NERC project: Are tropical uplands regional hotspots for methane and nitrous oxide?
This project was funded by the Natural Environment Research Council (NERC) with the following grant references; NE/H007849/1, NE/H006753/1 and NE/H006583/2. These grants were led by Professor Pete Smith, Professor Patrick Meir and Dr Yit Arn Teh respectively.
Tropical ecosystems are major sources of the greenhouse gases (GHGs) methane (CH4) and nitrous oxide (N2O), which are 25 and 298 times more effective than carbon dioxide (CO2), respectively, in trapping long-wave radiation in the atmosphere. Increases in CH4 and N2O concentrations since the start of the Industrial Revolution are responsible for over one-third of global warming, and future changes in the atmospheric budgets of these GHGs have implications for the Earth's climate and environmental conditions. N2O emissions, in particular, are projected to rise in the future due to agricultural expansion and enhanced atmospheric nitrogen deposition.
Recent studies of the global budgets of CH4 and N2O using satellite imagery, atmospheric measurements, and modelling suggest that significantly more CH4 and N2O are released from the tropics than previously thought due to unaccounted sources of CH4 and N2O. It is critical for us to identify and characterise these 'missing' sources if we wish to understand the current contribution of the tropics to GHG budgets. Knowledge of these 'missing' sources is also necessary for predicting how tropical GHG emissions are likely to respond to future environmental or climatic change. One strong potential candidate for these 'missing' sources of CH4 and N2O are tropical uplands. Tropical uplands have been conspicuously absent from existing atmospheric budgets, because scientific attention has largely focused on CH4 and N2O emissions from lowland forests, savannas, or wetlands. Studies from tropical uplands suggest that they are potentially large sources of CH4 and N2O, with emissions that are equal to or greater than those from lowland environments. Upland rainforests in Puerto Rico, for example, emit more CH4 than lowland forests, with emission rates that are on par with northern wetlands, the largest natural sources of CH4 worldwide.
To address these gaps in knowledge, a comprehensive study of CH4 and N2O cycling in the Peruvian Andes, using a mixture of field measurements, controlled environment studies, and mathematical modelling was undertaken.
The projects aims were to:
1. Investigate how CH4 and N2O fluxes vary in space and time along an environmental gradient that spans 3000 m of altitude, from lowland rainforest to upper montane rainforest.
2. Explore how key environmental variables (e.g., plant productivity, climate, soil moisture, carbon and nitrogen availability, oxygen) influence CH4 and N2O emissions.
3. Determine if existing mathematical models are able to simulate CH4 and N2O emissions from tropical ecosystems, adapting these models as necessary to better simulate field observations.
4. Determine if GHG emissions from the Andes are able to account for some of the 'missing' tropical sources of CH4 and N2O by extrapolating our field observations to the regional scale using a combination of mathematical modelling, satellite imagery, and land cover databases (i.e., GIS).
This research greatly advanced our understanding of CH4 and N2O emissions for an important but understudied region, and helped to determine the relative contribution of Andean ecosystems to the CH4 and N2O budgets for South America. Knowledge of the emission rates and environmental controls on CH4 and N2O fluxes from upland Andean ecosystems also helped evaluate whether other tropical uplands are likely to be sources of CH4 and N2O, and assess their potential contributions to the global atmospheric budgets of CH4 and N2O. Lastly, the development and adaptation of mathematical models that accurately simulate tropical CH4 and N2O fluxes allowed the project to predict the likely response of tropical uplands to future environmental or climatic change.
The goal of this research was to use the Andes as a model system for understanding the rates, spatial and temporal patterns, and environmental controls on methane (CH4) and nitrous oxide (N2O) fluxes in upland tropical environments. They aimed to quantify CH4 and N2O fluxes along a 3000 m environmental gradient in the Andes, spanning lowland rainforest (220 masl) to upper montane rainforest (3000 masl).
Empirical data were used to parameterise or adapt models to extrapolate CH4 and N2O fluxes to larger spatial scales, and to predict the response of trace gas fluxes to environmental change.
The project also aimed to evaluate the importance of upland Andean ecosystems as regional sources of CH4 and N2O, exploring the extent to which CH4 and N2O emissions from these environments account for discrepancies in regional CH4 and N2O budgets. They used an integrated, multiscale approach, combining field measurements, controlled environment studies, modelling, and spatially weighted upscaling.
Specifically, objectives include:
1. Determine spatial and temporal patterns of CH4 and N2O exchange from key ecosystems along an altitudinal gradient in the Andes.
2. Investigate how natural variations in key driving variables (plant productivity, climate, soil moisture, soil C, soil N, and oxygen) influence rates of CH4 and N2O exchange using field observations and controlled environment studies.
3. Parameterise or adapt existing models (ECOSSE, DNDC) with empirical data from a lowland to upland gradient in order to accurately simulate CH4 and N2O flux processes in tropical ecosystems.
4. Use a combination of modelling and remote sensing imagery to upscale our observations to evaluate the importance of Andean uplands as regional sources of CH4 and N2O.