Assessing Long-term Plant Availability of Biosolids-borne Heavy Metals Accumulated in Cropland Soils

In land application of biosolids, the majority of the introduced metals are associated with the solid phase. Plant absorption of biosolid-borne metals would take place in the soil and root interface, i.e. the rhizosphere where root exudates play important roles in dissolving metals and making them available for plant uptake. Plant roots exude a variety of soluble substances that include sugars, amino acids, organic acids, lipids, coumarins, flavonoids, proteins, enzymes, aliphatics, and aromatics. In addition, the root exudates may be decomposed by microbial metabolism in soils. Among them, organic acids are by far the most abundant and the most reactive with metal elements. The low-molecular-weight organic acids such as acetic, citric, oxalic, lactic, fumaric, tartaric, and succinic acids, which were often found in rhizosphere may have a primary role on availability of metals.Rhizosphere typically consists of the 1 to 5 mm layer of soil surrounding the root cylinder. At a given growing season, therefore not all of metal containing particles in soil will be under the influence of organic acids present in the rhizosphere. In continuous cultigation, however, all of the metal containing particles will be exposed to the influence of rhizosphere over time, one season or another. Plants grown on biosolids-amended fields, typically absorb ≪ 1% of the metals presents in the soil. It would take a long time to deplete the biosolids-borne metals that are available for plant uptake. Availability of biosolid-borne metals must account for not only the plant uptake of one growing season but also the total amounts available over time. A new approach is needed to describe the availability of metals in biosolids-treated soils.A kinetics model was developed to describe long-term availability of metals to plants grown in biosolid-treated soils that:Mt = C0 x (1 − e−kt)where Mt was the cumulative annual metal removal (kg ha−1) from biosolid-treated soils by plants at tth year, t was the time, and C0 is the site-specific total available metal pool (kg ha−1) represented by the root exudates extractable metal of the biosolids-amended soils and k is the site- and metal-specific plant uptake rate constant (t−1)from which the half-life bioavailable metal of biosolids-amended soil may be calculated. Methods to obtain C0 and k were tested.Archived soil and plant tissue samples from the biosolids-amended fields at Metropolitan Water Reclamation District of Greater Chicago, Fulton County IL Dedicated Land Reclamation Site and at the Rosemount Experiment Station, University of Minnesota were used validation and evaluation. The plant availabilities of Cd, Cr, Cu, Pb, Ni, and Zn in the biosolids-amended soils at the above referenced site were determined. Using Fulton County sites as an example, the results showed that 39, 42, 2, and 50% of the Cd, Ni, Pb, and Zn respectively are plant available and the availability half-life are 64, 36, 24, and 23 years. It will take 448, 203, 258, and 161 years of continuous cultivation, respectively to reduce the metal accumulations by 90 percent.