SCOPE 13 - The Global Carbon Cycle

Group Reports and Recommendations of the Ratzeburg Workshop

GROUP 1 REPORT

Atmosphere, CO₂ source analysis, CO₂ level forecasts, transport kinetics 

Chairman: B. Bolin

The reservoir size and overall increase of atmospheric CO₂ is well known. The present mean mixing ratio of atmospheric CO₂ is about 330 ppm which is equivalent to 702 x 10¹⁵ g C as CO₂ in the atmosphere, assuming 5.14 x 10²¹ g as mass of the atmosphere and 29 as mole weight of air. Measurements on the variability of atmospheric CO₂ in space and time are, however, inadequate. The increase of atmospheric CO₂ measured at Mauna Loa, Hawaii and at the South Pole may not represent the global increase.

Various sources may account for the excess of CO₂ in the atmosphere. The input of fossil CO₂ due to fossil-fuel burning and cement production is now about 5 x 10¹⁵ g C/year. The uncertainty of this calculation is about +7 and 3%, because of possibly inaccurate data for China, differences between production and consumption data, etc. The additional input of non-fossil fuel CO₂ due to forest cutting, forest burning, soil management practices, etc., is relatively unknown. It may be of the same order of magnitude as the input of fossil fuel CO₂. The preindustrial (1860) atmospheric CO₂ level may have been between 285 and 305 ppm. The main sink of atmospheric CO₂ is the ocean.

Based on the decrease in the atmospheric nuclear-bomb ¹⁴ C, the gross uptake of atmospheric CO₂ in the oceans may be between 60 and 200 x 10¹⁵ g C/year. Based on the airborne fraction of fossil fuel CO₂ and the uncertainty of the input of non-fossil fuel CO₂ to the atmosphere, the net CO₂ uptake of the oceans may be between 2 and 8 x 10¹⁵ g C/year. It is not known whether an additional net uptake of excess CO₂ by long-lived land biota exists, due to stimulation of photosynthesis by the enhanced CO₂ partial pressure.

Recommendations

1. Additional stations (approximately 10) should be set up in order to observe the three-dimensional spatial distribution of atmospheric CO₂, to detect the horizonal and vertical CO₂ transport, and possibly to obtain information on the exchange of atmospheric CO₂ between land biota and the oceans. Measurements should be done mostly by aircraft, in order to avoid the noise on ground level.

2. Additional ¹³C measurements on atmospheric CO₂ have to be performed to obtain more information about the origin of atmospheric CO₂ variations.

3. The ¹³C method of recording within tree rings may be used as another helpful tool to detect atmospheric CO₂ variations in the past.

4. For calculations of the non-fossil fuel CO₂ input, it is very important to have reliable data on the decrease in land biota.

5. Measurements on long-term changes of oceanic total inorganic and organic carbon, their ¹³C values and pH variations, have to be performed in order to detect the net uptake of excess CO₂ by the oceans.

6. The physico-chemical characteristics of the oceanic carbonate system are fairly unknown. New measurements have to be performed on the buffering factor of different oceanic waters as a function of CO₂ partial pressure and temperature. The cold oceanic waters of the polar regions should especially be included.

7. In order to understand a shifting of CO₂ between the atmosphere and the ocean by means of ¹³ C recording, kinetic and exchange isotopic effects of the system as a function of temperature have to be measured.

GROUP 2 REPORT

Primary production and the carbon-budget of the sea 

Chairman: G. M. Woodwell

Rapporteur: C. G. N. de Vooys

The oceans could be an important sink for much of the carbon generated by combustion of fossil fuel and generated in the soil. The major pools of carbon in the ocean are the inorganic carbon system (total CO₂), the dissolved organic matter, and the sediments. Because sedimentation rates are low, the sediments do not appear to be an appreciable short-term sink for carbon in the deep oceans. In shelf seas and coastal sea areas sedimentation is much higher and there would be a possibility here for carbon fixation. Small changes, however, in the dissolved organic carbon or the total inorganic CO₂ pools involve large quantities of carbon and knowledge of flux rates to and from these pools is extremely important to the world carbon-budget. These pools are estimated to be about 10²⁰ g C for the CO₂ and 10¹⁸ g for the dissolved organic carbon, dissolved organic carbon being the largest reservoir of organic matter in the sea. It is in dynamic equilibrium with input from various terrestrial and oceanic sources and with losses due to chemical and biological degradation and to sorption and other physical processes. In spite of the size and importance of the pool of dissolved organic carbon, relatively little is known about its molecular composition, sources, transformation, recycling, mineralization and sedimentation. Surprisingly, even the size of this pool is uncertain. Direct measurements appear accurate, at present, to within a factor of two.

The dissolved organic carbon is thought to originate predominantly from net primary production in the oceans. Primary production in the oceans is nearly exclusively generated by unicellular algae. The contribution of kelps, weeds and angiosperms, although very high in itself, is insignificant on a global scale. Dissolved organic carbon can be formed by extracellular excretion, which is an average of 30% of net primary production, or from the deterioration of dead algae cells. Total net primary production in the seas and oceans can be given approximately as 40 x 10¹⁵ g C/year.

Fluxes of net production to and from pools of dissolved organic carbon, particulate organic carbon, inorganic carbon, and carbon in the sediments are not well known.

One of the most important tools used in appraising nutrient and organic fluxes of the oceans is the C:N:P ratio; this was recorded by A. C. Redfield in 1958 as 106:15:1. The validity of this ratio has been confirmed many times for the average concentrations of C, N, and P in the oceans. Deviations from the average are common, however. Algal species, for instance, vary in nutrient requirements and in composition. The dissolved organic carbon of the oceans is enriched in carbon above the ratio for the biota; the organic matter of the deep sea is also enriched in carbon relative to phosphorus. The ratios seem to apply to the average living biota, but not to significant fractions of the remainder of the organic carbon pool.

These observations are taken to indicate that nitrogen and phosphorus atoms are regenerated to be used repeatedly in net production and that a simple interpretation of the Redfield ratios as limiting net primary production would be misleading. Apparently, one atom of phosphorus may be recycled several times before it is removed by sedimentation from the euphotic zone. In this way, organic carbon could be fixed and removed in organic matter, faecal pellets, or dissolved organic carbon from the euphotic zone, at rates considerably greater than have been previously estimated. This could constitute an important potential mechanism for removing carbon from the short-term circulation of the atmosphere.

Recommendations

1. Because of uncertainties in the pool size of dissolved organic carbon and particulate organic carbon in the oceans, the discrepancies between the high temperature combustion and the wet oxidation techniques should be resolved. It is possible that completely new techniques will be necessary, provided they can be used at sea, to minimize the problems associated with the deterioration of water samples. They should be able to provide new real-time data, so that apparent patchiness and other anomalies in deep water dissolved organic carbon concentrations might be pursued.

2. There should be a standardization of the methods in measuring primary production so that a comparison of data becomes possible.

3. The hypothesis of carbon fixation in marine sediments should be carefully tested, because of its importance as a potential sink for carbon.

GROUP 3 REPORT

The geologic carbon cycle 

Chairman: R. Wollast

Rapporteurs: A. Björkström, S. Kempe

The rock cycle between lithosphere and hydrosphere is composed of fluxes: (i) at the surface (i.e., from weathering, erosion, river transport, and sedimentation in pelagic, coastal, and intercontinental basins); and (ii) at depth (i.e. from the sea floor spreading, volcanism, intrusion, subduction, and uplifting). Only surface fluxes may be influenced by man, while the exact sizes of the carbon pools in rocks and hydrosphere are of lesser importance in the evaluation of human perturbations.

In this part of the carbon cycle, several possible sinks for anthropogenic CO₂ may exist:

1. fertilized photosynthesis may bind substantially more carbon in fresh water and estuarine systems, which is transferred to limnic or coastal sediments; 

2. the weathering, and hence the alkalinity, of rivers may have increased, binding additional CO₂ in solution;

3. the transfer of biogenic CO₂ from soil to ground water may have increased; and

4. shallow oceanic high-magnesium calcites may be redissolved.

It should be noted that none of these sinks would exceed the size of a few 10¹⁴ g of carbon.

Recommendations

1. The dissolution kinetics of calcium carbonates, and especially of highmagnesium calcite in sea-water, should be investigated; this includes in situ dissolution experiments, studies of organic grain coatings and their role as solution prohibitors.

2. The regional distribution of calcite, aragonite and organic compounds, especially in coastal regions, should be better mapped, and long-term sedimentation rate measurements should be conducted.

3. The degradation of organic particles (biogenic carbonates and organic matter) in the oceanic water column should be investigated more thoroughly, particularly with respect to kinetics and chemistry.

4. The dissolution from the sediment interface into water, including pore-water processes, needs more detailed studies.

5. Alkalinity changes in rivers, caused by land management or by CO₂ accumulation in the atmosphere, should be periodically checked. Stations should be set up to follow river loads of dissolved inorganic, dissolved organic and particulate carbon. 

6. The transfer of CO₂ to ground-water cannot be quantified at all; it is certainly larger than the inorganic carbon load in rivers, which is the resulting net-flux. Carbon-budgets for ground-water test areas and in close relationship with quantitative photosynthesis studies should be calculated.

7. The role of acidic rains for chemical weathering should be assessed. 

GROUP 4 REPORT

Chairman: S. C. Pandeya

Although a tremendous amount of data on net primary production (NPP) and phytomass was presented, it was agreed that much revision, addition, and refining was necessary. New sources, including the UNESCO-MAB state-of-knowledge reports, the IBP synthesis volumes and other publications from research centres were mentioned. The present figures have been pooled from various sources, which have given different types of classification of biomes, and hence the first step is to make a standard classification of biomes based on vegetational, climatic, and broad soil types. It is extremely difficult to subclassify each of the major biomes and the group, therefore, only took major biomes into consideration when calculating the respective biomass figures. It is hoped that classification of the biomes will soon be available, in order to obtain a further subdivision of the figures. Particular points came up in the discussion:

1. Experiments have shown that an increase in ambient CO₂ concentration either increases NPP or decreases transpiration under optimal conditions. It is not known what happens when nutrients constitute the limiting factor, as is usually the case in natural communities and even on many agricultural lands. 

2. In contrast to C₃ plants, C₄ plant communities have higher water use and photosynthetic efficiencies.

3. With time, NPP increases to a maximum and then decreases gradually (in natural communities). Although exact figures are not available, it was felt that the total phytomass has been decreasing throughout the world as a result of the transfer of forested areas into arable lands and the rapid urbanization and degradation of several marginal lands due to human interference. Indeed, a rapid increase in human population has an adverse effect on forested lands and on wildlife, and positive effects on monocultures and cattle population. This brings about an imbalance in the ecosystems.

Recommendations

1. There is a great need for collecting information on above- and below-ground biomass on a regional basis, together with information on soil organic carbon. There is insufficent data on transition-vegetation types such as woodlands and shrublands. For some vegetation types (e.g., forests, grasslands) there is not enough data on NPP and phytomass from the different regions.

2. Vegetation maps (including arable land) on a continental scale, and based on standardized classification, need to be improved at frequent intervals. This will allow for an assessment of changes in area distribution of vegetation types, as well as changes in phytomass. Remote-sensing technology can be used for this purpose.

3. Analyses on man's activities are needed, in order to assess the effects of the increase in urbanization and of the conversion of land from biologically productive into non- or low- productive.

4. More data on decomposition processes is needed in order to understand the fluxes of carbon dioxide from the soils.

5. Research into the amounts of carbon locked up in peat lands should be intensified. The available figures vary from 110 to 1123 x 10¹⁵ g C.

6. It is also recommended that links be established between studies on the carbon cycle and other cycles, such as those of water, nitrogen, phosphorus, and potassium.

7. The stimulation of land biota photosynthesis by an enhanced CO₂ partial pressure should be checked. If atmospheric CO₂ is the limiting factor for plant growth in several sectors of the land biota, its global extent should be investigated. 

GROUP 5 REPORT

Soil microbiology and organic geochemistry of soils and sediments 

Chairman: A. Nissenbaum

Rapporteur: T. Bramryd

The effects of man's activity on terrestrial ecosystems, as well as on aquatic environments, can lead to severe disturbances in the carbon cycle. Some ecosystems where man's influence on the carbon cycle could be of special interest are tropical rain forests, boreal forests, deciduous forests, grasslands, agricultural lands, peat lands, and swamps.

Deforestation for agricultural purposes, especially in tropical and subtropical countries, could severely affect the global carbon cycle. In rain forests, most of the organic matter and the nutrients are bound up in the above-ground biomass. Increased exploitation of these forests will lead to a large removal of nutrients, which can ultimately lead to impoverishment of the soil and a disturbance of the ecosystem. When the forests are cut down, an increased loss of organic matter through erosion also results. The rain forests also served as large assimilators of CO₂, converting it to organic matter. A disruption of this equilibrium can have unforeseeable consequences.

New forestry management practices such as, for example, whole tree utilization, will decrease the input of organic matter to the soil and could thus lead to a decrease in its humic content, resulting in a smaller water and nutrient-holding capacity. Furthermore, clearcutting and fertilization could have unforeseeable effects on the carbon cycle in temperate and boreal forests.

Drainage of peat land for foresty or agriculatural purposes increases the oxidation of the peat layer. Due to the energy crisis, many countries with large peat areas have started to explore the mires for fuel-peat production. Peat is also used for soil improvement. Peat lands are normally large carbon pools and their exploitation by man can severely affect the carbon cycle.

New agricultural methods, e.g., deeper plowing and burning of straw and litter, could result in an increased net flux of carbon into the atmosphere. The use of synthetic fertilizers increases the microbial activity in the soil, and could thus speed up the oxidation of soil organic matter.

Land exploitation for roads and cities could also affect the local carbon balance. The increased use of incinerators and pyrolysis in garbage and sewage-sludge disposal plants would lead to a rapid oxidation of organic materials. As most products from agriculture and forestry go into human societies, the amount of organic matter turned into garbage is significant.

In addition to the rapid release of CO₂, a large quantity of air pollutants such as heavy metals, nitrogen oxides, and sulphur oxides are emitted. This can cause a decreased productivity of the biomass due to poisoning and pH change in soil.

Making compost from sludge and garbage would be desirable; however, until this becomes possible, the best method would be to use well-compacted landfills. In these landfills, carbon is stored for a long time and could, therefore, function as a sink in a way similar to peat lands.

Climatic variations also cause changes in the productivity of the biota.

The global amount of soil organic matter seems to have decreased, during the last few centuries, due to human activity.

The marine environment can be divided into three subcompartments: the deep sea, the shelf area and the estuarine environment. The deep sea represents a long-term stable environment with input consisting mostly of particulate carbon, and minor contributions from windborne organics and slumping of continental slope sediments. The organic matter is probably rather stable, and microbial turnover (with the possible exception of the ocean-sediment interface) is quite minimal. The deep sea represents a long-term sink for organic carbon and is expected to be perturbed to a minimum degree by human interference. The shelf areas represent a situation where input by human activity (wastes and soil spills) is high in places. In addition, introduction of nutrients could be expected to increase the formation of cellular material, and hence the organic input into the sediments. The biological turnover is rather high, and part of the organic matter may be recycled rather quickly (on a geological time scale). The estuarine environment represents the most labile environment, where the turnover of carbon is the highest. Most estuaries are now heavily contaminated by human activity.

The lacustrine environment represents a situation where the carbon cycle is of great importance locally, but its global significance is possibly quite small.

In studying subaquatic sediments, it is obvious that most of the present organic geochemical data (scarce as it is) deals with the molecular nature of organic carbon, and, in particular, with the lipid-soluble and acid-hydrolysable compounds. Virtually nothing is known on the rates of formation, transformation and diagenesis of the organic matter in sediments.

Recommendations 

Terrestrial ecosystems

1. A greater research effort must be directed towards the understanding of carbon fluxes between the different ecosystems.

2. Increased knowledge on the dynamics of rain forests is needed.

3. More research must be done on the effects of new forestry management practices on, e.g., carbon content in soil. More must be known about the effects of other nutrients, e.g. nitrogen, as these nutrients affect productivity in forests, and thus photosynthesis.

4. More must be known about the effect of management practices on mineralization rates of organic matter.

5. The importance of drainage of peat lands, as well as utilization of peat as fuel and soil improvement, should be investigated.

6. Incineration and pyrolysis of garbage and sewage-sludge should be severely restricted. The best method would be to use well-compacted landfills. Making compost from sludge and garbage would be desirable.

7. The effects of heavy metals, sulphur dioxide, pesticides, and other pollutants on carbon mineralization in soil and productivity must be investigated.

Aquatic ecosystems

8. More quantitative data must be obtained on the total organic carbon input to each subcompartment.

9. The molecular nature of organic matter should be studied in each environment, with special emphasis on the quantitative estimation of autochthonous vs. allochthonous organic matter.

10. Multidisciplinary groups should be established, with the specific aim of choosing key sites in the shelf and estuarine environment in which long-term in situ experiments can be carried out, utilizing labelled compounds and/or geochemical markers. The experimental design should be such as to provide data on the transformation and mineralization of organic carbon on the molecular scale and on time scales involved in such transformations.

11. The microbial population in sediments should be studied with the aim of defining the environmental conditions which influence the overall behaviour of the community, thus allowing an estimation of how changing environmental conditions would influence microbial activity.

12. Special emphasis should be placed on studying estuarine and lacustrine sediments, where the rate of deposition is such that sediments laid down after the beginning of the era of anthropogenic intervention can be studied and compared to pristine sediments. Study of such effects could be of the utmost importance in predicting the future behaviour of the systems during perturbation of the present steady-state situation of the carbon cycle.

13. River systems, and in particular the tropical and subtropical big river systems, should be investigated to obtain data on the quantity and nature of organic matter carried by those systems into the marine environment.

14. An attempt should be made to quantify the amount of organic matter carried via the atmosphere from continents into the marine environment. Information obtained through such studies would allow us to introduce the correct values into the model of the global cycle. Validation of such models could be obtained by the proposed study of sediments from the preanthropogenic eras.