Data_Sheet_1_Effects of Nitrogen Addition on Plant Properties and Microbiomes Under High Phosphorus Addition Level in the Alpine Steppe.docx

Nitrogen (N) addition can increase the vegetative growth, improve the plant production, and restore the degraded terrestrial ecosystems. But, it simultaneously aggravates the soil phosphorus (P) limitation for plant growth, thus affecting its positive effects on ecosystems. However, how plants and soil microorganisms will change under conditions of high P content in soil is still unknown. In this study, we explored the effects of three levels of N addition (0, 7.5, and 15 g.N.m–2.year–1) on plants and microorganisms at the high P addition level (13.09 g.P.m–2.year–1) in the alpine steppe. We found that the soil microbial community composition had no significant difference between different N addition levels, and the soil AN and AP had a significant effect on the phospholipid fatty acid (PLFA) composition. The abundance of the core PLFAs (i.e., 16:1ω7c, 16:0, a17:1, i17:0, 18:1ω9c, and 18:1ω7c) also remained unchanged after N addition, and microbes at individual, population, and community levels were all correlated with SOM, AK, AN, and pH. Conversely, plant biomass and nutrient content showed linear trends with increasing N addition, especially the dominant functional groups. Specifically, the biomass and plant tissue N content of Gramineae, and the total N content of aboveground biomass were all improved by N addition. They were correlated with soil ammonium and AP. The structural equation modeling (SEM) demonstrated that N addition had a direct negative effect on soil microbial biomass, but an indirect positive effect on aboveground biomass via soil ammonium. These findings clarify the importance of N-amendment in regulating plants and microorganisms under high P conditions and provide a better understanding of the N-added effects in the alpine steppe.

氮（N）添加可促进植物营养生长、提升植物生产力，并修复退化陆地生态系统。但与此同时，其会加剧植物生长所面临的土壤磷（P）限制，进而削弱其对生态系统的正向调控作用。然而，在土壤磷含量较高的条件下，植物与土壤微生物将产生何种响应，目前仍不明确。本研究以高寒草原为研究对象，在高磷添加水平（13.09 g·P·m⁻²·年⁻¹）下，设置三个施氮水平（0、7.5和15 g·N·m⁻²·年⁻¹），探究了施氮对植物与土壤微生物的影响。研究结果表明，不同施氮水平下土壤微生物群落组成无显著差异；土壤有效氮（AN）与有效磷（AP）对磷脂脂肪酸（PLFA）组成具有显著调控作用。核心磷脂脂肪酸（即16:1ω7c、16:0、a17:1、i17:0、18:1ω9c及18:1ω7c）的丰度在施氮后亦未发生明显变化；微生物在个体、种群及群落水平均与土壤有机质（SOM）、速效钾（AK）、有效氮及pH值存在显著相关性。与之相对，植物生物量与养分含量随施氮量增加呈现线性变化趋势，其中优势植物功能群的响应尤为显著。具体而言，禾本科（Gramineae）植物的生物量、植株组织氮含量以及地上生物量总氮含量均因施氮得到提升，且这些指标与土壤铵态氮及有效磷存在显著相关性。结构方程模型（SEM）结果显示，施氮对土壤微生物生物量具有直接负向作用，但可通过土壤有效氮对地上生物量产生间接正向影响。本研究明确了高磷条件下施氮调控植物与微生物过程的重要性，为深入理解高寒草原的施氮生态效应提供了新的认知。