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
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- Koehler, B., Corre, M.D., Steger, K. et al. Biogeochemistry (2012) 111: 695. doi:10.1007/s10533-012-9711-6
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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 15N-N2O signatures, low water-filled pore space (35–60%), and similar 15N signatures of N2O and NO3−. In the subsoil (0.40–2.00 m), we detected evidence of N2O reduction to N2 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 N2, as indicated by: lighter 15N-N2O than 15N-NO3− throughout the profile, and increasing N2O concentrations with simultaneously decreasing 15N-N2O enrichment with depth. These interpretations were supported by an isotopomer map and by a positive correlation between 18O-N2O and 15N-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 15N signatures of NO3−, and reduced wet-season 15N signatures of N2O compared to the control plots. These suggest that the increased NO3− concentrations have stimulated N2O production and decreased N2O-to-N2 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.