An in-depth look into a tropical lowland forest soil: nitrogen-addition effects on the contents of N2O, CO2 and CH4 and N2O isotopic signatures down to 2-m depth

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 (N2O), carbon dioxide (CO2) and methane (CH4), as well as natural abundance isotopic signatures of N2O, nitrate (NO3 (-)) and ammonium (NH4 (+)). In the control plots during dry season, we deduced limited N2O production by denitrification in the topsoil (0.05-0.40 m) as indicated by: ambient N2O concentrations and ambient N-15-N2O signatures, low water-filled pore space (35-60%), and similar N-15 signatures of N2O and NO3 (-). In the subsoil (0.40-2.00 m), we detected evidence of N2O reduction to N-2 during upward diffusion, indicating denitrification activity. During wet season, we found that N2O at 0.05-2.00 m was mainly produced by denitrification with substantial further reduction to N-2, as indicated by: lighter N-15-N2O than N-15-NO3 (-) throughout the profile, and increasing N2O concentrations with simultaneously decreasing N-15-N2O enrichment with depth. These interpretations were supported by an isotopomer map and by a positive correlation between O-18-N2O and N-15-N2O site preferences. Long-term N addition did not affect dry-season soil N2O-N contents, doubled wet-season soil N2O-N contents, did not affect N-15 signatures of NO3 (-), and reduced wet-season N-15 signatures of N2O compared to the control plots. These suggest that the increased NO3 (-) concentrations have stimulated N2O production and decreased N2O-to-N-2 reduction. Soil CO2-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 CO2-C contents. The N-addition plots showed reduced dry-season soil CH4-C contents and threshold CH4 concentrations were reached at a shallower depth compared to the control plots, revealing an N-induced stimulation of methanotrophic activity. However, the net soil CH4 uptake rates remained similar between treatments possibly because diffusive CH4 supply from the atmosphere largely limited CH4 oxidation.