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 NO 3-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 NO 3-N day −1 kg −1 in the presence of 380 mg kg −1 NH 4 +-N. The half-saturation concentration of nitrification was more than 200 mg NH 4 +-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 NH 4 +-N concentration up to 5500 mg kg −1 and pH > 8.5. This led to inhibition of nitrification, which persisted after NH 4 + concentration decreased below 200 mg NH 4 + kg −1. Nitrification occurs in soil containing high concentration of aged oil.