Effects of forest fire smoke deposition on C, N, P stoichiometry, physicochemical and biological properties of litter/soils in Schima superba biological firebreaks

The deposition of nutrient elements such as C, N, and P carried by forest fire smoke is a crucial process in the material cycle of forest ecosystems, yet the mechanisms underlying its impacts of smoke deposition on the soil system remain unclear. Biological firebreaks are unique environment where the effects of heat from fire is minimal and only the effect of dispersed smoke that has a clear impact. Thus, taking Schima superba biological firebreaks in subtropical China as the research object, this study conducted a field simulation experiment of low- and high-concentration forest fire smoke deposition, and continuously monitored the stoichiometric characteristics, physicochemical properties, soil enzyme activities, and bacterial community structure of litter and soil layers (0–10 cm and 10–20 cm layers) for 12 months. The results showed that: (1) in the short term, smoke deposition significantly increased the contents of C, N, and P in litter and soil while reducing the C: P and N: P ratios in the litter layer. However, in the long term (12 months), it exacerbated soil P limitation, leading to a significant increase in C: P and N: P ratios in the 0–10 cm and 10–20 cm soil layers. (2) Smoke deposition exhibited a concentration-dependent effect on soil enzyme activities; i.e., "low concentration-promoting and high concentration-inhibiting" effect on activities of enzymes related to litter decomposition (e.g., cellulase and peroxidase), nitrogen cycle enzymes (e.g., protease, urease, and nitrate reductase) and phosphorus cycle enzymes (e.g., alkaline phosphatase and acid phosphatase). (3) The bacterial community in the litter layer changed rapidly and drastically, with smoke-tolerant and degrading taxa (e.g., Bacteroidetes) enriched in the short term; the soil layer was dominated by Acidobacteria and Proteobacteria, and stress-tolerant taxa (e.g., Actinobacteria) were enriched in the deep soil layer in the later stage. High-concentration smoke drove the community to reorganize into heat-tolerant and efficient organic matter-degrading groups. (4) Nutrient elements, such as NO-N, OC, TN, and EC were the key environmental factors regulating bacterial community structure. Structural Equation Modeling revealed that smoke deposition indirectly affected the C:N:P stoichiometric balance of the litter-soil system by directly altering bacterial community structure and enzyme activities, and this impact showed significant variation with soil depth. This study clarifies the multi-dimensional impacts and regulatory mechanisms of forest fire smoke deposition on soil ecological processes in S. superba firebreaks, providing a theoretical basis for post-fire restoration of subtropical forests and scientific management of biological firebreaks. • First study on smoke deposition effects on soil in a Schima superba firebreak. • Smoke deposition alters soil carbon, nitrogen, and phosphorus stoichiometry. • Smoke inputs cause significant shifts in the soil bacterial community. • Findings are critical for assessing biological firebreak sustainability. • Links atmospheric fire emissions to soil health and forest management.