, Volume 111, Issue 1-3, pp 695-713
Date: 22 Feb 2012

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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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.

An erratum to this article can be found at http://dx.doi.org/10.1007/s10533-012-9780-6.