Abstract:

Due to intensified human activities and industrial development, the input of reactive nitrogen (N) and phosphorus (P) has been continuously increasing. However, there is a lack of studies on soil physicochemical properties under N and P enrichment in the alpine grassland of Sejila Mountain, and little attention has been paid to the interactive effects of N and P. This study examined the response mechanisms of soil enzyme activities and stoichiometric ratios to nutrient addition using a field-control experiment of N and P addition in the alpine grassland of Sejila Mountain. Results showed that: 1) compared with the control, the activities of β-1,4-glucosidase and cellobiohydrolase increased by 71.3% and 62.0%, respectively, under combined N and P addition (p<0.05). Nitrogen-acquiring enzymes displayed heterogeneous responses: β-1,4-N-acetylglucosaminidase increased by 90.6% and 59.8% under P addition and combined N-P addition, respectively, but decreased by 43.8% under N addition. Leucine aminopeptidase increased by 111.2% only under combined N-P addition. Alkaline phosphatase increased by 55.2% under combined N-P addition (p<0.05). 2) Enzyme stoichiometry and vector analysis showed that the soil microorganisms were predominantly limited by P, as the vector angles under all treatments exceeded 45°. Although N and P addition elevated the metabolic upper limit of the microbial community, the dominant limitation still tended to be P. 3) Correlation analysis and redundancy analysis indicated that exogenous N, P, and their combined addition jointly regulated soil enzyme activities and their stoichiometric structure by altering soil C꞉N ratio, available phosphorus, microbial biomass carbon, and dissolved organic carbon. Under P addition and combined N-P addition, available phosphorus increased markedly by 103.1% and 427.2%, respectively; microbial biomass carbon declined by 32.9% under combined N-P addition; and dissolved organic carbon decreased by 8.2% under combined N-P addition. The sampling sites were distinctly separated along the gradient of “available phosphorus-carbon-nitrogen ratio” (p<0.05). In summary, this study demonstrates the characteristics of “stoichiometric rebalancing-metabolic threshold” of alpine grasslands under N and P enrichment: single-factor addition modifies enzymatic investment at specific sites, whereas combined addition enhances the overall metabolic potential without shifting P from its role as the dominant limiting nutrient.

Key words: nitrogen and phosphorus addition, soil enzyme activity, enzyme stoichiometry ratio, alpine grassland, Sejila Mountain

摘要：

由于人类活动和工业发展活性氮磷的输入持续增多。目前，色季拉山高寒草地还缺乏氮磷富集下土壤理化性质的研究，且较少关注氮磷的交互作用。以色季拉山高寒草地为对象，通过开展野外氮磷添加控制实验，探究土壤酶活性和化学计量比对养分添加的响应机制。结果显示，1）相较对照，氮磷复合添加下β-1,4-葡萄糖苷酶与纤维二糖水解酶分别上升71.3%与62.0%（p<0.05）；氮获取酶呈“异向”响应：β-1,4-N-乙酰基氨基葡萄糖苷酶在磷与氮磷下显著升高90.6%、59.8%，在氮处理下降43.8%；亮氨酸氨基肽酶仅在氮磷复合处理下显著升高111%；碱性磷酸酶主要在氮磷复合处理下上升55.2%（p<0.05）。2）酶化学计量与矢量分析表明，微生物总体受磷限制：各处理矢量角均大于45°；氮磷虽提升群落代谢上限，但主导限制仍偏向磷端。3）相关分析与冗余分析显示，外源氮、磷及氮磷通过改变土壤碳氮比、有效磷、微生物量碳与有机碳供给等因子共同调控酶与酶计量结构；磷与氮磷下有效磷显著提高103%、427%，微生物量碳在氮磷处理下降低了32.9%，溶解性有机碳氮磷处理下降低8.21%，样点沿“有效磷－碳氮比”梯度清晰分离（p<0.05）。综上所述，该研究揭示了高寒草地在氮、磷富集下的“计量再平衡－代谢阈值”特征：单因子添加改变特定位点的“酶学投资”，复合添加放大代谢潜力但仍未改变磷主导限制。

关键词: 氮磷添加, 土壤酶活性, 酶化学计量比, 高寒草地, 色季拉山