High CH4 and N2O emissions from soil and stems of disturbed swamp forests in Peruvian Amazon

Peatlands are an enormous sink of carbon and nitrogen. Natural and human disturbances may release them as greenhouse gases (GHGs) or water pollutants. Tropical peatlands have especially intensive matter cycling. Amazonia holds almost a half of tropical peatlands. Most of it is inaccessible to current forestry and drainage machinery and thus untouched by man. Tropical rainforest has been labelled ’lungs of the Earth’. While photosynthesis in mature forests does sequester carbon in biomass, they respire an equal amount of carbon dioxide (CO₂). Only swamp forests may sequester carbon in wet anoxic peat for centuries. However, anoxic decomposition of peat yields methane (CH₄) and suboxic processes release nitrous oxide (N₂O). Both have high global warming potential. In undisturbed peatlands, carbon sequestration outweighs GHG emissions. GHG budgets are more complicated in disturbed peatlands.

With an objective to clarify the greenhouse gas budget of tropical peatlands, the Department of Geography, University of Tartu held a measurement campaign in Iquitos, Peruvian Amazon in September 2019. We observed fluxes of the three GHGs using opaque chambers and measured potential environmental factors in three sites under various disturbance histories: 1) a Mauritia flexuosa palm-dominated swamp forest, 2) toe-slope swamp forest grown in 12 years on fallow pasture and banana plantation, and 3) slash-and-burn cassava field.

The toe-slope swamp respired the largest amounts of CO₂ while site differences were small and may have been offset by photosynthesis (which we did not measure). The wet swamp forest sites, especially palm trunks, emitted large amounts of CH₄. The dry slash-and-burn cassava field emitted little methane. The CH₄ emissions were strongly correlated with nitrogen content of the peat. Previous literature links high soil nitrogen content with lability of soil organic carbon and high microbial activity. The swamp forest floor emitted an average of 390 µg N₂O-N m^–2 h^–1 after torrential rainfall. The downpour may have carried just enough oxygen into the peat to trigger N₂O production by nitrification or hamper the full pathway of denitrification to N₂. High peat Ca⁺⁺ and Mg⁺⁺ content and pH>4 favoured nitrification. High NH₄⁺-N concentration in the swamp peat (190 mg kg^–1), which can be related to N₂ fixation and litter from three species of leguminous trees, formed a solid base for nitrification. The slash-and-burn cassava field emitted a sizable 37 µg N₂O-N m^–2 h^–1. In conclusion, the variety of disturbances produced an interesting pattern of GHG emissions in relationship with environmental conditions. Thus, Amazonian peatlands demand elevated attention.