Tropical forests are one of the most important biomes on Earth, providing a variety of key resources and ecosystem services, and supporting ~50% of all species worldwide (Mahli & Grace, 2000). These forests have a greater impact on riverine carbon and nutrient transport to the ocean than any other biome (Meybeck, 2006) and they are extremely sensitive to changing climate and land use (Balch et al., 2008; Nepsted et al., 2008). Globally, river networks annually receive an estimated 2.9 Petagrams (Pg) carbon from the terrestrial environment, but only deliver around one third of this to the World’s oceans (Tranvik et al., 2009). This leaves about 2 Pg of carbon per year to be recycled within rivers, which is similar to current estimates of carbon emissions from tropical deforestation (Le Quéré et al, 2009). While estimates of inland water carbon cycling are challenging to reconcile, current estimates suggest that streams and small rivers annually emit 1.8 Pg carbon (+/-0.25; Raymond et al., 2013). This CO2 potential is remarkable – equivalent to ~18 % of fossil fuel emissions (Regnier et al., 2013) – and justifies a major effort to better quantify and understand the role of headwater streams, which account for 70-80 % of the total river network on only 20% of the global land surface (Raymond et al., 2013).
Tropical headwater rivers represent a true “hotspot” of biogeochemical cycling. However, there are large uncertainties in tropical headwater regions due to a lack of empirical data and a poor representation of the processes that control carbon supply to the river and subsequent riverine outgassing. New evidence from our research in central Guyana, northern Amazonia, suggests that tropical rain events are fundamental for carbon mobilisation, recycling, and potentially CO2 emissions along the land-ocean river continuum. Critically, the carbon mobilised from these headwater rivers is optically ‘invisible’ to commonly used monitoring equipment and has the potential to be highly labile, and thus accessible, for microbial and photochemical remineralisation (Pereira et al., 2014).
This PhD studentship will investigate the uncertainties of conditions in which rainforests act as a carbon source or sink in response to extreme weather events (e.g. droughts and floods¬). A focus will be to unravel the key processes that primarily affect carbon storage and/or mobilisation at the terrestrial-aquatic interface in both particulate and dissolved phases (e.g. mineral interaction with complex organic compounds within the soil, microbiological turnover, and photo-oxidation after mobilisation). In combination, these processes determine the storage and mobilisation of carbon in soil and river systems, with the mineralisation of organic matter (OM) contributing to the outgassing of CO2 into the atmosphere (Mayorga et al., 2015).
Based in Edinburgh, Scotland at Heriot-Watt University, the student will undertake research visits to Newcastle University, NEIF Radiocarbon Laboratory in East Kilbride, Glasgow, and Iwokrama International Centre for Rainforest Conservation and Management (IIC), Guyana in South America. Under supervision the student will lead experimental design and execution, and be responsible for collecting new empirical data to test linkages of OM transformation in the SHEtran hydrological model (Ewen et al 2000; Birkinshaw & Ewen 2000).