| Popis: |
Three experimental sites were set up on grassland farms in Lanarkshire to study the effects of liquid sewage sludge applications on heavy metal behaviour, agricultural productivity, and long term soil fertility. Over the period 1985–1993, sewage sludge was applied at 0, 67, 135 and 270 m 3 ha −1 y” 1 to grassland soils maintained at pH 5.5 or 6.5. After four years, the high rate of metal sludge applications increased soil concentrations of Cu, Zn, Pb, Ni and Cr to a depth of 100–150 mm. The largest accumulation was in the top 0–25 mm. Approximately 33% of each metal applied was recovered in this top layer. Soil pH did not affect the distribution of metals in the soil profile. Acetic acid extracted similar amounts of Zn, but significantly lower amounts of Cu and Pb compared to the levels obtained using EDTA. This indicates that relatively high amounts of Cu and Pb were bound to the soil organic matter. Total soil analysis for Cu and Pb revealed a correlation with the respective EDTA extractable component, but the exact relationship varied between the sites. Plant Cu and Zn concentrations were increased, but those of Cd, Ni, Cr, and Pb were unaffected by sludge application. Increasing the soil pH from 5.5 to 6.5 decreased plant uptake of Zn and Ni. A positive relationship between soil total Cu and herbage Cu was found at two sites, but this relationship was absent from the third. Although soil Cu, Zn, and Pb concentrations were close to or exceeded UK Regulations at one location, the herbage still did not contain toxic concentrations of these elements. Sewage sludge appears to represent a sustainable soil conditioner, although long term use on grassland may lead to a build up of metals in the top 25 mm of the soil profile giving rise to a potential risk for grazing animals through soil ingestion, especially during winter. |
