Abstract: This study focued on alfalfa fields in the Loess hilly region as research objects， with four experimental treatments were established： no fertilization （CK）， nitrogen fertilizer alone （T1）， phosphorus fertilizer alone （T2）， and combined nitrogen-phosphorus fertilization （T3）. By examining the ecological stoichiometry of soil-microbe-extracellular enzyme systems under nitrogen and phosphorus addition， this research explored the characteristics of soil nutrient limitations and the feedback mechanisms of microbial communities to metabolic constraints. The results showed that fertilization significantly decreased soil carbon to phosphorus ratio， microbial biomass carbon to phosphorus ratio （MBC∶MBP）， microbial biomass nitrogen to phosphorus ratio （MBN∶MBP）， and enzymatic stoichiometric carbon to nitrogen ratio （EEA_C∶N）. In contrast， soil nutrient content， microbial biomass， extracellular enzyme activities， and enzymatic stoichiometric nitrogen to phosphorus ratio （EEA_N∶P） were significantly increased. Enzyme vector modeling and vector analysis indicated that long-term alfalfa cultivation led to varying degrees of carbon and phosphorus limitation in soil microorganisms. Nitrogen and phosphorus addition alleviated the microbial phosphorus limitation and increased microbial carbon use efficiency （CUE）. In conclusion， appropriate supplementation of nitrogen and phosphorus fertilizers is recommended for soils under long-term alfalfa cultivation in the Loess hill region， to mitigate nutrient limitations on soil microorganisms.

Key words: Loess hills, Extracellular enzymes, Microbial nutrient limitation, Carbon utilization efficiency

摘要： 本研究以黄土丘陵区紫花苜蓿地为研究对象，设置不施肥（CK）、单施氮肥（T1）、单施磷肥（T2）和氮磷肥配施（T3）4个处理，通过分析氮磷添加下紫花苜蓿土壤-微生物-胞外酶生态化学计量特征，探讨土壤养分限制特征及微生物对代谢限制的反馈机制。结果表明，施肥降低了土壤碳磷比、微生物量碳磷比（MBC∶MBP）、微生物量氮磷比（MBN∶MBP）和酶化学计量碳氮比（EEA_C∶N），但增加了土壤养分含量、微生物量、胞外酶活性和酶化学计量氮磷比（EEA_N∶P）。酶向量模型和矢量分析结果表明，长期种植苜蓿土壤微生物受到不同程度的碳限制和磷限制，氮磷添加缓解了土壤微生物磷限制，提高了微生物碳利用效率（CUE）。综上，黄土丘陵区长期种植苜蓿土壤应适当补施氮磷肥，以缓解土壤微生物的养分限制。

关键词: 黄土丘陵, 胞外酶, 微生物养分限制, 碳利用效率