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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. [ABSTRACT FROM AUTHOR]

With the increasing demands of livestock production, grasslands are under pressure from over‐intensified grazing. Phosphorus (P) and nitrogen (N) fertilization is widely employed to meet the nutrient demands of heavy grazing. Although soil ammonia‐oxidizers play a critical role in determining N dynamics after fertilizer application, their responses to fertilization are still not well understood in steppe grassland systems. Here, the individual and combined effects of N (0, 7.5 and 15 g N m−2 year−1) and P (0, 3.27, 6.55 and 13.09 g P m−2 year−1) applications on soil ammonia‐oxidizers were explored in the Tibetan alpine steppe. Ammonia‐oxidizing archaea (AOA) and bacteria (AOB) abundance and community composition were examined using qPCR, terminal restriction fragment length polymorphism and clone libraries. Results showed that AOB diversity was significantly increased by N fertilization and decreased by P fertilization. The abundance of AOB was significantly increased by N fertilization and its interactive effects with P application. In contrast, AOA community diversity and abundance remained unaffected by either N or P application. The AOB abundance and diversity were affected by the direct effects of N fertilization, as well as its indirect effects of N application through available N and ammonium (NH4+), and further analysis showed that NH4+ and pH were the main factors. The changes to the ammonia‐oxidizing community altered the total N content of plants and the N:P ratio of Gramineae. Plant P properties were influenced by P addition, which also interacted with soil pH and available N to indirectly affect AOB. Overall, AOB exhibited stronger responses to N and P fertilization of alpine steppe grassland than AOA, and appear to play a critical role in plant nutrient absorption under fertilization management. Highlights: Effects of N and P application on AOA and AOB communities were evaluated.AOB, not AOA, responded to N or P, and their interaction, and NH4+ and pH were regulators of AOB.N application affected AOB community, resulting in changes to plant nutrient absorption.P addition affected plant tissue stoichiometry, and thus affected AOB community. [ABSTRACT FROM AUTHOR]