Use of dairy-industry sludge as fertiliser for grasslands in northwest Spain: heavy metal levels in the soil and plants

Abstract

The suitability of dairy-plant sludge for fertilisation of a Cambisolic soil was investigated in northwest Spain. Soil properties (pH, electrical conductivity, organic matter content, NO3−, P, Ca, Mg, Na, K and Al) and soil and plant tissue heavy metal contents (Hg, Pb, Cd, Cu, Zn, Ni and Cr) were determined in 12 grassland plots fertilised over a 1–4-year period with dairy sludge and conventional fertiliser (cattle slurry and mineral fertilisers), and in six meadows fertilised with conventional fertilisers only. Heavy metal contents were also determined in plant tissues from different plots. There were no significant differences in soil heavy metal concentrations between the sludge-amended plots and the control plots, except for Pb concentration, which was significantly lower in the sludge-amended plots. A significant correlation between metal content in the soil and total sludge dose applied was detected only for Cr. Even in plots to which the dairy-plant sludge had been applied for 4 years, heavy metal levels in both soil and plants were within European Union and Spanish legislative limits (Council of the European Communities Council Directive of 12 June 1986 on the protection of the environment, and in particular of the soil, when sewage sludge is used in agriculture (86/278/EEC), Official J. Eur. Communities 1986;L 181:6–12; Royal Decree 1310/1990, del 29 de octubre por el que se regula la utilizacion de los lodos de depuracion en el sector agrario, BOE 1990;262:32339–32340.) However, a number of significant metal-metal correlations (Ni–Zn, Ni–Cr, Ni–Cu, Cu–Cr, Zn–Cr and Pb–Cd) were detected in both the sludge and the sludge-amended soils, but not the non-amended soils, suggesting that the sludge is an important source of these metals. In addition, soils that had received the dairy-plant sludge had significantly higher conductivity, probably because of the NaOH used for cleaning in dairy plants. Salinity-related variables (K, NO3−) were significantly correlated with Cr and Zn concentrations in plant tissues. These findings suggest a need to assess the possible long-term effects of dairy sludge application, with the aim of developing guidelines for its use as a fertiliser.