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Canal networks regulate aquatic losses of carbon from degraded tropical peatlands

Nature Geoscience volume 17, pages 213–218 (2024)Cite this article

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Abstract

Southeast Asian peatlands cover 0.2% of Earth’s land surface, but store one-tenth of all peat soil carbon globally. Recent deforestation and drainage have destabilized these carbon stores, increasing carbon inputs to aquatic and atmospheric reservoirs. Here we investigate the impact of anthropogenic disturbance on the aquatic fate of peat dissolved organic carbon (DOC) within networks of drainage canals overlying disturbed peatlands. We measured microbial respiration rates alongside photochemical mineralization of DOC for canal waters collected across West Kalimantan, Indonesia, and found that both pathways lead to rapid DOC oxidation to carbon dioxide in the water column. Carrying out a systematic assessment of the controls on peat DOC processing, we identify key variables needed to predict daily rates and show that DOC oxidation may range from 15 to 310 mgC m−2 d−1 in drainage canals across Southeast Asia, depending on the water chemistry, hydrology and meteorology on any given day. DOC oxidation averaged 70 mgC m−2 d−1 under typical conditions, indicating that this process may reduce canal export of peat DOC by ~35%. Findings from this study demonstrate that drainage canal networks are a hotspot for terrestrial carbon loss following land disturbance and strongly regulate aquatic loss of peat carbon across the landscape.

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Fig. 1: DOC oxidation rates were scaled across drainage canals, which cover the peatland landscape in Southeast Asia.
Fig. 2: Microbial respiration and photomineralization of tropical peat DOC.
Fig. 3: Mechanistic controls on DOC lability.
Fig. 4: Photomineralization depends on changing photon flux and DOC lability throughout the day.
Fig. 5: Key drivers of DOC oxidation in Southeast Asian drainage canals.

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Data availability

All data are available in the paper or the Supplementary Information. The datasets generated during the current study are available at the Zenodo repository under ‘Water properties and DOC oxidation rates in Southeast Asian drainage canals’ (https://doi.org/10.5281/zenodo.10557638).

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Acknowledgements

We thank E. Walker, M. Durazo, G. Randa, S. Perry, T. Nguyen, R. Rohim and V. Calistha for field and laboratory assistance. We thank N. Novita (The Nature Conservancy) for access to a spectrophotometer, T. Martz (Scripps Institution of Oceanography) for access to a DIC analyser and the AsiaFlux Palangkaraya drained forest station for public access to photosynthetically active radiation data. Research was supported by the Scripps Institution of Oceanography Postdoctoral Scholarship (J.C.B.) and by the Precourt Institute for Energy (A.M.H.).

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Authors and Affiliations

  1. Geosciences Research Division, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA, USA

    Jennifer C. Bowen & Lihini I. Aluwihare

  2. Department of Earth System Science, Stanford University, Stanford, CA, USA

    Jennifer C. Bowen & Alison M. Hoyt

  3. Department of Soil Science, Universitas Tanjungpura, Pontianak, Indonesia

    Putri J. Wahyudio & Gusti Z. Anshari

  4. Magister of Environmental Science, Universitas Tanjungpura, Pontianak, Indonesia

    Gusti Z. Anshari

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Contributions

J.C.B. and P.J.W. carried out sample collection, experimental work and data analysis. J.C.B. wrote the paper. J.C.B., G.Z.A., L.I.A. and A.M.H. contributed to the study design and paper revisions.

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Correspondence to Jennifer C. Bowen.

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Extended data

Extended Data Fig. 1 DOC oxidation rates were quantified under optimal laboratory conditions and then scaled in the water column of each canal studied in West Kalimantan.

To quantify the DOC oxidation rates under optimal conditions, the microbial incubations and sunlight exposure experiments were carried out over timescales that followed water residence times across drainage canals (Supplementary Section 1) and where enough dissolved O2 was available to support the DOC oxidation measured (Supplementary Section 2.1). Microbial respiration rates under optimal conditions were quantified using the concentration of DIC production measured and the incubation time (solid green arrow; see Methods). The vial pathlength, loss of aCDOM,λ in each water sample, and the photon flux reaching the canal water during each sunlight exposure experiment (dashed orange arrows) were used to quantify the optimal rates of DOC photomineralization (see Methods, Eqs. 1–2). These optimal laboratory-obtained rates were then scaled to water column rates in each canal based on approximate dissolved O2 availability in the water column (Supplementary Section 2.2), the water depth of each canal (Extended Data Table 1), the fluxes of ultraviolet and visible photons reaching canal waters on the day of the experiment (Extended Data Fig. 3), and the aCDOM,λ present in each canal to absorb those photons.

Extended Data Fig. 2 Approximate water residence times for four canals varying in size throughout Southeast Asia.

(a) Maximum water residence times for the Meranti ditch (light blue), Tugangnusa canal (blue), Badas canal (dark blue), and Kalampangan canal (black) were estimated using the (b) width, depth, length, and daily discharges reported in past studies2,27,28. (b) The annual rainfall and average daily discharge were lowest for the Meranti ditch27. The residence time of water in each canal when maximum residence times were less than 12 hours (shaded in gray) are plotted in (c). Maximum residence times in the Kalampangan canal consistently exceeded ~1 day2 and thus, are not shown in (c). Data in (a) are plotted on the y-axis as log10 values of maximum residence time (days). Box plots in (c) show the median and interquartile range, with x showing the mean and whiskers showing the upper and lower limits. The upper limit was calculated as the third quartile plus the interquartile range multiplied by 1.5 and the lower limit was calculated as the first quartile minus the interquartile range multiplied by 1.5.

Extended Data Fig. 3 Daily fluxes of ultraviolet and visible photons reaching West Kalimantan.

(a) The photon doses reaching West Kalimantan (280–700 nm) from 6 April 2022 to 23 January 2023 (solid line) were at least 20% lower than the dose modeled under clear sky conditions (dashed line; modeled with SMARTS). (b) Photon doses reaching West Kalimantan were close to the modeled clear sky conditions on 28 April 2022. When photon doses were measured by radiometry (teal triangles; error bars are smaller than symbols, ± 1 s.e.m. of replicate measurements, n = 4), they were in close agreement with those estimated from pyranometry (solid line) between 6:00 and 18:00. (c) The maximum daily photon flux spectrum (dark orange) plotted alongside the average (light orange) and minimum (light gray) photon flux spectra show how ultraviolet and visible photon doses varied 4.2- and 4.1-fold across time, respectively. (d) When these light wavelengths reach the surface of drainage canals, the total photon flux (integrated across wavelengths) is absorbed within 0.2 to 1.0 m depths in the water column (average of 0.45 m). The maximum percent of the daily photon flux reaching each canal depth was calculated using the minimum aCDOM and maximum photon flux spectra, whereas the minimum percentage was calculated using the maximum aCDOM and minimum photon flux spectra (Eq. 1).

Extended Data Fig. 4 DOC aromaticity and pH influence microbial respiration across peatland streams globally.

(a) The percentages of DOC oxidized to CO2 during microbial incubations per day were significantly lower in Southeast Asian drainage canals (green) compared to peatland streams or canals at other latitudes (white; p = 0.0003 from a two-tailed paired t-test, n = 20 and 38, respectively; Extended Data Table 2). These differences in microbial DOC lability could only be attributed to the (b) significantly lower water pH (p < 0.0001 from a two-tailed paired t-test, n = 19 and 11, respectively; Extended Data Table 2) and (c) the significantly higher aromaticity of Southeast Asian peat DOC, as measured by the specific ultraviolet absorbance at 254 nm (SUVA254; p < 0.0001 from a two-tailed paired t-test, n = 20 and 38, respectively; Extended Data Table 2). Given that the capacity for microbes to degrade and respire aromatic-containing DOC compounds depends strongly on pH39,40,41, DOC aromaticity and pH are likely major controls on the magnitude of respiration rates. Box plots in (a-c) show the median and interquartile range, with x showing the mean and whiskers showing the upper and lower limits (see Methods).

Extended Data Fig. 5 Microbial respiration rates measured after 3-day and 7-day incubations of canal water.

In eight of the drainage canals sampled in West Kalimantan (Fig. 1), 3-day (light green bars) and 7-day (dark green bars) incubations were carried out alongside each other to test whether a ~ 7-day incubation time used for the rest of the drainage canals studied was representative of rates taking place over shorter timescales (Supplementary Section 1.2). (a) Microbial respiration rates obtained during 7-day incubations were higher, the same, or lower than those obtained following 3-day incubations of the same canal water DOC with native microbes. In two drainage canals with the lowest respiration rates measured, microbial respiration rates were significantly higher following 7 days compared to 3 days (p = 0.0007 and p = 0.0006, respectively from two-tailed paired t-tests). In two drainage canals with the highest respiration rates measured, rates were significantly lower following 7 days compared to 3 days (p = 0.0002 and p = 0.05, respectively from two-tailed paired t-tests). (b-c) Rates are reported as the average ± 1 s.e.m. of replicate experimental vials (n = 4 unless otherwise indicated). The white bar on the far left of panel (a) and (b) indicates a microbial respiration rate that was not significantly different from zero (p = 0.06 from a two-tailed paired t-test). Box plots in (b-c) show the median and interquartile range of the replicate rate measurements for each canal water, with x showing the mean and whiskers showing the upper and lower limits when higher or lower than the interquartile range, respectively. The upper limit was calculated as the third quartile plus the interquartile range multiplied by 1.5 and the lower limit was calculated as the first quartile minus the interquartile range multiplied by 1.5.

Extended Data Fig. 6 Scale factors used to estimate in situ canal respiration rates.

Two different scale factors were used to translate laboratory-derived microbial respiration rates to in situ rates in the canals: a scenario where microbial respiration is not limited by available O2 in the water column (pink dashed line) and a scenario where respiration rates become limited as O2 becomes depleted with canal water depth (red dashed line). We assume that 100% on the x-axis is equivalent to the rates of aerobic microbial respiration measured during the laboratory incubations under optimal O2 concentrations (Supplementary Table 1). Decreasing percentages on the x-axis indicate a mixture of aerobic and anaerobic respiration, where anaerobic respiration rates are assumed to be negligible compared to aerobic respiration rates (Supplementary Section 2.3). Gray symbols show the percent of dissolved O2 measured at each water column depth relative to surface concentrations previously reported in Gandois et al. (2020) (average ± 1 standard deviation of replicate field measurements; n = 1 at a 20-cm depth, n = 4 at 0.5-, 15-, and 40-cm depths, n = 5 at a 110-cm depth, n = 6 at 1-, 3-, and 5-cm depths, n = 7 at a 10-cm depth, and n = 11 at a 25-cm depth)56, whereas the red dashed line shows the fit with a least-squares exponential model. We assume that these previous measurements of O2 depletion with depth in a 2-m blackriver in West Kalimantan is similar to drainage canals in the region because it had similar water chemistry (dissolved O2 = 1.9 mg L−1, DOC = 3000 µM, SUVA254 = 6.0 L mg−1 C m−1)56 as the water chemistry reported in this study and previous studies for Southeast Asian drainage canals (Extended Data Table 1). In situ microbial respiration rates in each of the canals were estimated assuming that dissolved O2 becomes depleted with water column depth half of the time on average (red dashed line; for example, during the dry season or during the day) and for the rest of the time, dissolved O2 concentrations are homogeneous with depth (light pink dashed line; for example, during the wet season or at night; Supplementary Section 2.3).

Extended Data Table 1 Water and DOC chemistries of drainage canals across Southeast Asian peatlands
Full size table
Extended Data Table 2 Microbial respiration rates for DOC in streams, ditches, and canals overlying peatland soils globally
Full size table
Extended Data Table 3 Sunlight exposure times, photon doses, and amounts of light absorbed by CDOM during each sunlight exposure experiments to quantify apparent quantum yields for DOC photomineralization
Full size table
Extended Data Table 4 Physical and chemical conditions of drainage canals used to estimate the range of potential daily DOC oxidation rates across Southeast Asia
Full size table

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Supplementary Discussion and Table 1.

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Bowen, J.C., Wahyudio, P.J., Anshari, G.Z. et al. Canal networks regulate aquatic losses of carbon from degraded tropical peatlands. Nat. Geosci. 17, 213–218 (2024). https://doi.org/10.1038/s41561-024-01383-8

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