Geoscience Reference
In-Depth Information
Carbon sequestration in soils
NEW DEVELOPMENTS
Soil organic matter (SOM) consists of 58 per cent carbon, and we have seen its importance in maintaining chemical
and biological fertility, water retention, and aeration in soils, and hence in promoting crop production. Soil carbon (C)
is part of the global carbon cycle, which is very important for global climate because it regulates the atmospheric
content of two important greenhouse gases: carbon dioxide (CO
2
) and methane (CH
4
) (p. 188). Climate change
scientists have suggested that the
sequestration
of atmospheric CO
2
in SOM could absorb an amount of CO
2
equivalent to the total CO
2
emissions from agriculture, industry and the burning of fossil fuels (the anthropogenic
CO
2
). Sequestration literally means to 'remove carbon from circulation or access, into a store'. The C cycle, like any
other global cycle, consists of major pools or stores, with fluxes or flows between the pools. The pools can act as
sinkswhen they sequestrate C, or as sourceswhen they provide C. The major pools in the cycle are the ocean, the
atmosphere and the terrestrial biosphere (plants, animals and soils). The carbon cycle is very complex, and there are
still major uncertainties about it, especially with regard to the loss of CO
2
to a number of as yet unidentified sinks
in the terrestrial biosphere and the oceans. According to calculations of the C cycle, sources of carbon in the
atmosphere are 1ยท1 gigatonnes (Gt) greater than the sinks, indicating that there are as yet unidentified C sinks (1
Terrestrial ecosystems exchange CO
2
rapidly with the atmosphere. Carbon dioxide is removed by plants from the
atmosphere through photosynthesis (p. 515). It is returned to the atmosphere by the respiration of the plants
themselves, by the respiration of soil microbes which feed on soil organic matter and by disturbances like fires which
oxidize living and dead organic matter. On a global basis, C in terrestrial biomass and soils is three times greater than
the CO
2
in the atmosphere. The bulk of terrestrial C is found in forests, and so trees are a potential sink for
anthropogenic CO
2
in the future. However, as atmospheric CO
2
increases, the biochemical ability of plant enzymes
to fix C decreases, and thus plants will become less of a sink in future. There are also greater demands on nutrients
and water in soils; increased respiration by microbes would also result from increased temperatures and plant remains.
Thus there is considerable uncertainty about the ability of terrestrial ecosystems to dampen down rising CO
2
in coming
years. However, some recent research is optimistic, as it predicts that boreal forests that are now C sinks will remain
sinks in the foreseeable future. All plant communities and soils are not the same, though, and whilst some C-rich
soils like tundra and boreal forest soils will remain sinks under higher temperatures, more fragile and vulnerable soils
such as those in semi-arid lands and the tropics would suffer serious damage in structure and in nutrient-holding
capacities with any loss of SOM; these regions are likely to become C sources.
Cultivation of soils over the centuries has generally led to significant losses of SOM, and hence to additions of CO
2
to the atmosphere. One suggestion for reversing the flow is to convert large areas of traditional cropland to organic
farming and conservation farming, including non-plough practices, which could sequester up to 1 per cent of the
fossil fuel emissions in Europe and the United States. For example, experiments in the United States found a
sequestration of 1,250 grams of carbon per square metre (g C m
-2
) from maize under conventional farming and 1,740
g C m
-2
under no-plough farming. However, nitrogen fertilizers were applied, and the C emissions incurred in the
manufacture, transport and application of the fertilizer cancel out any net gain of the maize as a C sink.
Similarly, cropping of marginal, semi-arid land is another method frequently advocated to increase the C store in soils.
This requires irrigation, though, and irrigation is potentially associated with large CO
2
emissions. Fossil-fuel energy
is also used in pumping irrigation water. Also ground water in arid regions contains large amounts of dissolved calcium
and CO
2
which releases CO
2
to the atmosphere and causes calcium carbonate to precipitate in soils. Irrigation actually
transfers CO
2
from soil and rocks to the atmosphere. The application of manures is also assumed to increase C
sequestration in soils. This happens in experimental plots, but the amounts of manure required are so large that they
are unrealistic on the field scale. Fertilizer use, irrigation and manuring have many advantages, but C sequestration
is not one of them. The regrowth of natural vegetation on abandoned agricultural land offers the best opportunity,
but the atmosphere-plant-soil system is so complex that predictions of future sinks and emissions are very
hazardous.
