This is
because the increase in the organic matter can lead to an
increase in the portion of the specific adsorption of Cu(II)
and Pb(II) by the soil, and the fraction of the Cu(II) and
Pb(II) adsorbed can not be desorbed by the unbuered
salts. Hence, the long-term rice cultivation probably may
cause an accumulation of organic matter in the paddy soils
and thus increase the stability of heavy metals such as
Cu(II) and Pb(II) in these soils through the formation of
complexes between the metals and the organic matter.