The efficiency of Cd phytoextraction by S. plumbizincicola increased with the addition of rice straw to polluted soils: the role of particulate organic matter

The addition of organic amendments to soils can significantly increase soil organic matter (SOM) content as well as the mass of large (e.g. 2000–250 μm) particulate organic matter (POM) fractions. Here we address whether the addition of organic amendments might affect both POM pools and the phytoextraction efficiency of Sedum plumbizincicola (S. plumbizincicola) in cadmium (Cd) polluted soils. We carried out a 442-days long pot experiment and measured Cd uptake by S. plumbizincicola for four successive plant crops in two polluted paddy soils subjected to rice straw (RS) amendment. We also measured soil pH, nutrient availability, pore water dissolved organic carbon (DOC) and Cd concentrations across different soil particle-size fractions. We found that RS amendments had no significant effects on soil pH, DOC or Cd concentrations in bulk soils during repeated phytoextraction. However, RS amendments were associated with significant (1) increases (i.e. 14.3 to 20.7%) in Cd uptake by S. plumbizincicola plants from the first crop harvest, (2) increases in available soil nutrients content, (3) increases in the percentage of oxidizable Cd fractions and the mass of soil POM fractions, and (4) decreases in POM Cd concentrations after repeated phytoextraction. These findings suggest that POM fractions may represent important accessible pools of Cd for uptake by S. plumbizincicola plants. RS amendments enhance both Cd bioavailability within soil POM pools and Cd uptake by the hyperaccumulator S. plumbizincicola thus providing an effective management practice to improve Cd removal from polluted soils through phytoextraction.