Background and aims While large differences in microbial biomass and extracellular enzyme activities (EEAs) between rhizosphere and bulk soils have been demonstrated, the potentially different response of microbes and their EEAs in rhizosphere and bulk soils to nitrogen (N) deposition is still not elucidated. Methods We analyzed the microbial biomass and EEAs in the rhizosphere and bulk soils of Sibiraea angustata in an alpine shrubland on the eastern Qinghai-Tibet Plateau after chronic N application. We also analyzed the stoichiometric linkages between plants, microbes, enzymes and soils to clarify the coupled responses of aboveground plants and belowground ecological processes. Results Microbial nutrient concentrations and activities of EAAs responded differently to N addition in the rhizosphere and bulk soils. In the rhizosphere, N addition caused a significant increase in microbial biomass carbon (C), N and phosphorus (P) concentrations and greater P-degrading enzyme activity (relative to the activities of C- and N-degrading enzymes), which induced a significant reduction in enzyme C:P and N:P ratios. The rhizosphere enzyme N:P ratio was negatively correlated with the N:P ratios of plant, soil and microbe, implying that increased plant and microbial P uptake under N addition may gradually aggravate rhizosphere P limitation. However, for the bulk soil, N addition did not affect microbial biomass but significantly enhanced C-degrading enzyme activity and decreased the enzyme C:N ratio. Meanwhile, the bulk-soil enzyme C:N ratio was negatively correlated with the soil C:N ratio but independent of the plant C:N ratio, implying that N addition may enhance bulk-soil microbial C limitation. Conclusions Our study suggests that elevated N deposition may induce differential microbial nutrient limitation between the rhizosphere and bulk soils due to the plant-microbe-soil interactions in the rhizosphere. This study highlights the importance of incorporating rhizosphere microbial processes into biogeochemical models describing environmental changes.

中文翻译：

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氮添加对高山灌丛根际和大块土壤中微生物和细胞外酶化学计量的不同影响

背景和目的虽然已经证明根际和大块土壤之间微生物生物量和细胞外酶活性 (EEA) 存在巨大差异，但根际和大块土壤中的微生物及其 EEA 对氮 (N) 沉积的潜在不同反应仍未阐明。方法我们分析了青藏高原东部高山灌丛中西伯利亚长春花在长期施氮后根际和大块土壤中的微生物生物量和EEA。我们还分析了植物、微生物、酶和土壤之间的化学计量联系，以阐明地上植物和地下生态过程的耦合反应。结果 EAA 的微生物养分浓度和活性对根际和大块土壤中的 N 添加有不同的反应。在根际，N添加导致微生物生物量碳（C）、N和磷（P）浓度和更大的P-降解酶活性（相对于C-和N-降解酶的活性）显着增加，这导致酶的显着降低C:P 和 N:P 比率。根际酶 N:P 比与植物、土壤和微生物的 N:P 比呈负相关，这意味着在 N 添加下植物和微生物 P 吸收的增加可能会逐渐加剧根际 P 限制。然而，对于大块土壤，N添加不影响微生物生物量，但显着增强C降解酶活性并降低酶C：N比。同时，土体酶 C:N 比与土壤 C:N 比呈负相关，但与植物 C:N 比无关，这意味着 N 添加可能会增强大块土壤微生物 C 限制。结论我们的研究表明，由于根际中植物-微生物-土壤的相互作用，增加的氮沉积可能会导致根际和大块土壤之间微生物养分的差异限制。这项研究强调了将根际微生物过程纳入描述环境变化的生物地球化学模型的重要性。