Terrestrial versus aquatic carbon fluxes in a subtropical agricultural floodplain over an annual cycle
Graphical abstract
Introduction
Understanding the contribution of different flux components in ecosystem carbon budgets is crucial for monitoring how carbon dynamics respond to environmental change. This can be achieved by quantifying the carbon inputs and outputs within defined boundaries and time scales. This approach represents the core concept of the net ecosystem carbon budget (NECB), for which complete accounting of all carbon flux pathways determines the net carbon accumulation or loss rate of an ecosystem (Chapin et al., 2006). Because the NECB approach includes all relevant physical, biological, and anthropogenic processes, carbon flux measurements must often be integrated across different discipline boundaries. This is especially important in ecosystems at the terrestrial-aquatic interface, where traditional land-based measurements fail to identify carbon lost via the aquatic pathway (Chapin et al., 2006; Barr et al., 2010). Fully integrated carbon budgets that account for all ecosystem processes have redefined the source/sink interpretation of some terrestrial-aquatic ecosystems (Genereux et al., 2013; Chu et al., 2015; Lundin et al., 2016).
The current role of agricultural ecosystems in the global carbon cycle is controversial. On a global scale, modelled estimates suggest that most areas of intensively managed croplands increase biomass production, yet overall agriculture has reduced total soil carbon stocks by between 8 to 13% over the last 100 years (Bondeau et al., 2007; Sanderman et al., 2017). At the finer scale, individual agricultural carbon studies spanning temperate and tropical regions show a wide range of carbon balances from net carbon sources to sinks (Kutsch et al., 2010; Bhattacharyya et al., 2014; Eichelmann et al., 2016). Many agricultural areas have been developed with extensive drainage networks, where the effect of land use and hydrological modification has altered the load and composition of carbon exported within streams and rivers (Royer and David, 2005; Raymond et al., 2008; Kaushal et al., 2014). Aquatic fluxes are often quantified in waters draining catchments with mixed land use, or are rarely inclusive of all aquatic carbon species (Royer and David, 2005; Raymond et al., 2008; Smith et al., 2010). Indeed, the aquatic pathway in the form of lateral export has not been accounted for within global models and fine scale studies of agricultural carbon budgets (Bondeau et al., 2007).
Cultivated wetlands represent a large area of productive agricultural environments in low lying landscapes. Agriculture is the primary land use change responsible for the estimated 50% loss in global wetland area (Verhoeven and Setter, 2009). Much research has been dedicated to understanding carbon cycling in natural and regenerated wetlands, which are often ecosystem hotspots for carbon cycling both in terms of their carbon sequestration capacity and greenhouse gas feedback (Mitra et al., 2005; Neubauer et al., 2014). The transition phase from natural wetland to agricultural production can either reduce the carbon sink or switch modified wetlands to carbon sources (Armentano and Menges, 1986). In warmer climates, this loss of soil carbon via CO2 emissions may be particularly heightened in drained wetlands as ecosystem respiration increases (Raich and Schlesinger, 1992; Knox et al., 2015). However, very little attention has been paid to the functioning of cultivated wetlands as carbon sinks or sources after initial disturbance, or the role of the aquatic pathway as a conduit for terrestrial carbon leakage in these drained landscapes.
In Australia, many drained coastal floodplains and wetlands have been cultivated for intensive sugarcane production (Arthington et al., 1997). Sugarcane is grown in sub-tropical to tropical regions with generally high annual rainfall, and is one of the highest yielding crops in agricultural soils (De Vries et al., 2010). A limited number of studies have measured sugarcane carbon fluxes directly, yet those that have reported very high annual NEE rates of 1800 to 2685 g C m−2 yr−1 (Cabral et al., 2013; Anderson et al., 2015). It is widely accepted that sugarcane crops are very efficient at biomass carbon accumulation during their growing cycle, however isolated measurements do not provide a full assessment of farm-scale carbon budgets. Recent studies have shown that aquatic CO2 and CH4 fluxes from drained coastal wetlands in Australia tend to be larger than most natural aquatic environments (Gatland et al., 2014; Ruiz-Halpern et al., 2015; Jeffrey et al., 2016), and represent an unresolved flux in many agro-ecosystems. Given the preference for sugarcane in biofuel production (De Vries et al., 2010), understanding the full ecosystem carbon balance of such systems is required to achieve carbon neutral farming practices.
To advance our understanding of the carbon cycle of cultivated wetlands and the role of the aquatic pathway in carbon budgets, a complete seasonal assessment of the NECB was undertaken by integrating the aquatic carbon flux with terrestrial net ecosystem exchange (NEE). This work was carried out in a subtropical, extensively drained coastal floodplain under sugarcane production, and represents the first landscape of this type to undergo an integrated terrestrial and aquatic carbon budget assessment. The chosen site represents a well constrained “model catchment”, with distinct surface and groundwater flow paths originating within the catchment boundary (Webb et al., 2017), and a relatively controlled terrestrial carbon uptake pathway by one vegetation type (sugarcane crop).
Section snippets
Study site
The present study was undertaken in an agricultural wetland used for sugarcane production, situated in the sub-tropical coastal region of eastern Australia (28°17′1.69″S, 153°30′15.02″E). The site was originally a freshwater tidal wetland connected to the Tweed River estuary, which has been converted to sugarcane production for the past 40 years through drainage construction and implementation of floodgates (see Webb et al., 2016, 2017 for description of study site). The sub catchment is
Eddy covariance measurements
The daily averaged net CO2 and daily integrated carbon flux reflected a distinct seasonal trend associated with biomass growth (Fig. 2, Fig. 3). Average peak daytime CO2 uptake rates of −40 μmol m−2 s−1 (maximum −58 μmol m−2 s−1) occurred during the months of February and March 2015 (Fig. 2), which is equivalent to some of the highest rates of CO2 assimilation observed for C4 plants (Yin and Struik, 2009). Over these two months, the cumulative carbon uptake was −394 g C m−2 which represents 44%
Acknowledgements
Analytical instrumentation (LE120100156) and field investigations (DE140101733) were funded by the Australian Research Council and research funds from a CSIRO Postgraduate scholarship (R-06417) to JRW. We are grateful to the knowledge and support of Robert Quirk who allowed monitoring to take place from his property. We thank Paul Kelly, Douglas Tait, and Luke Jeffrey for their continuous fieldwork assistance as well as Phillip Riekenberg, James Sippo, Lea Taylor, and Mitchell Call for flux
References (104)
- et al.
Long-rotation sugarcane in Hawaii sustains high carbon accumulation and radiation use efficiency in 2nd year of growth
Agric. Ecosyst. Environ.
(2015) - et al.
Seasonal exports and drivers of dissolved inorganic and organic carbon, carbon dioxide, methane and δ13C signatures in a subtropical river network
Sci. Total Environ.
(2017) - et al.
Use of change-point detection for friction–velocity threshold evaluation in eddy-covariance studies
Agric. For. Meteorol.
(2013) - et al.
Tropical low land rice ecosystem is a net carbon sink
Agric. Ecosyst. Environ.
(2014) - et al.
Fluxes of CO2 above a sugarcane plantation in Brazil
Agric. For. Meteorol.
(2013) - et al.
Assessing carbon and water dynamics of no-till and conventional tillage cropping systems in the inland Pacific Northwest US using the eddy covariance method
Agric. For. Meteorol.
(2016) - et al.
Resource use efficiency and environmental performance of nine major biofuel crops, processed by first-generation conversion techniques
Biomass Bioenergy
(2010) - et al.
Emissions of methane and nitrous oxide from Australian sugarcane soils
Agric. For. Meteorol.
(2010) - et al.
Comparison of carbon budget, evapotranspiration, and albedo effect between the biofuel crops switchgrass and corn
Agric. Ecosyst. Environ.
(2016) - et al.
North American prairie wetlands are important nonforested land-based carbon storage sites
Sci. Total Environ.
(2006)
Seasonality of ecosystem respiration and gross primary production as derived from FLUXNET measurements
Agric. For. Meteorol.
Long-term decomposition of sugarcane harvest residues in Sao Paulo state, Brazil
Biomass Bioenergy
A method for measuring free CO2 in upland streamwater using headspace analysis
J. Hydrol.
Life cycle assessment of bagasse waste management options
Waste Manage.
The net biome production of full crop rotations in Europe
Agric. Ecosyst. Environ.
Long-term agricultural drainage stimulates CH4 emissions from ditches through increased substrate availability in a boreal peatland
Agric. Ecosyst. Environ.
Gaseous nitrogen losses from coastal acid sulfate soils: a short-term study
Pedosphere
Insights into estuarine benthic dissolved organic carbon (DOC) dynamics using δ13C-DOC values, phospholipid fatty acids and dissolved organic nutrient fluxes
Geochim. Cosmochim. Acta
Application and test of a simple tool for operational footprint evaluations
Environ. Pollut.
Recommendations for autonomous underway pCO2 measuring systems and data-reduction routines
Deep Sea Res. Part II: Top. Stud. Oceanogr.
Measurements necessary for assessing the net ecosystem carbon budget of croplands
Agric. Ecosyst. Environ.
Respiration of three Belgian crops: partitioning of total ecosystem respiration in its heterotrophic, above- and below-ground autotrophic components
Agric. For. Meteorol.
Simulation of the effects of trash and N fertilizer management on soil organic matter levels and yields of sugarcane
Soil Tillage Res.
Annual carbon dioxide exchange in irrigated and rainfed maize-based agroecosystems
Agric. For. Meteorol.
Simulating the net ecosystem CO2 exchange and its components over winter wheat cultivation sites across a large climate gradient in Europe using the ORCHIDEE-STICS generic model
Agric. Ecosyst. Environ.
Assessing carbon dynamics at high and low rainfall agricultural sites in the inland Pacific Northwest US using the eddy covariance method
Agric. For. Meteorol.
Divergent drivers of carbon dioxide and methane dynamics in an agricultural coastal floodplain: post-flood hydrological and biological drivers
Chem. Geol.
Carbon dioxide in water and seawater: the solubility of a non-ideal gas
Mar. Chem.
Patterns of change in the carbon balance of organic soil-wetlands of the temperate zone
J. Ecol.
Potential impact of sugarcane production on riparian and freshwater environments, intensive sugarcane production: meeting the challenge beyond 2000
Climate Statistics for MURWILLUMBAH
Assessing the eddy covariance technique for evaluating carbon dioxide exchange rates of ecosystems: past, present and future
Glob. Change Biol.
Measuring fluxes of trace gases and energy between ecosystems and the atmosphere—the state and future of the eddy covariance method
Glob. Change Biol.
Controls on mangrove forest-atmosphere carbon dioxide exchanges in western Everglades National Park
J. Geophys. Res.: Biogeosci.
Soil greenhouse gas emissions and carbon budgeting in a short-hydroperiod floodplain wetland
J. Geophys. Res.: Biogeosci.
Estimates of N2O and CH4 fluxes from agricultural lands in various regions in Europe
Nutr. Cycl. Agroecosyst.
Modelling the role of agriculture for the 20th century global terrestrial carbon balance
Glob. Change Biol.
Reconciling carbon-cycle concepts, terminology, and methods
Ecosystems
The recovery of 15N from labelled urea fertilizer in crop components of sugarcane and in soil profiles
Aust. J. Agric. Res.
Net ecosystem methane and carbon dioxide exchanges in a Lake Erie coastal marsh and a nearby cropland
J. Geophys. Res.: Biogeosci.
Climatic variability, hydrologic anomaly, and methane emission can turn productive freshwater marshes into net carbon sources
Global Change Biol.
Dynamics of component carbon fluxes in a semi-arid Acacia woodland, central Australia
J. Geophys. Res.: Biogeosci.
Extreme storms and changes in particulate and dissolved organic carbon in runoff: entering uncharted waters?
Geophys. Res. Lett.
Sampling error in eddy correlation flux measurements
J. Geophys. Res.: Atmos.
Carbon dioxide and methane emissions from an artificially drained coastal wetland during a flood: implications for wetland global warming potential
J. Geophys. Res.: Biogeosci.
A connection to deep groundwater alters ecosystem carbon fluxes and budgets: example from a Costa Rican rainforest
Geophys. Res. Lett.
The biogeochemistry of northern peatlands and its possible response to global warming
Measurements of carbon sequestration by long-term eddy covariance: methods and a critical evaluation of accuracy
Glob. Change Biol.
The effect of rain on air-water gas exchange
Tellus B
Uncertainty in eddy covariance measurements and its application to physiological models
Tree Physiol.
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