Litter quality and microbial biomass mediate the enhancement of soil enzyme activities under facilitator shrub canopies in fire-affected Mediterranean forests (Algeria)

Background and Aims: Wildfires severely degrade Mediterranean soils by impairing enzymatic functions crucial for nutrient cycling. While previous studies have documented facilitation effects on soil properties, the mechanistic pathways through which facilitator shrubs restore post-fire soil enzymatic functions remain poorly understood. Both aboveground litter inputs and belowground root exudates have been proposed as drivers of soil recovery, yet their relative contributions have rarely been disentangled in post-fire contexts. This study investigated whether native facilitator shrubs can accelerate post-fire recovery of soil enzymatic machinery in Algerian forests, and elucidated the underlying mechanisms linking plant inputs including litter quality and rhizosphere processes to enzymatic restoration. Methods Across a post-fire chronosequence (1–5 years) and severity gradient in Chlef forest, Algeria, we compared soil enzymatic activities (n = 5400 samples) under four facilitator shrubs (Retama sphaerocarpa, Pistacia lentiscus, Cistus monspeliensis, Rosmarinus officinalis) versus adjacent open soil. We analyzed litter quality, microbial biomass, nutrient fluxes, and conducted litter incubation and seedling transplant experiments. Structural Equation Modeling (SEM) confirmed causal pathways, and a novel Enzymatic Facilitation Index (EFI) was cross-validated on independent plots. Key Results Facilitator canopies significantly increased enzyme activities: β-glucosidase + 56.9%, acid phosphatase + 54.0%, and urease + 75.9% (all p < 0.001). Microbial biomass carbon was 85% higher under facilitators. Recovery followed exponential kinetics with rate constants k = 0.38 year⁻¹ under facilitators versus k = 0.15 year⁻¹ in open soil. Litter C:N ratio explained 54% of β-glucosidase variance, while litter N content explained 65% of urease variance. The EFI predicted Pinus halepensis seedling performance (R²=0.58) and was successfully cross-validated (R²=0.55). SEM confirmed the pathway: facilitator → litter quality → microbial biomass → enzyme activity → seedling success (CFI = 0.985; RMSEA = 0.041). Conclusion Facilitator shrubs actively engineer soil functional recovery through both litter-mediated and rhizosphere-mediated improvements in microbial communities. While litter quality emerged as a strong predictor of enzymatic recovery, root exudates likely constitute an additional, complementary pathway that warrants further investigation. Leveraging N-fixing species like Retama sphaerocarpa offers an evidence-based solution for restoring fire-degraded Mediterranean ecosystems.