Because freshwater covers such a small fraction of the Earth’s surface area, inland freshwater ecosystems (particularly lakes, rivers, and reservoirs) have rarely been considered as potentially important quantitative components of the carbon cycle at either global or regional scales. By taking published estimates of gas exchange, sediment accumulation, and carbon transport for a variety of aquatic systems, we have constructed a budget for the role of inland water ecosystems in the global carbon cycle. Our analysis conservatively estimates that inland waters annually receive, from a combination of background and anthropogenically altered sources, on the order of 1.9 Pg C y−1 from the terrestrial landscape, of which about 0.2 is buried in aquatic sediments, at least 0.8 (possibly much more) is returned to the atmosphere as gas exchange while the remaining 0.9 Pg y−1 is delivered to the oceans, roughly equally as inorganic and organic carbon. Thus, roughly twice as much C enters inland aquatic systems from land as is exported from land to the sea. Over prolonged time net carbon fluxes in aquatic systems tend to be greater per unit area than in much of the surrounding land. Although their area is small, these freshwater aquatic systems can affect regional C balances. Further, the inclusion of inland, freshwater ecosystems provides useful insight about the storage, oxidation and transport of terrestrial C, and may warrant a revision of how the modern net C sink on land is described.

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Plumbing the Global Carbon Cycle: Integrating Inland Waters into the Terrestrial Carbon Budget

I. CONTEXT * The Ecosystem Concept * Earth's Climate System * Geology and Soils * II. MECHANISMS * Terrestrial Water and Energy Balance * Carbon Input to Terrestrial Ecosystems * Terrestrial Production Processes * Terrestrial Decomposition * Terrestrial Plant Nutrient Use * Terrestrial Nutrient Cycling * Aquatic Carbon and Nutrient Cycling * Trophic Dynamics * Community Effects on Ecosystem Processes * III. PATTERNS * Temporal Dynamics * Landscape Heterogeneity and Ecosystem Dynamics * IV. INTEGRATION * Global Biogeochemical Cycles * Managing and Sustaining Ecosystem * Abbreviations * Glossary * References

Principles of Terrestrial Ecosystem Ecology

Dissolved organic matter (DOM) in soils plays an important role in the biogeochemistry of carbon, nitrogen, and phosphorus, in pedogenesis, and in the transport of pollutants in soils. The aim of this review is to summarize the recent literature about controls on DOM concentrations and fluxes in soi

Controls on the dynamics of dissolved organic matter in soils: a review.

This report is FAO's latest assessment of the long-term outlook for the world's food supplies, nutrition and agriculture. It presents the projections and the main messages. The projections cover supply and demand for the major agricultural commodities and sectors, including fisheries and forestry. This analysis forms the basis for a more detailed examination of other factors, such as nutrition and undernourishment, and the implications for international trade. The report also investigates the implications of future supply and demand for the natural resource base and discusses how technology can contribute to more sustainable development.

One of the report's main findings is that, if no corrective action is taken, the target set by the World Food Summit in 1996 (that of halving the number of undernourished people by 2015) is not going to be met. Nothing short of a massive effort at improving the overall development performance will free the developing world of its most pressing food insecurity problems. The progress made towards this target depends on many factors, not least of which are political will and the mobilization of additional resources. Past experience underlines the crucial role of agriculture in the development process, particularly where the majority of the population still depends on this sector for employment and income.

World Agriculture: Towards 2015/2030: An Fao Perspective

The terrestrial biosphere is a key component of the global carbon cycle and its carbon balance is strongly influenced by climate. Continuing environmental changes are thought to increase global terrestrial carbon uptake. But evidence is mounting that climate extremes such as droughts or storms can lead to a decrease in regional ecosystem carbon stocks and therefore have the potential to negate an expected increase in terrestrial carbon uptake. Here we explore the mechanisms and impacts of climate extremes on the terrestrial carbon cycle, and propose a pathway to improve our understanding of present and future impacts of climate extremes on the terrestrial carbon budget.

Climate extremes and the carbon cycle

A review of the export of carbon in river water: fluxes and processes.

The relationship between stream water DOC concentrations and soil organic C pools was investigated at a range of spatial scales in subcatchments of the River Dee system in north-east Scotland. Catchment percentage peat cover and soil C pools, calculated using local, national and international soils databases, were related to mean DOC concentrations in streams draining small- ( 700 m) catchments, suggesting that disturbance and land use may have a small effect on DOC concentration. Our results therefore suggest that the relationship between stream water DOC concentration and catchment soil C pools exists at a range of spatial scales and this relationship appears to be sufficiently robust to be used to predict the effects of changes in catchment soil C storage on stream water DOC concentration. Copyright © 1999 John Wiley & Sons, Ltd.

The relationship between dissolved organic carbon in stream water and soil organic carbon pools at different spatial scales

Many previous studies have highlighted the importance of peatland soils to the global greenhouse gas (GHG) balance through their role in carbon sequestration and methane emission. However such studies often overlook the drainage system as a pathway for dissolved organic carbon (DOC) and GHG release. Surface waters associated with peatlands have repeatedly been shown to be highly and consistently supersaturated in CO2 and CH4 with respect to the atmosphere. Therefore in addition to downstream losses, degassing from the water surface (evasion) has the potential to act as an important pathway directly linking the peatland carbon pool to the atmosphere, and hence altering the perceived greenhouse gas and carbon balances of the catchment. 
Here we present both the carbon and GHG budgets for Auchencorth Moss, an ombrotrophic peatland in South East Scotland, including losses through the aquatic pathway. The catchment functioned as a net sink for GHGs and a net source of carbon. Downstream DOC export was the largest component of the carbon budget; the greatest flux of GHGs was via net ecosystem exchange (NEE). Terrestrial emissions of CH4 and N2O combined returned only ~5% of CO2-equivalents captured by NEE to the atmosphere, whereas evasion of CO2, CH4 and N2O from the stream surface returned ~40%. The budgets clearly show the importance of lateral (downstream) and vertical (evasion) aquatic fluxes at Auchencorth Moss and highlight the potential for significant error in source/sink strength calculations if they are omitted. Furthermore, a process based understanding of soil-stream connectivity, suggests that the aquatic flux pathway may play an increasingly important role in the sink-source function of peatlands under future management and climate change scenarios.

Role of the aquatic pathway in the carbon and greenhouse gas budgets of a peatland catchment

Peatland streams potentially represent important conduits for the exchange of gaseous carbon between the terrestrial ecosystem and the atmosphere. We investigated how gaseous evasion of carbon from the stream surface compared with downstream carbon transport at three locations on a Scottish headwater stream. Carbon dioxide was consistently above atmospheric saturation in the stream, with mean concentrations of 159.1, 81.8, and 29.5 mmol L 21 at the lower, middle, and upper sites, respectively (i.e., 7.6, 3.9, and 1.2 times in excess of atmospheric equilibrium concentrations). Methane concentrations in stream water were much lower but showed a similar pattern. Rates of gaseous evasion from the stream surface to the atmosphere, determined experimentally using direct measurement of dissolved gas concentrations in conjunction with coinjection of conservative solute and volatile gas tracers, also declined downstream. Combined stream losses of all forms of carbon from the entire catchment (i.e., degassing from the stream surface and exports downstream) totaled 54,140 kg C yr 21 . Evasion of carbon dioxide from the stream surface accounted for 34% of this total, compared to 57% lost as dissolved organic carbon via export downstream. When expressed per unit area of watershed, the gaseous C evasion from the stream represents a loss of 14.1 g C m 22 yr 21 , which equals 28‐70% of the estimated net carbon accumulation rate for such peatlands. This study shows that gaseous carbon loss from the surface of temperate headwater streams can be both spatially variable and significant in terms of rates of net annual land surface‐atmosphere exchange at the catchment scale.

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.2001.46.4.0847

Carbon dioxide and methane evasion from a temperate peatland stream

Any change in the ability of northern peatlands to act as a sink for atmospheric CO2 will play a crucial part in the response of the Earth system to global warming. We argue that a true assessment of the sink-source relationships of peatland ecosystems requires that losses of C in drainage waters be included when determining annual net C uptake, thus connecting measurements of stream C fluxes with those made at the land surface-atmosphere interface. This was done by combining estimates of net ecosystem exchange (NEE) with stream water measurements of TOC, DIC, and gaseous C loss, in a 335-ha lowland temperate peatland catchment (55°48.80′N, 03°14.40′W) in central Scotland over a 2-year period (1996–1998). Mean annual downstream C flux was 304 (±62) kg C ha−1 yr−1, of which total organic carbon (TOC) contributed 93%, the remainder being dissolved inorganic carbon (DIC) and free CO2. At the catchment outlet evasion loss of CO2 from the stream surface was estimated to be an additional 46 kg C ha−1 yr−1. Over the study period, NEE of CO2-C resulted in a flux from the atmosphere to the land surface of 278 (±25) kg C ha−1 yr−1. Net C loss in drainage water, including both the downstream flux and CO2 evasion from the stream surface to the atmosphere, was therefore greater or equal to the net annual C uptake as a result of photosynthesis/respiration at the land surface. By combining these and other flux terms, the overall C mass balance suggests that this system was either acting as a terrestrial C source or was C neutral.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2003GB002058

Michael F. Billett

Papers

A review of the export of carbon in river water: fluxes and processes.

The relationship between dissolved organic carbon in stream water and soil organic carbon pools at different spatial scales

Role of the aquatic pathway in the carbon and greenhouse gas budgets of a peatland catchment

Carbon dioxide and methane evasion from a temperate peatland stream

Linking land-atmosphere-stream carbon fluxes in a lowland peatland system