Indications of nitrogen-limited methane uptake in tropical forest soils

Abstract

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 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 4 replicate plots (40 m × 40 m each) per treatment using vented static chambers (4 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 forests will lead to widespread inhibition of CH₄ uptake

Converting forests into rubber plantations weakened the soil CH4 sink in tropical uplands

“…The oxidation of CH 4 in soils is the only known biotic CH 4 sink, which is approximately three times larger than the latest estimate of the mean net annual CH 4 emission during 2003–2012 (Saunois et al, ). The majority of tropical upland forest soils are net CH 4 sinks (Dalal & Allen, ; Ishizuka et al, ; Veldkamp, Koehler, & Corre, ; Werner et al, ), but converting forest into pastures, cacao agroforestry systems, rubber plantations, and oil palm plantations in humid tropics in Indonesia already showed a tendency of declining CH 4 uptake by soils (Hassler et al, ; Pendall et al, ; Veldkamp, Purbopuspito, Corre, Brumme, & Murdiyarso, ).…”

Humid Subtropical Forests Constitute a Net Methane Source: A Catchment‐Scale Study

“…Yet as net CH 4 fluxes are controlled by both CH 4 oxidation and production (Conrad, ), the smaller CH 4 emissions in the valley bottom may also indicate a greater role of CH 4 oxidation there, as supported by high uptake rates in the drier period 2012–2013. An alternative explanation for the lower CH 4 emission in the valley bottom may be the inhibition of methanogenesis by other electron acceptors such as NO 3 − and SO 4 2− (Conrad, ; Le Mer & Roger, ; Veldkamp et al, ). Along the TSP catchment, the concentrations of NO 3 − and SO 4 2− in soil remain high in the valley bottom (NO 3 − ; Figures h and S3) until significant denitrification (Yu et al, ) and sulfate reduction (Yu, Si, et al, ) take place in the groundwater discharge zone.…”

“…Nutrient availability may be an additional factor affecting methanotrophy in forest soils (Kolb, ). For example, increased N input has been shown to stimulate CH 4 consumption in nitrogen (N)‐limited forest soils (Goldman et al, ; Veldkamp et al, ). However, several studies have pointed out that N enrichment in forest soils inhibits soil CH 4 uptake (Aronson & Helliker, ; Liu & Greaver, ; Steudler et al, ).…”

Canopy soil greenhouse gas dynamics in response to indirect fertilization across an elevation gradient of tropical montane forests

“…, Veldkamp et al . ), but none of those studies included measurements of canopy soil activity. Vance and Nadkarni () and Wardle et al .…”

“…, Veldkamp et al . ), and improve litter quality, increasing CO 2 emissions. Because these canopy soils exhibit conservative N cycling (Matson et al .…”