Organic matter biogeochemistry of tropical rainforest rivers


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).


The project will focus on identifying key OM pools using the latest liquid chromatography organic carbon and nitrogen detection systems (LC-OCD-OND; Huber et al., 2011) and ramped/oxidation molecular sieve (ROMS) radiocarbon preparation system to explore OM transformations under changing environmental conditions using the climate-controlled facilities at the Lyell Centre ( Newcastle University will support the student to construct a new empirical model simulating the relationships between OM pools across the terrestrial-aquatic interface using existing samples and new data to be obtained from hill-slope transect within the Iwokrama forest. The SHEtran hydrological transport model will simulate the newly derived relationships between labile and recalcitrant pools of OM and test the implications of changing climate regimes on rain events at increasing (spatial and temporal) scales. New, quantitative understanding from river and hillslope experimental studies will be used to assess the effect of climatic variability from storm to seasonal/multi-annual timescales on riverine CO2 outgassing across spatial scales.

Project Timeline

Year 1

• Carry out literature review of headwater river carbon dynamics in tropical rainforests
• Construct initial SHEtran model to assess current weaknesses in our understanding
• Meet both supervisors to finalise PhD objectives and plan fieldwork for Year 2
• Identify training needs to complete ongoing workplan
• Receive training in relevant dissolved and particulate OM analytical methods
• Set up initial OM climate-controlled laboratory experiments
• Submit NEIF Radiocarbon proposal for 14C sample analysis support

Year 2

• Analyse the results on year 1 climate-controlled experiments
• Complete hillslope transects in Iwokrama Rainforest with support from IIC
• Continue new experiments for OM transformations
• Analyse samples for 14C and receive training support
• Meet supervisors to assess progress and plan for year 3.
• Input new results into SHEtran model
• Present results at national conference

Year 3

• Participate in additional fieldwork if necessary
• Complete outstanding experiments
• Run climate simulations of SHEtran model during extended visit to Newcastle University
• Make substantial progress towards thesis preparation and define a detailed timeline to ensure completion with 3.5 years
• Present results at international conference
• Prepare first publication

Year 3.5

• Complete and submit thesis
• Prepare additional data for publication

& Skills

The supervisors and IIC will provide full training in their individual expertise areas, enabling the student to combine current best practice and develop the skills necessary to advance this science. Supervision will be provided jointly by Heriot-Watt (RP) and Newcastle (GP) universities through an ongoing collaboration. Directly relevant training will be provided by both institutes to equip the student with the skills necessary for all aspects of the project. IAPETUS fosters a strong sense of “community” that encourages students to organise a range of activities (e.g. annual conference) and identify additional training needs to be addressed via tailored opportunities. The student will have opportunities for additional training at the partner institution. The student should develop strong collaborations that will potentially result in further opportunities.

References & further reading

Raymond et al (2013) Nature 503, 355 – 359; Regnier et al (2013) Nat. Geosci. 6, 597 – 607; Mayorga et al (2005) Nature 436, 538-541; Malhi & Grace (2000) Trends Ecol Evol 15, 332-337; Meybeck (2006) Origins and behaviours of C species in world rivers. Balch et al (2008) GCB 14, 2276-2287; Nepstad et al (2008) Phil. Trans. Roy. Soc. B 363, 1737-1746; Tranvik et al (2009) L&O 54, 2298-2314; Le Quéré et al (2009) Nat. Geosci. 2, 831 – 836; Huber et al (2011) Wat. Res. 45, 879-885; Ewen et al (2000) ASCE J. Hyd. Eng., 5, 250-258; Birkinshaw & Ewen (2000) J. Hydrol. 230, 1-17.

Further Information

Dr Ryan Pereira,
The Lyell Centre, Institute of Life and earth Sciences, School of Energy, Geoscience and Society, Heriot Watt University, EH14 4AS.
Telephone: 0131 451 3537

Dr Geoff Parkin,
School of Engineering, Newcastle University, NE1 7RU.
Telephone: 0191 208 6146

Dr Mark Garnett,
NEIF Radiocarbon Laboratory, Rankine Avenue, East Kilbride, G75 0QF, United Kingdom
Tel: 01355 270024

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