Carbon and trace element mobility in an urban soil amended with green waste compost

The amendment of degraded urban soils using recycled organic wastes offers potential improvements to physicochemical status and functionality, but there is a paucity of knowledge on the potential impact on residual contaminants in soil. The aim of this study was to evaluate the mobility of trace metals and arsenic (As) through an urban soil following amendment with green waste compost over an annual cycle. Rhizon soil pore water samplers were inserted into the sides of a soil pit located in a relatively undisturbed urban lawn, which was amended with a surface mulch of green waste compost. Regular sampling and analysis of pore water from the compost and at varying soil depths was carried out, in comparison to an adjacent profile, which had not been amended. Rapid dissociation of As from compost was found following its surface deposition as a mulch. Periods of high rainfall enhanced As mobility through upper soil horizons to deeper technogenic layers with higher pH. Copper (Cu) concentrations in soil beneath compost were substantially elevated and strongly correlated with dissolved organic carbon (DOC) released from compost into pore water. Seasonal biological activity, rather than soluble carbon, determined the patterns of mobility and release of zinc (Zn) in both compost-amended and control soil. Location of more stable concentrations of lead (Pb) at lower depths reflected reduced atmospheric fallout or the composition of the lower strata. Flushes of DOC from compost also enhanced mobility of residual cadmium (Cd) in soil. Amendment of urban soils with green waste compost mulches impacts on residual trace element contaminants and may enhance mobilisation of harmful elements to the wider environment, meaning that caution is needed when applying compost to soils with elevated levels of trace metals.