Lherroux et al. (1997) studied the behavior of metals
following pig manure application to a natural field and
drew the same conclusion. Achiba et al. (2009) also
found the exchangeable fraction of Cu and Zn increased
significantly after application of manure and compost.
After the application of BRs, the exchangeable fraction
of Cu and Zn from these materials was released into the
soil system during incubation. Thus the concentration of
the exchangeable fractions in soil might be elevated in this
aspect. On the other hand, chelating groups might complex
with these heavy metal ions to reduce the concentration
of the exchangeable fractions in soil. These processes
lead to species transformation of heavy metals and their
final distribution was a combined result of the changes. The results of our study could be explained as resulting
from the amount of heavy metal leaching exceeding that
combining with the PBR due to its saturated binding
sites. The binding sites on CBR might be unsaturated
and the exchangeable heavy metal concentration was not
high, so the function of complexing was more effective
than leaching for CBR. Comparing the two BRs, PBR
presented more agricultural risk than CBR as a fertilizer.
The soil environment might also affect the distribution of
heavy metal species. The exchangeable fraction would be
released into the environment when soil conditions became
more acidic (Nemati et al., 2010). This was the reason that
all the relative values in the YS treatments were higher
than in LS, according to Table 3. Thus BRs were more
adaptable to LS than YS. There are more acidic soils in the
south and alkaline soils in the north of China. Thus BRs are
not appropriate for the soils in the south of China. They are
more suitable for the North.