Microbial Metabolic Limitation in Response to Phosphorus Enrichment: Implications for Carbon Sequestration in a Nitrogen-Enriched Desert Steppe

The availability of nitrogen (N) and phosphorus (P) significantly influences microbial metabolism, thereby affecting soil carbon (C) sequestration. However, it remains unclear how microbial resource limitation and C turnover dynamics respond to P availability under elevated N loads in dryland ecosystems. To address this, we conducted a 7-year experiment in a desert steppe in northern China, applying a gradient of P additions (0–16 g P m⁻² year⁻¹) under conditions of N loading (atmospheric N deposition +5 g N m⁻² year⁻¹). Our aim was to investigate microbial nutrient limitations and their impact on microbial carbon use efficiency (CUE) based on stoichiometry theory. Our findings revealed that, under N loading, microbial metabolism in both the surface (0–10 cm) and subsurface (10–20 cm) layers of the topsoil was limited by both C and P. Interestingly, with increasing P addition, microbial C limitation initially increased and then decreased at the surface but remained unchanged in the subsurface. Under conditions of C limitation, P enrichment did not alleviate microbial P limitation in either soil layer. Surprisingly, the microbial communities in both the surface and subsurface layers maintained plastic stoichiometric homeostasis despite aggravated C:P and N:P imbalances. Furthermore, P enrichment decreased microbial CUE in both soil layers, with the surface experiencing the most significant decline. Further analysis showed that the factors driving microbial nutrient limitation and CUE varied between soil layers under P enrichment, with enhanced microbial C limitation strongly inhibiting CUE. Our study indicates that increased C limitation due to P enrichment can reduce CUE and exacerbate stoichiometric imbalances. This could potentially lead to greater C loss in N-enriched dryland soils.