（2）In terms of bacterial community structure,N2 addition significantly decreased the Chao1 and Shannon index in the 0-5 cm and 5-10 cm soil layers.Both N1 and N2additions significantly decreased Acidobacteria,Acidobacteriia,Chloroflexi and Verrucomicrobiae relative abundance,and significantly increased Actinobacteria relative abundance,while P addition had no significant effect.N and P additions had no significant effects on fungal diversity,richness and community composition.The RDA showed that NO₃^--N,p H,NH₄⁺-N were the key factors affecting the bacterial community structure.For P solubilizing microorganisms,N2 addition significantly increased Ralstonia relative abundance in the 0-5 cm and 5-10 cm soil layers;P additions significantly decreased Ralstonia relative abundance in the 10-20 cm soil layer.The correlation results showed that Ralstonia was negatively correlated with liable P.It can be seen that the effect of N addition on bacterial community composition is greater than that of fungi,N addition inhibits the transformation of soil P by P solubilizing microorganisms,thereby reducing the availability of soil P,while P supplementation had no significant effect on bacterial and fungal community structure.（3）N addition significantly increased the activities ofβ-glucosidase（BG）,cellobiase（CB）and acid phosphatase（AP）in each soil layer,and significantly decreased the activity ofβ-acetylglucosaminidase（NAG）in the 0-5 cm soil layer.P addition significantly increased the activities of BG and NAG in each soil layer,as well as the leucine aminopeptidase（LAP）activity in the 0-5 cm and 5-10 cm soil layers,and significantly decreased the AP activity in each soil layer.The average E_C:N:P was 1:0.79:1.76.RDA showed that Olsen-P,NH₄⁺-N,NO₃^--N,TN and TP were important factors affecting soil enzyme activities and its stoichiometric ratio,AP activity was an important factor affecting soil P fractions,and it was negatively correlated in 0-5 cm and 5-10 cm soil layers.The above results indicated that N and P additions significantly affected soil enzyme activities and their stoichiometric ratios,N addition aggravated soil P limitation,while P addition alleviated P limitation.AP activity was the key to driving P availability.In summary,the effects of N and P additions on soil physicochemical properties,soil P fractions,microbial community structure and dominant enzyme activities were not consistent.N addition increased soil P restriction,while P addition increased soil P availability.N and P additions regulate soil P supply by affecting bacterial community structure and AP activity.The results deepened the understanding of the microbiological mechanism of N and P additions affecting soil P cycling in subtropical plantations,and provided a scientific basis for the management of subtropical Chinese fir plantations.