Grassland Soil Microbial Biomass and Accumulation of Potentially Toxic Metals from Long-Term Slurry Application

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(1) A randomized-block field experiment compared three application rates (50, 100 and 200 m3 haI year1) of pig and cow slurries with fertilized and control plots of perennial ryegrass for 17 years. Soil samples were analysed for pH, biomass carbon and nitrogen, soil organic carbon, total nitrogen and EDTA-extractable copper and zinc concentrations at different depths within the profile. Herbage copper and zinc contents were determined at the first cut of 1986. (2) Treatment effects on all variables studied were less pronounced with increasing soil depth. (3) The amounts of carbon and nitrogen in the soil biomass were strongly correlated with soil pH. These relationships varied with soil depth within the top 15 cm of the profile. No lime was applied and the soil was acidified under all treatments except the highest application rate of cow slurry, which maintained the pH in the top 15 cm at 6 3. The soil became more acidic with increasing application rate of pig slurry but the opposite trend was shown under cow slurry. (4) The highest application rate of pig slurry lowered the soil biomass carbon content and increased soil organic carbon content in the top 5 cm of the profile after 16 years. Biomass nitrogen and soil total nitrogen showed similar but less pronounced trends, indicating a reduction in both the size of the microbial biomass and the turnover of soil carbon and nitrogen in this treatment. These effects cannot be attributed solely to differences in soil pH. (5) The highest application rate of pig slurry also led to marked accumulations of EDTA-extractable copper (> 80 mg kg') and zinc (50 mg kg-') in the top 5 cm of the soil after 17 years. Pig slurry also increased the proportions of soil copper and zinc present in extractable form, particularly at the highest application rate. Herbage concentrations of copper and zinc reached 10 and 44 mg kg-', respectively, after 16 years. Herbage copper could be toxic to sheep at concentrations > 10 mg kg. (6) Slurry application rates typical of farm practice did not reduce the size of the soil biomass, lead to phytotoxic concentrations of copper or zinc in the soil, or increase herbage copper or zinc to concentrations likely to be toxic to grazing animals. (7) The nitrifying microbial population in the soil did not appear to be inhibited by high application rates of pig slurry. Increasing application rate of both types of slurry increased potential nitrification rates in the soil as shown by the proportion of nitrogen mineralized, which was in the form of nitrate in incubated soil samples.