Mangrove forests are hot spots in the global carbon cycle, yet the fate for a majority of mangrove net primary production remains unaccounted for. The relative proportions of alkalinity and dissolved ...CO2 CO2* within the dissolved inorganic carbon (DIC) exported from mangroves is unknown, and therefore, the effect of mangrove DIC exports on coastal acidification remains unconstrained. Here we measured dissolved inorganic carbon parameters over complete tidal and diel cycles in six pristine mangrove tidal creeks covering a 26° latitudinal gradient in Australia and calculated the exchange of DIC, alkalinity, and CO2* between mangroves and the coastal ocean. We found a mean DIC export of 59 mmol m−2 d−1 across the six systems, ranging from import of 97 mmol m−2 d−1 to an export of 85 mmol m−2 d−1. If the Australian transect is representative of global mangroves, upscaling our estimates would result in global DIC exports of 3.6 ± 1.1 Tmol C yr−1, which accounts for approximately one third of the previously unaccounted for mangrove carbon sink. Alkalinity exchange ranged between an import of 1.2 mmol m−2 d−1 and an export of 117 mmol m−2 d−1 with an estimated global export of 4.2 ± 1.3 Tmol yr−1. A net import of free CO2 was estimated (−11.4 ± 14.8 mmol m−2 d−1) and was equivalent to approximately one third of the air‐water CO2 flux (33.1 ± 6.3 mmol m−2 d−1). Overall, the effect of DIC and alkalinity exports created a measurable localized increase in coastal ocean pH. Therefore, mangroves may partially counteract coastal acidification in adjacent tropical waters.
Key Points
DIC, alkalinity, and CO2 fluxes from mangroves were determined across a latitudinal gradient
Global DIC and alkalinity export were estimated at 3.6 Tmol yr‐1 and 4.2 Tmol yr‐1, respectively
Alkalinity exports from mangroves can increase coastal ocean pH
The blue carbon paradigm has evolved in recognition of the high carbon storage and sequestration potential of mangrove, saltmarsh and seagrass ecosystems. However, fluxes of the potent greenhouse ...gases CH4 and N2O, and lateral export of carbon are often overlooked within the blue carbon framework. Here, we show that the export of dissolved inorganic carbon (DIC) and alkalinity is approximately 1.7 times higher than burial as a long-term carbon sink in a subtropical mangrove system. Fluxes of methane offset burial by approximately 6%, while the nitrous oxide sink was approximately 0.5% of burial. Export of dissolved organic carbon and particulate organic carbon to the coastal zone is also significant and combined may account for an atmospheric carbon sink similar to burial. Our results indicate that the export of DIC and alkalinity results in a long-term atmospheric carbon sink and should be incorporated into the blue carbon paradigm when assessing the role of these habitats in sequestering carbon and mitigating climate change.
•Urban land significantly elevates dCH4, FCH4 and nutrient levels.•Fine-grained riverbed increases dCH4 concentration when compared with boulder/cobble channels.•River waters at all 27 sample sites ...are oversaturated in CH4 relative to the atmosphere.•dCH4 is significantly related to nutrients, Chl-a, dCO2, stream flow velocity, pH and DO.•dCH4 can be predicted with urbanization intensity and water quality variables.
Rivers and streams play a central role in global carbon budget, but our knowledge is limited on the magnitude and extent of urbanization influence on riverine methane (CH4) dynamics. In this study, we investigated dissolved CH4 (dCH4) concentration and CH4 diffusive fluxes in 27 river segments of two 4th-order and three 3rd-order tributary rivers to the Yangtze River in China, which drained land areas with varied urbanization intensities. We found that urban development was the key factor responsible for high fluvial dCH4 concentration and diffusive flux, exceeding the influence of agricultural farming, and these headwater rivers were over-saturated in CH4 with respect to atmospheric equilibrium. dCH4 concentration (3546 ± 6770 nmol L−1) in the river segments draining higher urban area (20% ≤ urban land proportion ≤ 46%) was 5-6 times higher than those (615 ± 627 nmol L−1 and 764 ± 708 nmol L−1) in the river segments draining less urban area (0.1% ≤ urban land proportion < 2% and 2 ≤ urban land proportion < 20%). River segments draining higher urban area also acted as important sources of CH4 to the atmosphere (8.93 ± 14.29 mmol m−2 d−1). Total nitrogen (TN) concentration in river water showed the best prediction capacity when compared to other water parameters. Based on urban land use grouping, nutrient elements could predict dCH4 well in rivers draining higher urban areas (urban ≥ 2%), which also reflected the lateral input of pollutants (TN, ammonia nitrogen, and total phosphorus). River bottom sediment fraction contributed to trapping organic matter and nutrients as well as to oxic and anoxic conditions, thereby determining reach-scale spatial patterns of dCH4 concentration. These findings highlight that combining distal geomorphic and hydrologic drivers can be effective in determining the relationship between riverine CH4 and the proximal controls (e.g., nutrients, dissolved oxygen, dissolved organic carbon), as well as in identifying their key drivers. Being rapid urbanization a common feature of catchments worldwide, our results suggest riverine CH4 emissions will increase into the future.
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Mangrove- and saltmarsh-dominated estuaries have high rates of organic carbon burial. Here, we estimate soil, pore water, and surface-water carbon fluxes in an Australian estuarine tidal creek to ...assess whether (1) advective pore water exchange releases some of the soil carbon, (2) outwelling (lateral exports) represents a major carbon sequestration mechanism, and (3) methane emissions offset soil carbon sequestration. A radon (222Rn) mass balance implied tidally driven pore-water exchange rates ranging from 5.5 ± 3.6 to 15.6 ± 8.1 cm d−1. Pore water exchange explained most of the dissolved organic carbon (DOC) and methane surface-water fluxes but not dissolved inorganic carbon (DIC) and alkalinity. Organic carbon burial in soils derived from 239 + 240Pu dating was 11–63 g C m−2 yr−1. Methane and carbon dioxide emissions at the water–air interface were 0.27 ± 0.03 and 63 ± 166 mmol m−2 d−1, respectively. When calculated as CO₂-equivalents, aquatic CH₄ emissions converted to 19–94 g C-CO₂ m−2 yr−1. Upscaling methane and soil carbon fluxes to representative areas revealed that CH₄ emissions could offset < 5% of soil carbon burial. DIC outwelling (12 ± 6 mmol m−2 catchment d−1) was less than five-fold greater than DOC and particulate organic carbon (POC) outwelling and four-fold greater than catchment-wide carbon burial. Because much of this DIC remains in the ocean after air–water equilibration, lateral DIC exports may represent an important long-term carbon sink. Recent research has focused on quantifying carbon burial rates in blue carbon habitats such as saltmarshes and mangroves. We suggest that DIC outwelling and methane outgassing should also be considered when assessing the carbon sequestration capacity of these coastal vegetated habitats.
Freshwater ecosystems play a major role in global carbon cycling through the breakdown of organic material and release of greenhouse gases (GHGs). Carbon dioxide (CO2) and methane (CH4) emissions ...from lakes, wetlands, reservoirs and small natural ponds have been well studied, however, the GHG emissions of highly abundant, small‐scale (<0.01 km2) agricultural dams (small stream and run‐off impoundments) are still unknown. Here, we measured the diffusive CO2 and CH4 flux of 77 small agricultural dams within south‐east Australia. The GHG emissions from these waterbodies, which are currently unaccounted for in GHG inventories, amounted to 11.12 ± 2.59 g CO2‐equivalent m2/day, a value 3.43 times higher than temperate reservoir emissions. Upscaling these results to the entire state of Victoria, Australia, resulted in a farm dam CO2‐equivalent/day emission rate of 4,853 tons, 3.1 times higher than state‐wide reservoir emissions in spite of farm dams covering only 0.94 times the comparative area. We also show that CO2 and CH4 emission rates were both significantly positively correlated with dissolved nitrate concentrations, and significantly higher in livestock rearing farm dams when compared to cropping farm dams. The results from this study demonstrate that small agricultural farm dams can be a major source of greenhouse gas emissions, thereby justifying their inclusion in global carbon budgets.
Carbon dioxide (CO2) and methane (CH4) emissions from lakes, wetlands, reservoirs and small natural ponds have been well studied; however, the GHG emissions of highly abundant, small‐scale (<0.01 km2) agricultural dams (small stream and run‐off impoundments) are still unknown. Here, we measured the diffusive CO2 and CH4 flux of 77 small agricultural dams within south‐east Australia. The GHG emissions from these waterbodies, which are currently unaccounted for in GHG inventories, amounted to 11.12 ± 2.59 g CO2‐equivalent per m2 d−1, a value 3.43 times higher than temperate reservoir emissions.
Groundwater may be highly enriched in dissolved carbon species, but its role as a source of carbon to coastal waters is still poorly constrained. Exports of deep and shallow groundwater-derived ...dissolved carbon species from a small subtropical estuary (Korogoro Creek, Australia, latitude −31.0478°, longitude 153.0649°) were quantified using a radium isotope mass balance model (²³³Ra and ²²⁴Ra, natural groundwater tracers) under two hydrological conditions. In addition, air-water exchange of carbon dioxide and methane in the estuary was estimated. The highest carbon inputs to the estuary were from deep fresh groundwater in the wet season. Most of the dissolved carbon delivered by groundwater and exported from the estuary to the coastal ocean was in the form of dissolved inorganic carbon (DIC; 687 mmol m⁻² estuary day⁻¹; 20 mmol m⁻² catchment day⁻¹, respectively), with a large export of alkalinity (23 mmol m⁻² catchment day⁻¹). Average water to air flux of CO₂ (869 mmol m⁻² day⁻¹) and CH₄ (26 mmol m⁻² day⁻¹) were 5- and 43-fold higher, respectively, than the average global evasion in estuaries due to the large input of CO₂- and CH₄-enriched groundwater. The groundwater discharge contribution to carbon exports from the estuary for DIC, dissolved organic carbon (DOC), alkalinity, CO₂, and CH₄ was 22, 41, 3, 75, and 100 %, respectively. The results show that CO₂ and CH₄ evasion rates from small subtropical estuaries surrounded by wetlands can be extremely high and that groundwater discharge had a major role in carbon export and evasion from the estuary and therefore should be accounted for in coastal carbon budgets.
Tree stems are an important and unconstrained source of methane, yet it is uncertain whether internal microbial controls (i.e. methanotrophy) within tree bark may reduce methane emissions. Here we ...demonstrate that unique microbial communities dominated by methane-oxidising bacteria (MOB) dwell within bark of Melaleuca quinquenervia, a common, invasive and globally distributed lowland species. In laboratory incubations, methane-inoculated M. quinquenervia bark mediated methane consumption (up to 96.3 µmol m
bark d
) and reveal distinct isotopic δ
C-CH
enrichment characteristic of MOB. Molecular analysis indicates unique microbial communities reside within the bark, with MOB primarily from the genus Methylomonas comprising up to 25 % of the total microbial community. Methanotroph abundance was linearly correlated to methane uptake rates (R
= 0.76, p = 0.006). Finally, field-based methane oxidation inhibition experiments demonstrate that bark-dwelling MOB reduce methane emissions by 36 ± 5 %. These multiple complementary lines of evidence indicate that bark-dwelling MOB represent a potentially significant methane sink, and an important frontier for further research.
Tree stem mediated methane emissions represent a potentially important yet poorly constrained source of atmospheric methane. Here we present the first ever quantification of tree stem methane ...emissions from Melaleuca quinquenervia, a widespread iconic Australian lowland tree and globally invasive species. Under two distinct hydrological conditions (wet and dry) we captured 431 tree stem flux measurements encompassing six different vertical stem heights along a 50 m topo-gradient transect, separated into three distinct hydrological zones (upper, transitional and lower). The tree stem methane fluxes closely reflected local topography/hydrology and ranged from - 30.0 to 123,227 lmol m⁻² day⁻¹, with the maximum values amongst the highest values reported to date. The highest methane emissions were observed during wet conditions, within the inundated lower zone and from near the tree stem bases and water table. The average methane flux per tree (scaled to 1 m of stem) for the transitional and lower zones was 52-fold and 46-fold higher during wet conditions compared to dry, whereas the upper zone emissions changed little between seasons. Adjacent soil fluxes followed similar trends along the hydrology gradient with the upper zone tree stem emissions offsetting the adjacent soil methane sink capacity. A clear trend of sharply decreasing methane emissions with stemheight suggests a soil methane origin. A 45-h timeseries of two trees within the lower zone revealed three to fourfold diel variability, with elevated morningtime fluxes. Overall, the study revealed that seasonal hydrological conditions and topo-gradient substantially regulated the methane emissions from M. quinquenervia and that this previously overlooked pathway should be accounted for within wetland methane budgets, especially during inundated conditions.
•Daily sampling of aquatic chemistry after megafires in a large catchment.•Significant increases in major parameters and highly distorted CQ relationships.•Extreme dynamism and transient behaviour ...with monotonic recovery in ∼3–12 months.•Greatest sensitivity to erosion during initial flow events following fire.•Initial dominance of NH4 and rapid biogeochemical cycling of N species.
Increased frequency and intensity of drought, wildfires and flooding due to climate change has major implications for river water quality, yet there are limited high-temporal resolution data capturing the combined transient impacts of these extreme events at large catchment scales. We present flow-stratified water quality data from a large coastal catchment (Macleay River, Australia) spanning severe drought and extensive fires followed by flooding. We examine concentrations (C), discharge (Q) and flux of suspended sediment, major ions, dissolved organic carbon (DOC) and key nutrients (NO3 and PO4), with a focus on the critical first-flush period after the fires. Highly elevated suspended sediment (∼5500 mg L−1; >100x median pre-fire levels) during the initial post-fire period reflected enhanced erosion from fire-impacted, high-relief landscapes, with peak monthly suspended sediment loads of ∼1.1–3.7 t ha−1. The greatest sensitivity to erosion was during initial flow events following fire, highlighting the compounding effect of sequential extreme events on sediment transport. Maximum solute concentrations typically occurred during the first hydrograph peak post-fire with significantly (P = 0.01) elevated major ions following the order of K>Ca>SO4>HCO3≈Mg>Cl>Na, broadly reflecting the composition of ash materials. Distorted CQ relationships for major ions, DOC and nutrients indicated mobilisation behaviour and enhanced surface runoff during initial hydrograph peaks post-fire, with mean concentrations and CQ relationships progressively shifting to those approximating pre-fire within ∼3–12 months. Elevated DOC (∼7x; P = 0.01) displays distinct changes in fluorescence excitation-emission matrix spectral characteristics, attributable to both fire and drought. Both NO3N (160 μM) and PO4 (7.5 μM) were significantly elevated after the fires (∼15–22x; P = 0.01), with maximum monthly loads of 0.82 and 0.14 kg ha−1 respectively. Fast biogeochemical cycling of dissolved inorganic nitrogen (DIN) species occurred during initial flow events following fire, with NH4N initially dominant (>80% of DIN) and exceeding ecosystem guideline threshold values (>100 μM NH4N), followed by rapid (∼1 week) nitrification. The extreme dynamism and transience of water quality parameters highlights the critical importance of high-frequency sampling to adequately capture the compound impacts drought, fires and floods on aquatic systems.
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The growing importance of resolving ecosystem carbon budgets has resulted in more studies integrating terrestrial and aquatic carbon fluxes. Although recent estimates highlight the importance of ...inland waters in global carbon budgets, the extent to which aquatic pathways contribute to the net ecosystem carbon budget (NECB) of different ecosystems remains poorly understood. Here, we provide a cross-ecosystem review of annual carbon budgets integrating terrestrial and aquatic fluxes. Large variability in the proportion of aquatic carbon offset to terrestrial net ecosystem productivity (NEP) was observed, with aquatic offsets ranging from < 1% in a boreal forest to 590% in a freshwater marsh. The total aquatic carbon flux was positively correlated with terrestrial NEP, suggesting highly productive ecosystems will have greater aquatic carbon offsets. However, due to an order of magnitude difference in the range of terrestrial NEP (~ 1000 g C m⁻² y⁻¹) compared to aquatic fluxes (~ 100 g C m⁻² y⁻¹), ecosystems with small NEP’s had greater relative aquatic carbon offsets overall in their NECB’s. Northern hemisphere peatlands and forests represented 54% of all integrated carbon budget studies collected, indicating a severe ecosystem and spatial bias. Mangroves, agricultural, and disturbed ecosystems were the most underrepresented, yet had extreme ranges in terrestrial NEP and NECB (– 638 to 1170 g C m⁻² y⁻¹). To improve our mechanistic understanding of the role of aquatic pathways in NECB’s, more site-specific integrative studies need to be undertaken across a broader range of climatic regions and ecosystem types.
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