| Soil microorganisms play an important role in nutrient cycling.The rhizosphere is a hotspot for microbial activity due to microbiome-root interactions.The structure and function of the rhizosphere microbial community are different from those in nonrhizosphere.Nitrogen(N)and phosphorus(P)inputs can regulate microbial community structure and extracellular enzyme activity by changing soil nutrient availability in subtropical forests.Different mycorrhizal tree species adopt different nutrient acquisition strategies,resulting in inconsistent responses of rhizosphere microbial to nitrogen and phosphorus inputs.In the subtropical regions of China,there is a great diversity of tree species,and the nitrogen deposition is relatively large.However,the impact of N and P additions on the structure and function of rhizosphere microbial community of different mycorrhizal tree species is not yet clear.This study selected 13 year old Chinese fir(Cunninghamia lanceolata(Lamb.)Hook)and Masson pine(Pinus massoniana)plantations in Huitong County in Hunan Province in 2020,and set up 5 nitrogen and phosphorus addition treatments.Each treatment was repeated 4 times,with a total of 72 sample plots.After short-term N and P additions,rhizosphere soil samples in 2021 were collected to determine soil properties,microbial phospholipid fatty acids,and extracellular enzyme activities.We analyzed the effects of N and P additions on rhizosphere physicochemical properties and microbial nutrient limitations.We compared the differences in the response of rhizosphere microbial community structure and extracellular enzyme activity to N and P additions between two mycorrhizal tree species,and further explored the driving factors of microbial change.The main research results are as follows:(1)The nutrient content and microbial biomass of the rhizosphere soil of Chinese fir are higher,while the proportion of fungi and enzyme activity of Masson pine are higher.Chinese fir plantations in the rhizosphere exhibited higher total phosphorus(TP),nitrate nitrogen(NO3--N),the ratio of nitrate to ammonium,and microbial biomass,while Masson pine plantations showed higher rhizosphere soil organic carbon(SOC),the ratio of fungi to bacteria(F:B),and extracellular enzyme activities(BG,NAG+LAP,ACP,CBH,BX,AG)(p<0.05).(2)N addition significantly improved nitrogen availability in rhizosphere,but it showed a V-shaped change under P addition.In two plantations,N addition significantly increased rhizosphere available nitrogen(AN)and ammonium nitrogen(NH4+-N),and decreased pH values(p<0.05),while P addition significantly increased rhizosphere available phosphorus(AP),and rhizosphere AN and NH4+-N decreased first and then increased along the P addition gradient(p<0.05).(3)Compared with N addition,the influence of P addition on microbial community structure and extracellular enzyme activity in rhizosphere soil of Chinese fir plantations had a threshold value.In Chinese fir plantations,the relative abundance of fungi(F%)and F:B ratio decreased significantly under the N addition(p<0.05).However,along the P addition gradient,the F%and F:B ratio first decreased and then increased.This indicated that the fungal community in the rhizosphere soil of Chinese fir was more sensitive to the addition of N and P.In addition,the extracellular enzyme activity,BG:ACP,and vector length in Chinese fir plantations increased significantly with N addition rates(p<0.05),indicating that N input aggravated microbial C restriction and stimulated extracellular enzyme secretion.Under the P addition,extracellular enzyme activity,BG:(NAG+LAP),and vector length showed a single peak trend in Chinese fir plantations(p<0.05),indicating that there were thresholds for changes in extracellular enzyme activity and microbial C limitation.(4)Compared to Chinese fir plantations,the microbial community structure,microbial nutrient limitation,and extracellular enzyme activity in the rhizosphere of Masson pine plantations only responded significantly to the low N addition,indicating that its microorganisms were resistant to elevated N supply.However,in Masson pine plantations,P addition significantly increased microbial biomass,and decreased BG:(NAG+LAP)and vector length(p<0.05),indicating that P addition reduced significantly microbial C limitation and promoted microbial growth.Furthermore,rhizosphere(NAG+LAP):ACP and vector angle remained unchanged under P addition in two plantations,indicating that microbial P restriction did not immediately alleviate due to P application.In addition,there was no significant change in extracellular enzyme activity in Masson pine plantations after P addition.(5)The rhizosphere physicochemical properties and microbial C restriction had significant effects on microbial community structure and extracellular enzyme activity.In Chinese fir plantations,rhizosphere F:B ratio,vector length,and enzyme activity were linearly correlated with NH4+-N and NO3--N(p<0.05).In addition,the structural equation model(SEM)showed that changes in rhizosphere AN in Chinese fir plantations under N and P addition affected microbial C limitation and extracellular enzyme secretion,thereby driving the variation in microbial community structure.In Masson pine plantations,the SEM results showed that rhizosphere SOC had a direct impact on extracellular enzyme activity,while increases in rhizosphere TP alleviated microbial C limitation,thus changing the microbial community structure.This study supports the view that mycorrhizal types affect soil nutrient cycling,verifies that arbuscular mycorrhizal forests are more sensitive to inorganic nitrogen input.Moreover,it was found that N and P additions influenced the microbial community structure and extracellular enzyme activity by changing the rhizosphere N availability in Chinese fir plantations,while in Masson pine plantations P addition promoted the growth of rhizosphere microorganisms by alleviating microbial C restriction.The results of this study strengthen the understanding of the responses of microbial community structure and function in the rhizosphere in different types of forests to global nitrogen and phosphorus deposition. |
