Low-Altitude Soil Inputs Alter Carbon and Nitrogen Cycling in Permafrost Ecosystems

Permafrost-associated microbial communities have long been exposed to low temperatures and may therefore experience functional constraints. Although climate warming and permafrost degradation are expected to enhance carbon and nitrogen decomposition, these processes may still be limited by microbial functional capacity. To identify the functional constraints governing C–N release in permafrost soils, we conducted a 71-day incubation experiment using soils from alpine meadow, alpine steppe, and alpine wet meadow ecosystems on the Qinghai–Tibetan Plateau, to which low-altitude soils were added. We quantified changes in inorganic nitrogen, dissolved organic carbon (DOC), and CO 2 production before and after incubation. Nitrogen dynamics showed clear temporal patterns: NH 4 + –N exhibited consistent positive priming across all stages, with experimental treatments averaging 0.08 mg/kg higher than controls and reaching a maximum increase of 0.21 mg/kg at day 7. NO 3 – –N accumulated markedly during the mid-incubation period, whereas NO 2 – –N displayed a transient rise followed by a rapid decline. DOC concentrations declined throughout the incubation, decreasing from 105–110 mg/kg to about 55 mg/kg by mid-stage, with a slightly greater reduction in the experimental treatment (54 mg/kg) than in the control (48 mg/kg), indicating accelerated DOC consumption following low-altitude soil addition. Our findings suggest that inputs of low-altitude soils may alleviate nitrogen limitation, stimulate microbial mineralization, and promote a stage-dependent coordination between carbon release and nitrogen transformation. The strongest responses occurred in deep soil layers, where exogenous soil input was associated with stronger carbon turnover and enhanced nitrogen mineralization. Overall, the results suggest that permafrost soils are subject to both nutrient-related and functional constraints, and that warming-induced relaxation of these constraints may accelerate carbon and nitrogen losses from deep permafrost soils.