Isotope signature and ecoenzymatic stoichiometry as key indicators of urban soil functionality

This study aimed to assess the feasibility of the ecoenzymatic stoichiometry and isotope signature approaches as indicators of urban soil functionality, related to carbon and nutrient cycles.

In Pisa and Livorno (Italy), study sites with three degrees of urbanization (natural, peri-urban and central urban sites) were selected, where holm oak (Quercus ilex L.) was the most common evergreen species. The urban and peri-urban sites differed in terms of NO₂ emissions. At each site, topsoil and plant litter were sampled, pH, EC, TOC, and TN were measured in soil and δ¹³C and δ¹⁵N in soil and plant litter. The β-glucosidase, acid phosphatase and N-acetyl-β-D-glucosaminidase enzyme activities were also determined in soil and the ratios were calculated.

The δ¹⁵N in plant litter increased from peri-urban to urban sites, along with the NO₂ emissions, emerging as a sensitive indicator of atmospheric N deposition. The δ¹⁵N and δ¹³C increased in soil, indicating more rapid N cycles and organic matter degradation in peri-urban and urban areas than in natural areas. The ecoenzymatic stoichiometry revealed C and P microbial limitations for all the sites studied. However, the microbial needs of C and P increased and decreased, respectively, along the urbanization gradient. Isotope abundance and microbial nutrient limitations were found to correlate with soil properties. Specifically, soil δ¹⁵N was closely correlated with microbial C limitations.

The isotope signature and enzymatic stoichiometry used as indicators revealed that the soil characteristics affected the soil carbon and nutrient cycles as well as microbial energy and nutrient needs.