Indications of nitrogen-limited methane uptake in tropical forest soils

Abstract. It is estimated that tropical forest soils contribute 6.2 Tg yr⁻¹ (28%) to global methane (CH₄) uptake, which is large enough to alter CH₄ accumulation in the atmosphere if significant changes would occur to this sink. Elevated deposition of inorganic nitrogen (N) to temperate forest ecosystems has been shown to reduce CH₄ uptake in forest soils, but almost no information exists from tropical forest soils even though projections show that N deposition will increase substantially in tropical regions. Here we report the results from two long-term, ecosystem-scale experiments in which we assessed the impact of chronic N addition on soil CH₄ fluxes from two old-growth forests in Panama: (1) a lowland, moist (2.7 m yr⁻¹ rainfall) forest on clayey Cambisol and Nitisol soils with controls and N-addition plots for 9–12 yr, and (2) a montane, wet (5.5 m yr⁻¹ rainfall) forest on a sandy loam Andosol soil with controls and N-addition plots for 1–4 yr. We measured soil CH₄ fluxes for 4 yr (2006–2009) in four replicate plots (40 m × 40 m each) per treatment using vented static chambers (four chambers per plot). CH₄ fluxes from the lowland control plots and the montane control plots did not differ from their respective N-addition plots. In the lowland forest, chronic N addition did not lead to inhibition of CH₄ uptake; instead, a negative correlation of CH₄ fluxes with nitrate (NO₃^–) concentrations in the mineral soil suggests that increased NO₃^– levels in N-addition plots had stimulated CH₄ consumption and/or reduced CH₄ production. In the montane forest, chronic N addition also showed negative correlation of CH₄ fluxes with ammonium concentrations in the organic layer, which suggests that CH₄ consumption was N limited. We propose the following reasons why such N-stimulated CH₄ consumption did not lead to statistically significant CH₄ uptake: (1) for the lowland forest, this was caused by limitation of CH₄ diffusion from the atmosphere into the clayey soils, particularly during the wet season, as indicated by the strong positive correlations between CH₄ fluxes and water-filled pore space (WFPS); (2) for the montane forest, this was caused by the high WFPS in the mineral soil throughout the year, which may not only limit CH₄ diffusion from the atmosphere into the soil but also favour CH₄ production; and (3) both forest soils showed large spatial and temporal variations of CH₄ fluxes. We conclude that in these extremely different tropical forest ecosystems there were indications of N limitation on CH₄ uptake. Based on these findings, it is unlikely that elevated N deposition on tropical forest soils will lead to a rapid reduction of CH₄ uptake.