Grass-induced changes in properties of soils in urban green areas with emphasis on mobility of metals

Green areas in urban parks are special ecosystems because of social and ecological benefits. We investigated the ecological impacts of green spaces by comparing the properties of soils, with emphasis on metals, in green spaces and bare soil playgrounds in four urban parks in Murcia City (SE Spain). Organic matter (OM) contents in green areas are higher than playground soils due to litter decomposition from grass. Cation exchange capacity in green areas ranged from 10 to 14 compared to 2 cmol(+)kg(-1) in playground soils, showing the effect of OM (and clay) contents in green areas due to the presence of vegetation. Soil total metal contents in urban parks are below the environmental threshold values for soils in Spain and other European countries, and come from atmospheric deposition and fertilizers application. Diethylenetriaminepentaacetic acid (DTPA)-extractable soil Cd in green areas (4% of total Cd) is lower than in playground soils (58% of total Cd). We attributed the reduced DTPA- and water-extractable Cd, Co, Cr, Mn, and Mo to the formation of stable complexes between metals and OM produced from the continued decomposition of roots of grasses in green spaces. Grasses accumulate less metal than soils except in roots where Cr and Zn are higher than soils by as much as 31 and 17%, respectively. We conclude that grasses increased (up to 8x) soil OM content in green areas to significantly increase the ability of soils to accumulate nutrients for plants, and to retain and reduce the mobility and toxicity of metals in soils in urban parks.