Pasture and soil zinc evolution in forest and agriculture soils of Northwest Spain three years after fertilisation with sewage sludge

The main problem with the agricultural use of sewage sludge is the higher heavy metal (mainly zinc) concentrations in the sludge than in the soil. In Spain, R.D. 1310/1990 and European Directive 86/278 limit the total heavy metal concentration in soil but not the changes in heavy metal availability, which directly affect plant absorption and thereby represent a possible risk to human health. The heavy metal availability in soil depends on different factors (pH, soil organic matter, and weather) and the type of soil (agriculture or forest). This study evaluate the effects of two types of soils (forest and agriculture), two types of vegetation (natural and sown), and two types of fertilisation (sludge fertilisation and mineral fertilisation, with a no fertiliser control) in afforested and treeless pastures and in sown and unsown forestlands of Northwest Spain on the total and available Zn concentration in soil and the concentration of Zn in grasslands. The experimental design was completely randomised with nine treatments and three replicates. The fertilisation with sewage sludge increased the total, Mehlich 3 and sward Zn concentrations in forest and agriculture soils, and the levels of Zn were lower in the forest than in the agriculture soils probably because of the low soil pH in the former which probably limited the mineralisation of the sludge in the forest soil. Therefore, there is still a potential source of Zn in forest soils than in agriculture soils, when the incorporation conditions improve. This makes advisable to extend the period without grazing after sewage sludge inputs in forest than agriculture soils to avoid direct Zn intake by animals through the soil consumption. The effect of the application of sewage sludge on sward Zn concentrations was more relevant in unsown pastures of forest soils than in agriculture soils due to the different absorption capacity of unsown compared with sown species. Zinc inputs from sewage sludge to soils did not cause harmful effects on plants or animals. However, the management of heavy metal availability must be included by policy makers in further directives to better evaluate environmental risk. Moreover, Zn maintenance requirements of animals like goats, horses, bovines and ovine were not often reached, which may make advisable to provide Zn supplement to animals if their nourishment is derived solely from these pastures.

► Sewage sludge use as fertiliser increases silvopastoral productivity.
► Heavy metal soil availability depends on type of soil.
► Higher levels of Zn in sewage than in soil may cause environment and health problems.
► The concentrations of available Zn in soils should be considered for sewage applications further than soil pH.