| Atmospheric nitrogen (N) deposition increased continuously due to growingagricultural and industrial activities since the industrial revolution. N deposition hasinfluenced not only plant species diversity and productivity, but also soil microbialcommunity and enzyme activity in ecosystems. As the main drivers of nitrification anddenitrification, respectively, ammonia-oxidizing microorganisms and denitrifying bacteriaplay key role in soil N-cycling. Meanwhile, arbuscular mycorrhizal fungi (AMF), as thebiggest biomass in soil microbes, regulate the exchange of carbon (C) between plant rootsand soil, and thus play a crucial role in soil C-cycling. Therefore, for assessing the changein the ecosystem service function of alpine meadow on the Tibetan Plateau in future, it isvital to investigate the response of soil archaea, bacteria, and some of functional microbialgroups (i.e. AOB, AOA, denitrifying bacteria, and AMF), soil enzymes activities and soilmicrobial biomass carbon (SMBC) and nitrogen (SMBN) to different N.In this study, soil samples were collected from a controlled N addition experimentsystem on the alpine meadow ecosystem in Haibei Alpine Meadow Ecosystem ResearchStation (HAMERS) of Chinese Academy of Sciences. The abundance of soil archaea,bacteria, and specific microbial functional groups (AOA, AOB, and denitrifying bacteria)under a6-year different N fertilization treatments were detected through quantifying16SrRNA, amoA, nirK, nirS, nosZ genes, respectively. The AMF community and diversitywere also investigated by454pyrosequencing. Additionally, four soil enzymes (invertase,proteinase, phosphatase, and urease) activities, and the soil microbial biomass carbon(SMBC) and nitrogen (SMBN) were further measured using incubation methods.AOB abundance was more sensitive to different N-level additions compared to AOA.AOB abundance was higher in the high N (7.5g N m-2yr-1) treatment than in CK and lowN (1.5g N m-2yr-1) treatments. Under high N-level condition, significant difference inAOB abundance was observed among different N forms (Am-2, Ni-2, AN-2)(F=36.658,P=0.000). Meanwhile, AOA abundance was significant higher in high ammonia-Ntreatment (Am-2) than in CK and low N treatments (Ni-1and AN-1). These resultsindicated that N amendments may have more profound influence on the abundance of AOBthan AOA in the alpine grassland on the Tibetan Plateau. On the other hand, the abundanceof total archaea, bacteria, and denitrifying bacteria were not significantly affected by N additions.The results based on the454-pyrosequencing indicated that there was no significantdifference about AMF richness among the seven treatments. However, compared with lowN treatment, high N treatment has a tendency to increase the AMF species richness. NMDSanalysis showed that both N-form and N-level had no significant effects on the AMFcommunity composition, but the AMF community composition was significantly related tosoil pH, NO3-–N and plant biomass.Soil invertase, proteinase, phosphatase and urease activities were not significantlyaffected by N deposition. However, urease activity was significantly higher in late plantgrowth stage than in early plant growth stage (P <0.05). In addition, SMBC and SMBNsignificantly decreased with increasing of N deposition. However, N-form did notsignificantly affect SMBC and SMBN.In conclusion, N fertilization had different effects on the soil microbial abundance,enzyme activities, and microbial biomass in the alpine meadow on the Tibetan Plateau.N-level had more effects on soil microbes compared with N-form. Overall, this study willundoubtedly provide some scientific data support in the evaluation of the influence ofincreased N deposition on the ecosystem service function of alpine meadow in the TibetanPlateau in future. |
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