THE EFFECTS OF NATURAL AND HUMAN DISTURBANCES ON SOIL-NITROGEN DYNAMICS AND TRACE GAS FLUXES IN A PUERTO-RICAN WET FOREST

We examined the effects of two disturbances (Hurricane Hugo and forest clearcutting) on soil nitrogen dynamics and on the exchanges of N2O, CO2, and CH4 between soils and the atmosphere of a subtropical wet forest in Puerto Rico. The disturbances resulted in prolonged increases in ammonium pools and short-term increases in rates of net N-mineralization and net nitrification. Nitrous oxide emissions increased following both disturbances. The most dramatic increase was observed 4 mo after clearcutting; N2O emissions (109.49-mu-g N/m2-hr) from the cut plot were about two orders of magnitude higher than emissions from the reference plot (1.71-mu-g N/m2-hr). Carbon dioxide emissions from both disturbed plots (mean 102.47 mg C/m2-hr) were about 30 percent lower than the reference (mean 151.28 mg C/m2-hr). Soils at all sites were generally sinks for CH4. Methane uptake, however, was suppressed by both disturbances. This suppression may be related to disturbance-induced changes in the nitrogen cycle, as we have previously observed in temperate zone forests.