The greenhouse gas budget of a tropical ecosystem – has the global carbon sink been underestimated?

Biogeochemical Cycles

IAP2-19-023

Overview

When the Inter-governmental panel on climate change (IPCC) calculates the global terrestrial carbon budget it has to assume a value of the oxidation state of the terrestrial biosphere’s organic matter (this value is called the oxidative ratio – OR – Masiello et al., 2008). The value of OR that the IPCC uses is 1.1, but Worrall et al. (2013) has pointed out that this value was based on only one measurement and this value of OR was too high. A value of OR that is too high has meant that the IPCC has underestimated the sink of carbon in to the terrestrial biosphere and this overestimation of the value of OR can be seen, for example, in the necessary retraction of a recent Nature paper (Resplandy et al., 2018) where the IPCC’s value of OR had been used. Subsequently, Worrall et al. (2018) considered the greenhouse gas budget of a peatland and by tracking the macromolecular composition (lignin, carbohydrates and proteins) through the carbon cycle of the peatland it was possible to show that the value of OR was dominated by the carbohydrates and so OR for this ecosystem was as low 0.98 and could be even lower. This approach has only been tested in a peatland and so it needs to be tested in other terrestrial environments. The ideal test environment would be a terrestrial ecosystem rich in organic matter and other than peatlands the tropical rainforests are the most organic-rich environments in the terrestrial biosphere due to their high primary productivity. Tropical environments contrast from peatlands as the carbon is stored aboveground rather than in the soil as in peatlands.

The underlying hypotheses of this study are that:
• the interaction between the tropical biosphere and the atmosphere is controlled by oxidised forms of carbon leaving more reduced carbon in the terrestrial biosphere;
• the carbon cycle of the tropical ecosystem is controlled by preferential loss of carbohydrates and preferential retention of lignin derivatives;
• the OR experienced by the atmosphere from tropical forest is less than 1 because the tropical rainforest preferentially turns over carbohydrates; and
• the current estimates of the carbon sink in the terrestrial biosphere has been underestimated because OR has been misunderstood.

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Image Captions

Figure 1. Canopy walkway at La Selva Biological Station, Costa Rica.

Methodology

The project will work at the La Selva Biological Station in Costa Rica. The La Selva Biological Station is lowland tropical rain forest, which includes a number of catchments, and does not contain peat soils. Furthermore, ongoing and published work from La Selva means that there is a carbon budget for the site which can be used as a framework for understanding the transition of organic matter through and out of the tropical ecosystem. Therefore, the La Selva Biological Station is the ideal location to test our hypotheses.

The study will take several approaches to test the hypotheses:
– Add detail and maintain estimates of the carbon budget of the La Selva. The carbon budget is the framework by which we can assess the relative mass transfer of different types of macromolecules.
– Install gas collars at La Selva so that the CO2 and O2 exchange can be measured using the modified EGM-5 infra-red gas analyser (IRGA). The ratio of the CO2 exchange to the O2 exchange is the OR and so this represents a direct method for measuring OR.
– Analyse the elemental composition (C, H, N and O) of the major organic matter reservoirs and fluxes. The organic matter reservoirs and fluxes to be considered are: aboveground biomass, belowground biomass, soil profile, and the fluvial fluxes of organic matter (including both dissolved organic matter, DOM, and particulate organic matter, POM). In Durham, we have developed field methods for processing river water to extract sufficient mass of DOM and POM. From elemental composition it is easy to calculate both the oxidation state of the organic matter and its OR using the approach of Masiello et al. (2008).
– Use of calorimetry to assess the heat of formation of the sampled organic matter as a means of providing an alternative measure of the oxidation state. This allows for a range of thermodynamic properties of the organic matter to be derived.
– Measure the macromolecular composition of the organic matter by means of thermogravimetric analysis, which means that the lignin, carbohydrate and protein content of all samples can be analysed. Worrall et al. (2017) showed that if the macromolecular composition of the major organic matter reservoirs and fluxes can be measured within the context of an ecosystems carbon budget then the composition of the components lost through processing in the ecosystem can be reconstructed.
– The global OR values can be calculated based on flux and stock-weighted approaches as shown in Clay et al. (2018).

Project Timeline

Year 1

i) Literature review
ii) Training in analytical and statistical techniques
iii) Preparation and planning for fieldwork in Costa Rica
iv) Fieldwork at La Selva (typically expected in May-June when climatic conditions are normally most favourable but before US College summer field season)

Year 2

i) Analysis of collected samples by elemental analysis, bomb calorimetry, and thermogravimetric analysis
ii) Assessment of the La Selva carbon budget
iii) Develop testable hypotheses from results for the second season of fieldwork
iv) Second field season at La Selva to test hypothesis developed from analysis of first season and direct measurement of OR by closed chamber methods.

Year 3

i) Analysis of second phase of collected samples by the above methods
ii) Present results at European Geosciences Union General Assembly in Vienna
iii) Assess results in the model of global OR and so understand the magnitude of the terrestrial carbon sink.

Year 3.5

i) Finish and submit thesis
ii) Write papers based on results

Training
& Skills

The studentship will involve full training in the necessary field, laboratory and data analysis techniques. The field techniques include the use of infra-gas analysis with O2 electrodes; systematic sampling such techniques; and total greenhouse gas budget estimation. Laboratory analysis will include: thermogravimetric analysis, elemental analysis; and calorimetry.

References & further reading

Clay, G.D., Worrall, F., Plummer, R., C.S. Moody (2018). Organic matter properties of Fennoscandian ecosystems: Potential oxidation of northern environments under future change? Science of the Total Environment, 610-611, 1496 – 1504.
Masiello CA, Gallagher ME, Randerson JT, Deco RM, Chadwick OA (2008) Evaluating two experimental approaches for measuring ecosystem carbon oxidation state and oxidative ratio. Journal of Geophysical Research – Biogeosciences 113: G3, G03010.
Resplandy, L., Keeling, R.F., Eddebbar, Y., Brooks, M.K., Wang, R., Bopp, L., Long, M.C., Dunne, J.P., Koeve, W, and A.Oschlies, (2018). Quantification of ocean heat uptake from changes in atmospheric O2 and CO2 composition. Nature 563, 7729, 105 – 108.
Worrall, F., Clay, G.D., Masiello, C.A. & Mynheer, G. 2013. Estimating the oxidative ratio of the global terrestrial biosphere carbon. Biogeochemistry, 115, 23-32.
Worrall, F., Clay, G.D., Moody, C.S., Burt, T.P. & Rose, R. 2017. The flux of organic matter through a peatland ecosystem – the role of cellulose, lignin and their control of the oxidation state. Journal of Geophysical Research: Biogeosciences, 122, 1655-167.

Further Information

Fred Worrall, Fred.Worrall@durham.ac.uk; 0191 334 2295

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