Warming Does Not Change Vertical Variations in Microbial Resource Limitation in Subtropical Forests at China

Global warming can differentially alter ecosystem carbon, nitrogen, and phosphorus dynamics, regulating the balance between soil substrate supply and microbial metabolic demand. However, empirical research on how warming influences microbial resource limitation along the soil profile remains limited, particularly in tropical–subtropical regions. Here, we investigated vertical variations (0–60 cm soil layers) in microbial resource limitation and their corresponding responses to warming in subtropical forests in southern China, using a soil warming experiment with heating cables (+4 °C) and enzymatic stoichiometry. Alleviated carbon limitation but aggravated nutrient (nitrogen and phosphorus) limitation for microbial metabolism was observed along soil profiles, regardless of warming treatment. Among different soil depths, warming mitigated microbial carbon limitation conditions and exacerbated microbial nutrient limitation conditions in a 0–10 cm surface layer, but had no significant effect below the 20 cm soil depth. Moreover, vertical variations in microbial nitrogen limitation were primarily regulated by soil moisture and the fungal–bacterial ratio regardless of warming treatment. In contrast, vertical changes in microbial carbon and phosphorus limitation were driven by soil moisture and the fungal–bacterial ratio under ambient conditions, but by the soil carbon–phosphorus ratio and the fungal–bacterial ratio after warming. For surface soil, warming effects on microbial carbon, nitrogen, and phosphorus limitation were mainly explained by microbial biomass stoichiometry and the fungal–bacterial ratio. Overall, warming had diverse effects on microbial resource limitation along the entire soil profile. These findings provide important insights for accurately predicting biogeochemical cycles under global warming scenarios.