Environmental Engineering Reference
In-Depth Information
the respiration of the plants themselves, by the respiration of soil microbes which feed on
soil organic matter and by disturbances such as fires which oxidize living and dead
organic matter. On a global basis carbon in terrestrial biomass and soils is three times
greater than the CO
2
in the atmosphere. The bulk of terrestrial carbon 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 carbon
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 carbon sinks will remain sinks in the foreseeable future. All plant
communities and soils are not the same, though, and whilst some carbon-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 carbon 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 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 carbon emissions incurred in the manufacture, transport
and application of the fertilizer cancel out any net gain of the maize as a carbon sink.
Similarly, cropping of marginal, semi-arid land is another method frequently advocated
to increase the carbon 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 carbon 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 carbon 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.
