Do local non-statutory habitat designations reflect differences in below-ground biodiversity and functioning in urban green space?

Urban green spaces provide essential ecosystem services, and a range of statutory and non-statutory designations are used to protect them. However, these designations are often based on above-ground characteristics, and it remains unclear whether they also reflect belowground processes that underpin multiple ecosystem services. Here, we evaluated soil-based ecosystem processes and services in urban green spaces designated as Sites of Biological Importance (SBIs) and in adjacent, near-identical non-designated sites. We tested the hypothesis that aboveground characteristics used to designate SBIs would be mirrored by differences in soil processes supporting key ecosystem services. Seasonal dynamics of soil nutrient concentrations, soil CO2 efflux (respiration), and litter decomposition were measured using a reciprocal litter transplant design to separate litter and environment driven effects. SBI designation interacted with seasonal variation, showing significantly higher soil nutrient concentrations, microbial biomass carbon, dissolved organic carbon, and improved soil water retention. Activities of oxidative enzymes (peroxidase and phenol oxidase) were also higher in SBIs, indicating an enhanced microbial capacity to process complex carbon compounds. In contrast, soil respiration did not differ between SBIs and controls and was mainly driven by soil moisture and canopy cover. The litter transplant experiment revealed a positive home-field advantage in non-designated areas but the opposite response in SBIs. Overall, these findings demonstrate that SBI designation captures important belowground differences, particularly in nitrogen-related nutrient storage and enzyme-mediated decomposition, emphasizing the broader ecological value of non-statutory designations for sustaining urban soil functions.