Urea hydrolysis and inorganic N in a Luvisol after application of fertiliser containing rare-earth elements

Abstract

In recent decades, Chinese agriculturists have used rare-earth-containing fertilisers as basal fertilisers together with N fertilisers (e.g. urea). We studied urea hydrolysis and its hydrolysis products in a laboratory experiment using urea-N fertiliser with rare earths at rates from 0.5 to 50% (w/w). The results indicated that application of rare earths at a high rate could result in a short-term inhibition of urea hydrolysis and an increase in soil (NH₄⁺ + NO₃^– + NO₂^–)-N content. When the application rate of rare earths was higher than 5% of the applied urea-N (corresponding to 10 mg/kg soil), soil exchangeable NH₄⁺-N content increased significantly following the hydrolysis of the applied urea. Increasing the application rate of rare earths appeared to reduce the content of soil urea-derived (NO₃^– + NO₂^–)-N. A substantial reduction in soil pH was found immediately after application of rare earths and urea. We conclude that application of rare earths at >10 mg/kg may lead to a substantial increase in the content of urea-derived N in the soil, via the inhibition of urea hydrolysis and nitrification.