Nitrogen Transformations in a Tropical Virgin Grassland Oxisol

Urea (0,100,200 kg N/ha) and herbicides (10 kg/ha dalapon and 3.75 kg/ha paraquat) were applied to a grassland sward for renovation. Nitrogen fractions were determined in the soil upto 18 months. Urea was found to enhance total-and mineral -N, mineralizable-and availability-N indexes with a wide variations occurring from vegetational cover to no cover. Major part of organic-N in tropical soil comprised nonhydrolyzable-N, different from temperate soils, but the fractions of hydrolyzable-N, viz., amino acid> ammonium>unidentified>hexosamine-N, were identical to it. Urea increased nonhydrolyzable-N, and urea and paraquat increased hexosamine and unidentified-N fractions of the hydrolyzable-N in the grassland Oxisol.