SHORT-TERM LEAF SURFACE-ADHESION OF HEAVY-METALS FOLLOWING APPLICATION OF SEWAGE-SLUDGE TO GRASSLAND

In the short term, surface-applied, digested, liquid sewage sludge may remain on grass leaf surfaces, leading to enhanced direct intake of p otentially toxic elements by stock. The purpose of this field experi m ent was to investigate metal adhesion to herbage from liquid sewage sl udge applications. Liquid digested sludge was applied at 0, 55 or 110 m(3) ha(-1) to an Agrostis capillaris and Holcus lanatus award that ha d been cut to either 4 cm (short) or 13 cm (long). The application rat e of metals was relatively low. One day after application (day 1), 8-1 3% of the sludge solids applied had adhered to the short grass treatme nt, accounting for 35-62% of the dry-matter (DM) yield. On the long gr ass, 20-42% of the sludge dry solids adhered, accounting for 46-47% of the herbage DM yield. The content of all heavy metals in and on the g rass herbage (Cu, Fe, Ni, Pb, Zn, Cr, Cd) was significantly increased (P < 0.05) at day 1. Cu concentrations in and on the grass decreased t o 25 mg kg(-1) after 16-29 d, Fe took 33-45 d to decline to 1000 mg kg (-1) whereas Pb took 12-18 d to decline to 30 mg kg(-1). It took 36 d for Cd, 43 d for Fe, Cu and Ni, and 57 d for Zn and ph to be not signi ficantly (P < 0.05) different from the control. Grass growth was the m ost significant factor(P < 0.05 for all treatments and metals) influen cing the decline in grass metal concentration, explaining 65-96% of th e variation. The cutting treatment, sludge application rate and metal species also significantly affected the extent and rate of reduction i n metal contamination over time.