Optimization of pig slurry application to heavy metal polluted soils monitoring nitrification processes

Nitrification is often negatively affected by heavy metal pollution in soils, this limiting land revegetation. Thus, the potential use of pig slurry as a nitrogen-rich organic amendment in different heavy metal contaminated soils has been evaluated; this also being a way of recycling this waste. In order to identify the factors affecting nitrification processes in heavy metal polluted soils (soil pH, heavy metal solubility and the N source), incubation experiments were run using two polluted soils with different pH values (5.0 and 7.1) and a non-contaminated soil (pH 8.2). Ammonium was added as pig slurry or as ammonium sulphate for comparison (both added at 150 mg NH4+-N kg−1 of soil). Pig slurry provoked higher nitrification rates and N-immobilisation than ammonium sulphate, especially in the neutral-polluted soil, reflecting an improvement of the microbial activity in the soil. The microbial immobilisation of N led to an inverse relationship between the amount of N added and nitrate conversion in the neutral-polluted soil and in the non-contaminated soil amended with different pig slurry dosages (75, 150 and 225 mg NH4+-N kg−1 of soil). Low rates of nitrification and N-immobilisation were found in the acidic soil. Pig slurry addition to metal polluted soils enhanced soil nitrification, especially when metals were in low-solubility forms.