Elemental-enzymatic indicators of nutrient limitation in ombrotrophic peatlands across seasonal and spatial gradients

•Background: Blanket bogs are rare ombrotrophic peatlands that function as important long-term carbon stores. Their persistence depends on strong nutrient limitation, which maintains Sphagnum dominance and suppresses decomposition, yet mechanisms driving seasonal and spatial variability in microbial nitrogen and phosphorus limitation patterns remain poorly understood. This study investigated nutrient soil limitation dynamics by integrating plant elemental stoichiometry with microbial enzymatic stoichiometry.
•Results: We conducted snapshot sampling during two contrasting seasons (summer 2023 and winter 2024) along an elevational sequence (550–750 m) and across peat depths (0–60 cm). Given the high-water content of peat soils (~ 86% moisture), we focused on enzyme stoichiometric ratios rather than absolute enzyme activities, specifically the ratio of urease to acid phosphatase. Seasonal variation dominated patterns in both soil elemental stoichiometry and microbial enzymatic stoichiometry. Depth gradients were equally strong, with N:P increasing down-profile. This highlights rapid seasonal reorganisation at the surface versus slower, depth-driven changes from peat development. Soil nitrogen-to-phosphorus ratios were tightly coupled to microbial enzyme stoichiometry, following an inverse power-law relationship (exponent = − 1.56, R 2 = 0.60). From this relationship, we derived a urease-based microbial nitrogen acquisition threshold (urease:acid phosphatase = 88; 95% confidence interval: 64–118), which showed strong correspondence with the canonical plant nitrogen-to-phosphorus limitation threshold of 16, used here as a reference for ecosystem-level nutrient balance. Microbial community composition corroborated these patterns, with bacterial-to-fungal ratios 4.3-fold higher in winter than summer (18.9-fold in surface peat), consistent with observed N:P stoichiometric gradients, suggesting that seasonal changes in microbial community structure align with variation in nutrient availability, while also reflecting the influence of additional environmental controls.
•Conclusions: Our findings demonstrate a strong coupling between plant–soil nutrient stoichiometry and microbial enzyme allocation in blanket bogs, structured by the interaction of dynamic seasonal forcing and persistent depth-related constraits. Seasonal processes regulate short-term nutrient availability and microbial function in surface peat, whereas depth reflects long-term peat development that constrains the baseline stoichiometric framework of the system. Winter conditions were characterised by stoichiometric patterns consistent with relative N limitation at the ecosystem level, whereas summer conditions reflected stronger P constraint. We identified a quantitative microbial enzyme threshold that aligns with plant-derived nutrient limitation thresholds, directly linking microbial processes to ecosystem nutrient status and providing a robust framework for diagnosing nutrient limitation in water-saturated peat soils. Incorporating seasonal nutrient dynamics into peatland management may be critical for maintaining microbial functioning, vegetation structure, and long-term carbon sequestration.