Nitrogen oxide fluxes and nitrogen cycling during postagricultural succession and forest fertilization in the humid tropics

The effects of changes in tropical land use on soil emissions of nitrous ox ide (N2O) and nitric oxide (NO) are not well understood. We examined emissi ons of N2O and NO and their relationships to land use and forest compositio n, litterfall, soil nitrogen (N) pools and turnover, soil moisture, and pat terns of carbon (C) cycling in a lower montane, subtropical wet region of P uerto Rico. Fluxes of N2O and NO were measured monthly for over 1 year in o ld (more than 60 years old) pastures, early- and mid-successional forests p reviously in pasture, and late-successional forests not known to have been in pasture within the tabonuco (Dacryodes excelsa) forest zone. Additional, though less frequent, measures were also made in an experimentally fertili zed tabonuco forest. N2O fluxes exceeded NO fluxes at all sites, reflecting the consistently wet environment. The fertilized forest had the highest N oxide emissions (22.0 kg . N . ha(-1) . y(-1)). Among the unfertilized site s, the expected pattern of increasing emissions with stand age did not occu r in all cases. The mid-successional forest most dominated by leguminous tr ees had the highest emissions (9.0 kg N . ha(-1) . y(-1)), whereas the mid- successional forest lacking legumes had the lowest emissions (0.09 kg N . h a(-1) . y(-1)). N oxide fluxes from late-successional forests were higher t han fluxes from pastures. Annual N oxide fluxes correlated positively to le af litter N, net nitrification potential nitrification, soil nitrate, and n et N mineralization and negatively to leaf litter C:N ratio. Soil ammonium was not related to N oxide emissions. Forests with lower fluxes of N oxides had higher rates of C mineralization than sites with higher N oxide emissi ons. We conclude that (a) N oxide fluxes were substantial where the availab ility of inorganic N exceeded the requirements of competing biota; (b) spec ies composition resulting from historical land use or varying successional dynamics played an important role in determining N availability; and (c) th e established ecosystem models that predict N oxide loss from positive rela tionships with soil ammonium may need to be modified.