3. Results and discussion
3.1. Properties of soil and amendments
Table 2 addresses the main properties of the soil and amendments
used in this study. The soil has a sandy loam texture, acidic
pH and low content in essential nutrients (namely P and K). The
electrical conductivity is low, as expected for mining soils that are
exposed to weathering conditions (Alvarenga et al., 2009a,b).
Heavy metal pseudo-total concentration (associated with the soil
content extracted with aqua regia) is of special concern for Pb and
Zn, exceeding by far the legal limits set by Portuguese legislation
(Decreto-Lei no276/2009) for sewage sludge applications in agriculture
soils. In particular, the potentially bioavailable fraction of
Pb, assessed by the mobilisable content, represents about 77% of
the total content.
Composts CES and CWES were collected after 144 days of
composting. Both materials were phytotoxin-free (germination
indices close to 80%) and may be considered stable with moderately
slow respiration rates between 2 and 8 mg CO2 g1 C d1 (Gomez
et al., 2006). Whereas ES showed a lower GI (nearly 54%) but
higher respiration rate (25 mg CO2 g1 C d1). Heavy metals content
in the tested soil amendments (CWES, CES and ES) was low in
comparison to the limits set by the end-of-waste criteria for
biodegradable waste submitted to biological treatment (draft final
report) (JRC e European Commission, 2013).