Abstract
In tropical forests, multi-year studies on the impact of chronic nitrogen (N) enrichment on soil N-oxide fluxes are lacking. Our objectives were to: (1) assess the changes in soil N-oxide (NO+N2O) fluxes from a montane forest in response to 3–4 years of N addition and compare these to the response of a lowland forest with 11–12 years of N addition, (2) quantify the contributions of nitrification and denitrification to nitrous oxide (N2O) emissions, and (3) assess how deep in the soil profile the N2O concentrations are affected by N input. These measurements were conducted in montane and lowland forests in Panama in 2008–2009, which covered the last 2 years of a 4-year investigation that started in 2006. Each forest had a control and N addition treatment (125 kg urea N ha−1 year−1) with four replicate plots (1,600 m2 each) per treatment. N-oxide emissions from the montane forest started to increase within the first 2 years of treatment and continued to increase in the 3rd and 4th years of N addition, during which the emissions were already equivalent to those from the lowland forest with 11–12 years of N addition. The large N-oxide response of the montane forest to N addition were due to the large increases in gross nitrification rates in the organic layer and the high moisture contents of the mineral soil due to the high rainfall (5.5 ± 0.2 m year−1). In the lowland forest (2.7 ± 0.1 m rain year−1), N-oxide response to N addition was more pronounced in wet years (i.e. 2006–2007 with 5–17 % higher rainfall than average) than in dry years (i.e. 2008–2009 with 5–26 % lower rainfall than average). Denitrification was the dominant source of N2O not only for the organic layer and the top 5 cm of mineral soil but possibly also for the entire 2 m depth. Soil–air N2O concentrations were elevated by N addition down to at least 2 m. Our results suggest that the best indicators of these responses were the presence of an organic layer and rainfall quantity and seasonality.
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Acknowledgments
This study was funded by the Robert Bosch Foundation (Germany) for M.D. Corre’s independent research group, NITROF, and by the Deutsche Forschungsgemeinschaft (German Research Foundation, Co 749/1-1). The Smithsonian Tropical Research Institute and ANAM, Panama provided invaluable administrative and technical support. S. Joseph Wright hosted our study in the Gigante nutrient manipulation experiment. The help of the NITROF assistants (Rodolfo Rojas, Erick Diaz and Ignacio Del Cid), and the SSTSE laboratory technicians (especially Kerstin Langs) are highly appreciated.
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Corre, M.D., Sueta, J.P. & Veldkamp, E. Nitrogen-oxide emissions from tropical forest soils exposed to elevated nitrogen input strongly interact with rainfall quantity and seasonality. Biogeochemistry 118, 103–120 (2014). https://doi.org/10.1007/s10533-013-9908-3
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DOI: https://doi.org/10.1007/s10533-013-9908-3