Biological immobilization of fertilizer nitrogen in humid tropical soils of Mauritius

The mobility and fate of fertilizer nitrogen was studied by applying ammonium sulphate labelled with atom excess nitrogen-15 to rocky soil from a dry region and to rock-free soil from a high rainfall zone. Sorghum plants (Sorghum bicolor) were grown in soil columns and water was applied to represent the daily rainfall at each site. Analysis of the leachates, plant material and soil showed that biological immobilization of fertilizer nitrogen was almost complete 2 months after application, with most of this nitrogen remaining hydrolysable after 10 months, and there was rapid uptake of nitrogen by the sorghum. The fertilizer nitrogen lost by leaching was less than 5 per cent in rock-free soil and 12 per cent in rocky soil.