Potential release of metals from biosolids is often held to be the major reason why caution is needed in land re-use of these materials. The so called "time bomb" concern is that biosolid metal availability may increase dramatically with time, through decomposition of biosolid organic material that initially binds the metals. We examined biosolid metal availability using isotopic tracers - ¹⁰⁹Cd, ⁶⁵Zn, and ⁶⁴Cu - to determine the labile pool of metal in biosolid-amended soils. Particular emphasis was given to Cu as this metal binds most strongly to organic matter, and therefore has the greatest potential for release as biosolid organic matter decomposes. Even in soils incubated with biosolids for over 7 years, metal lability increased only marginally in soils as biosolid organic matter decomposed. Perhaps the greatest changes in metal lability were associated with changes in soil pH. pH normalisation of metal lability is essential to accurately distinguish metal release from organic matter from pH-induced dissolution of soil or biosolid minerals.

We suggest that the small increases in metal lability noted, despite significant decomposition of biosolid organic matter, are due to other competing reactions. Metals released from biosolid organic matter are likely to be sequestered by oxides and minerals present in both biosolids and in soils. Ageing reactions of metals with soil and biosolid minerals (micropore/solid state diffusion, occlusion, co-precipitation, etc.) will reduce metal lability over time. Note that while surface adsorption will reduce soil pore water concentrations of metals, it will not reduce metal lability as surface adsorbed metals are included in the measure of the labile pool. We conclude that the biosolid metal "time-bomb" is unlikely to be significant and that metal release from biosolids will be more related to acidification of soil leading to desorption of metal from soil surfaces.