Grassland soil microbial biomass and accumulation of potentially toxic metals from long-term slurry application.
Quantities of C and N in the microbial biomass of soils receiving long term applications of pig and cow slurries were compared, and changes in soil pH, soil organic C, total N and EDTA-extractable Cu and Zn were measured. After 17 years, differences in biomass C and N between treatments were strongly correlated with differences in soil pH. The soils became more acidic with increasing application of pig slurry but the opposite trend was shown for cow slurry. The highest annual application of pig slurry (200 m3/ha) lead to accumulations of EDTA-extractable Cu and Zn (> 8 mg/kg and 50 mg/kg resp.) in the top 5 cm of the soil while herbage concn of Cu and Zn reached 10 and 44 mg/kg, resp. Herbage Cu can be toxic to sheep at concn of > 10 mg/kg. However typical farm application rates did not reduce the size of the soil biomass, lead to phytotoxic concn of Cu and Zn in soil or increase herbage Cu and Zn to concn likely to be toxic to grazing animals. The nitrifying microbial population of the soil was not affected by high applications of slurry and potential nitrification rates increased with increasing slurry application rates.