Soil residuals and plant uptake of Cu and Zn from biosolids applied to a clay loam soil under field conditions in Victoria, Australia

A field experiment was conducted to investigate the effects of incorporating anaerobically digested dewatered biosolids (DWB) and composted biosolids (CB) on the availability of the heavy metals Cu and Zn to canola (Brassica napus cv. Beacon) and oats (Avena sativa cv. Echidna) grown in a clay loam soil. DWB at rates of 0, 5, 25, 45 and 65 t dry solids (ds) ha-1 and CB at rates of 0, 10 30, 50 and 70 t ds ha-1 were incorporated into the soil to a depth of approximately 15 cm. The plots were arranged in triplicate, in a randomised complete block design. Canola and oat seeds were sown at a seeding rate of 5 and 100 kg ha-1 , respectively. After the crop harvest, total and diethylenetriamine penta-acetic acid (DTPA)-extractable heavy metals in biosolids-amended soil were determined using X-ray fluorescence (XRF) and inductively coupled plasma mass spectrometry (ICP-MS), respectively. Total metals in plant leaves were analysed using ICP-MS. In these biosolids, only Cu and Zn were high enough to significantly increase their concentration in the soil and plant tissue. The results showed that application of biosolids significantly (P < 0.001) increased DTPA-extractable and total concentrations of Cu and Zn in the amended soil. The ratio of DTPA-extractable to total (XRF) Cu and Zn also changed with increasing application rates of DWB and CB. The change in the ratio was more pronounced in soils receiving DWB, probably associated with the significant (P < 0.001) decrease in soil pHw with their loading rate. Positive correlations were also noted between the application rate of biosolids and the Cu and Zn levels in the canola and oat leaves.