Nitrogen transformations in a range of tropical forest soils

Nitrogen transformations were measured in soil from 33 sites arranged in 15 classes in five tropical areas. Sites sampled included forests on fertile volcanic and sedimentary soils, infertile oxisols and ultisols, extremely infertile white-sand soils, montane sites, and a primary and a secondary successional sere. In comparison with temperate forests, net N mineralization and nitrification were rapid in intact lowland tropical forests (other than white sands) and microbial 15N uptake was relatively slow. In contrast, white-sand and upper montane sites had lower rates of mineralization and nitrification and greater microbial immobilization of 15N; N could be in short supply in many such sites. These patterns of N transformations reinforce inferences about soil N supply based on patterns of litterfall N in tropical forests.