NET NITROGEN MINERALIZATION IN SOILS UNDER 6 INDIGENOUS TREE SPECIES,AN ABANDONED PASTURE AND A SECONDARY FOREST IN THE ATLANTIC LOWLANDS OF COSTA-RICA

Nitrogen mineralization, nitrification potentials, pH, total N, C, ext ractable P and cations were measured in soils under 4-year-old, mono-s pecific stands of six fast-growing, native tree species, an abandoned pasture, and a 20-year-old secondary forest, as part of a study on the use of indigenous tree species for rehabilitation of soil fertility o n degraded pastures at the La Selva Biological Station in the Atlantic humid lowlands of Costa Rica. Soil net nitrification potential rates were higher under two N-fixing, leguminous species, Stryphnodendron mi crostachyum Poepp. et Endl. (1.1-1.9 mg kg(-1) day(-1)) and Dalbergia tucurensis Donn. Smith (0.7-1.5 mg kg(-1) day(-1)), than under the non -N-fixing trees in the plantation, Vochysia guatemalesis Don. Sm., Voc hysia ferruginea Mart, Dipteryx panamensis (Pittier) Record and Mell a nd Hyeronima alchorneoides Fr. Allemao (0.2-0.8 mg kg(-1) day(-1)). Va lues under the N-fixing trees were comparable to those found in second ary forest. There were no statistically significant differences in soi l total N or in other nurtients between the species. Results of pH mea surements done before and after incubation did not show any clear evid ence of a pH drop attributable to nitrification.