Are tropical uplands regional hotspots for methane and nitrous oxide?

Lead Research Organisation: University of Edinburgh


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, we will conduct a comprehensive study of CH4 and N2O cycling in the Peruvian Andes, using a mixture of field measurements, controlled environment studies, and mathematical modelling. Specifically, we will: 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). The proposed research will greatly advance our understanding of CH4 and N2O emissions for an important but understudied region, and will help us 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 will also help us 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 will allow us to predict the likely response of tropical uplands to future environmental or climatic change.


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Description We have made some of the first detailed measurements of trace gas fluxes from soil in tropical upland forests. We show that methane fluxes can be large, and emissions are controlled by oxygen availability/how full the soil is of water. Nitrous oxide emissions are instead controlled by nitrate availablity. On going work will focus on the regional importance of these emissions. A separately funded PhD on methane emissions has recently been awarded; papers are in review from this additional research which did not add cost to this grant. Recent new publications show that many of the forests in this region are net sources of N2O; augmented litter (carbon) input does not affect the fluxes, but the availability of nitrate does. The overall control of environment, water filled pore space in soil, carbon supply and nutrient availability imposes complex controls on overall emission rates. In two recent papers, we show that (i) C and N addition have contrasting effects on carbon mineralisation in soils in montane forests (little effect when C added; negative priming when N added) and lowland forests (positive priming when C added, but no effect of N addition); and (ii) that variability in methane emissions from montane soils was seasonal, and turned out to be best explained by the influence of oxygen concentration on methane production in superficial soil horizons.
Exploitation Route The measured emissions rates and the mechanisms of control of these emissions are of direct use to the earth system modellling community (ie science), and from there into climate-related policy (ie policy and society).
Sectors Education,Environment

Description The findings have/will be used by the earth system modelling community and from there to climate-related policy. The findings have also supported an additional PhD at no cost to the grant (=careers impact), and a successful NERC consortium application (=research impact).
First Year Of Impact 2013
Sector Education,Environment
Impact Types Societal,Economic

Title Soil and gas measurements from Puna grasslands under different land management over one year 
Description The dataset contains concentrations of total soil organic carbon, soil carbon fractions, soil CO2 fluxes, soil temperature and moisture in the Peruvian Andes. Measurements and sampling took place between 2010 and 2013. Data were generated as part of a larger NERC project: 'Are tropical uplands regional hotspots for methane and nitrous oxide' 
Type Of Material Database/Collection of data 
Year Produced 2017 
Provided To Others? Yes