Fluvial organic carbon fluxes from oil palm plantations on tropical peatland

Sarah Cook,Lip Khoon Kho,Mick J Whelan,Susan E Page,Mike Peacock,Chris D Evans,Yit Arn Teh,Vincent Gauci,Mark H Garnett

doi:10.5194/bg-15-7435-2018

Abstract

Abstract. Intact tropical peatlands are dense long-term stores of carbon. However, the future security of these ecosystems is at risk from land conversion and extensive peatland drainage. This can enhance peat oxidation and convert long-term carbon sinks into significant carbon sources. In Southeast Asia, the largest land use on peatland is for oil palm plantation agriculture. Here, we present the first annual estimate of exported fluvial organic carbon in the drainage waters of four peatland oil palm plantation areas in Sarawak, Malaysia. Total organic carbon (TOC) fluxes from the plantation second- and third-order drains were dominated (91 %) by dissolved organic carbon (DOC) and ranged from 34.4 ± 9.7 C m−2 yr−1 to 57.7 %, 16.3 g C m−2 yr−1 (± 95 % confidence interval). These fluxes represent a single-year survey which was strongly influenced by an El Ninõ event and therefore lower discharge than usual was observed. The magnitude of the flux was found to be influenced by water table depth, with higher TOC fluxes observed from more deeply drained sites. Radiocarbon dating on the DOC component indicated the presence of old (pre-1950s) carbon in all samples collected, with DOC at the most deeply drained site having a mean age of 735 years. Overall, our estimates suggest fluvial TOC contributes ∼ 5 % of total carbon losses from oil palm plantations on peat. Maintenance of high and stable water tables in oil palm plantations appears to be key to minimising TOC losses. This reinforces the importance of considering all carbon loss pathways, rather than just CO2 emissions from the peat surface, in studies of tropical peatland land conversion.

Highlights

Tropical peat carbon stocks are estimated to be 105 Gt C (105 × 1015 g; Dargie et al, 2017), with over half (57 Gt C) stored within the peatlands of Southeast Asia (Page et al, 2011a; Dargie et al, 2017)
4 Discussion Average dissolved organic carbon (DOC) concentrations in water draining the Sabaju oil palm plantations were lower than those reported by Moore et al (2013) for drained tropical peatlands, while those from the Sebungan plantation were in line with the values reported by Moore et al (2013)
The average annual Total organic carbon (TOC) fluxes (Fig. 6) for the third-order and second-order drains were less than TOC flux estimates for drained tropical peat swamp forests reported elsewhere (94 to 108 g C m−2 yr−1; Moore et al, 2013; Müller et al, 2015) and those reported for intact peat swamp forests in Indonesia and Sarawak, Malaysia (63– 64 g C m−2 yr−1), by Moore et al (2013) and Müller et al (2015)

Summary

Introduction

Tropical peat carbon stocks are estimated to be 105 Gt C (105 × 1015 g; Dargie et al, 2017), with over half (57 Gt C) stored within the peatlands of Southeast Asia (Page et al, 2011a; Dargie et al, 2017). Southeast Asian peatlands and tropical peatlands as a whole contain approximately 10 % and 20 % of the global peat carbon stocks, respectively (Page et al, 2011a; Dargie et al, 2017). Only 6 % of remaining peat swamp forest areas are considered pristine (Miettinen et al, 2016), whilst carbon emissions from peatlands converted to agriculture are globally significant and increasing (Wijedasa et al, 2017)

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