Long-Established Monospecific Tree Stands are Associated with Distinct Soil Microbial Community Structure and Nutrient-Cycling Enzyme Activities in a Semi-Arid Gypsiferous Mediterranean Soil

Abstract This study assessed whether long-established monospecific stands of Eucalyptus camaldulensis (EC), Eucalyptus occidentalis (EO), Pinus halepensis (PH), and Cupressus sempervirens (CS), established approximately 70 years ago, were associated with distinct patterns of soil microbial community structure and biochemical functioningin a semi-arid Mediterranean gypsiferous soil. Specifically, we examined differences in soil organic matter pools, microbial community structure, and potential enzyme activities related to C, N, and P cycling, and explored their associations with soil resource status and leaf/needle chemical traits. Mineral topsoil, leaves or needles, and litter were sampled from monospecific stands established on the same Gypsic Haploxerept near San Cataldo, Sicily. Soil chemical and biochemical properties were assessed, including microbial biomass C and N, respiration, microbial community structure, and potential enzyme activities. In addition, the elemental composition and fiber fractions of leaves and litter were analyzed. Differences among stands were assessed by one-way ANOVA, while exploratory stepwise discriminant function analyses were used to integrate soil, microbial, enzymatic, and plant-trait variables. The four stands differed markedly in soil organic matter pools, microbial community structure, and biochemical functioning. EC was associated with greater C and N availability, highest microbial biomass, respiration, and β-glucosidase activity, indicating greater potential for microbial C acquisition. PH and CS showed highest phosphatase activity consistent with greater potential for P acquisition, while their microbial profiles showed relatively greater contributions of fungal and/or actinomycete phospholipid fatty acid markers. EO showed a distinct profile, combining relatively high organic C availability with comparatively low urease and phosphatase activities. Exploratory discriminant analyses indicated that both soil resource status and residue structural traits contributed to stand separation. Long-established monospecific stands were associated with distinct soil microbial community structure and biochemical profile in this gypsiferous Mediterranean soil. Integrating enzyme activities, phospholipid fatty acid profiles, and plant-residue chemistry helped characterize these differences, which should be interpreted as associations rather than replicated causal species effects.