摘要 : Atmospheric nitrogen (N) deposition is rapidly increasing in tropical regions. We investigated how a decade of experimental N addition (125 kg N ha -1 year -1) to a seasonal lowland forest affected depth distribution and contents ... 展开 Atmospheric nitrogen (N) deposition is rapidly increasing in tropical regions. We investigated how a decade of experimental N addition (125 kg N ha -1 year -1) to a seasonal lowland forest affected depth distribution and contents of soil nitrous oxide (N 2O), carbon dioxide (CO 2) and methane (CH 4), as well as natural abundance isotopic signatures of N 2O, nitrate (NO 3-) and ammonium (NH 4+). In the control plots during dry season, we deduced limited N 2O production by denitrification in the topsoil (0.05-0.40 m) as indicated by: ambient N 2O concentrations and ambient 15N-N 2O signatures, low water-filled pore space (35-60%), and similar 15N signatures of N 2O and NO 3-. In the subsoil (0.40-2.00 m), we detected evidence of N 2O reduction to N 2 during upward diffusion, indicating denitrification activity. During wet season, we found that N 2O at 0.05-2.00 m was mainly produced by denitrification with substantial further reduction to N 2, as indicated by: lighter 15N-N 2O than 15N-NO 3- throughout the profile, and increasing N 2O concentrations with simultaneously decreasing 15N-N 2O enrichment with depth. These interpretations were supported by an isotopomer map and by a positive correlation between 18O-N 2O and 15N-N 2O site preferences. Long-term N addition did not affect dry-season soil N 2O-N contents, doubled wet-season soil N 2O-N contents, did not affect 15N signatures of NO 3-, and reduced wet-season 15N signatures of N 2O compared to the control plots. These suggest that the increased NO 3- concentrations have stimulated N 2O production and decreased N 2O-to-N 2 reduction. Soil CO 2-C contents did not differ between treatments, implying that N addition essentially did not influence soil C cycling. The pronounced seasonality in soil respiration was largely attributable to enhanced topsoil respiration as indicated by a wet-season increase in the topsoil CO 2-C contents. The N-addition plots showed reduced dry-season soil CH 4-C contents and threshold CH 4 concentrations were reached at a shallower depth compared to the control plots, revealing an N-induced stimulation of methanotrophic activity. However, the net soil CH 4 uptake rates remained similar between treatments possibly because diffusive CH 4 supply from the atmosphere largely limited CH 4 oxidation. 收起