Nitrification in polluted soil fertilized with fast- and slow-releasing nitrogen: A case study at a refinery landfarming site

The nitrifying activity and the effect of fertilization with urea and methylene urea were studied in a landfarming site. The site has been operative over 20 years and maintained by heavy nitrogen fertilization. The landfarming soil contained 4–6% (w/w) oil. The nitrate accumulation was 20–50 mg NO3-N day−1 kg−1 observed after methylene urea fertilization of 889 g N m−2. Nitrification ex situ (in laboratory conditions) was 8.8 mg NO3-N day−1 kg−1 in the presence of 380 mg kg−1 NH4+-N. The half-saturation concentration of nitrification was more than 200 mg NH4+-N kg−1. The results show that nitrification was active in soil with high oil concentration. Urea fertilization of 893 g N m−2 caused an increase of soil NH4+-N concentration up to 5500 mg kg−1 and pH > 8.5. This led to inhibition of nitrification, which persisted after NH4+ concentration decreased below 200 mg NH4+ kg−1.