Impact on crops, plants and soils of metal trace elements transfer and flux, after spreading of fertilizers and biosolids

In France, the yearly production of sludge from wastewater treatment plants is 900,000 metric tons dry matter and 60% of this is reused for land application. Today, the sustainability of this pathway is open to question. Among the different arguments cited are the levels of metal trace elements and the risks of accumulation in soils. With the ultimate aim of agronomic sludge recycling, the transfer of metal trace elements has been studied using vegetation containers planted with rye-grass under controlled conditions of temperature and humidity. Samples of a domestic sludge, an industrial sludge and a fertilizer have been mixed with the soil. By monitoring the growth of the rye-grass, we have been able to observe that the addition of sludge increases production of plant matter. It appears that the roots absorb higher quantities of metal trace elements and form a barrier to their transfer to the above ground parts of the rye-grass. For the group of metal trace elements studied, no significant differences have been observed between the rye-grass grown on soil alone and that on soils amended with fertilizer or urban sludge. For the majority of the vegetation containers studied, there has been no significant modification in the soil metal distribution over time, as a result of the addition of urban sludge, and no significant difference between fertilizers and sludges.