Nitrogen-oxide emissions from tropical forest soils exposed to elevated nitrogen input strongly interact with rainfall quantity and seasonality

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.