Urban soil quality is being deteriorated even with low heavy metal levels: An arthropod-based multi-indices approach

Urban-induced habitat conversion drastically changes soil life in a
variety of ways. Soil sealing, human disturbance, habitat fragmentation,
industrial and vehicular pollution are the main causes of urban soil
degradation. Soil arthropods, as the most abundant and diverse group of
soil fauna, are involved in many soil processes that are of great
importance in maintaining soil health and multifunctionality.
Nevertheless, soil quality is still mainly characterized by physical,
chemical, and microbiological parameters. Here, we assessed and compared
the biological soil quality in woody (REF: reference forest, REM: remnant
forest) and non-woody (TURF: public turfgrass, and RUD: ruderal habitat)
types of urban green spaces along a disturbance and management intensity
gradient in the Budapest metropolitan area (Hungary), using community
metrics and soil arthropod-based indicators. Vegetation cover and
landscape characteristics of study sites were quantified through
vegetation and urbanization indices, respectively. Basic soil properties,
total and bioavailable concentrations of the main heavy metals (Cd, Co,
Hg, Ni, Zn) were also measured. Acari, Collembola, and Hymenoptera (mainly
Formicidae) were the most abundant groups. Litter-dweller taxa,
particularly Protura, proved to be the most sensitive to urban
disturbance. Representatives of Hemiptera, Diptera, Symphyla, and
Pauropoda were common in low densities. Soil arthropod assemblages in RUD
and TURF were more diverse taxonomically than in REM and REF sites.
Although the integrated faunal indices showed no differences among soil
habitat types, they provided different responses and, consequently,
different information. Our findings demonstrated that the biological
quality and arthropod community structure of soils were strongly impacted
by soil C/N and heavy metal contamination. We found that low and moderate
levels of pollution have adverse effects on edaphic fauna, suggesting
biological degradation of soils, even below pollution limits.
Nevertheless, more disturbed urban green spaces have been shown to play a
significant role in maintaining belowground biodiversity, thereby soil
functions.