Biogeochemical Impacts of Dust on the Global Carbon Cycle - Mineral Dust: A Key Player in the Earth System - page 374

such as those that may have occurred under higher dust fluxes during the last glacial
period, have the potential to change ocean CO 2 uptake at a scale that may affect
atmospheric CO 2 and climate.
The second biogeochemical impact of dust arises in ocean regions where the iron
supply from the dust stimulates microbial nitrogen fixation. This process occurs
predominantly in warm tropical nutrient-poor waters and appears to be limited by
iron supply over most of the tropical Pacific, South Atlantic and Indian Ocean. Only
in the tropical North Atlantic does it seem that the iron supply is sufficient to remove
this limitation and induce phosphorus limitation of nitrogen fixation.
Thus, dust has a substantial biogeochemical impact on both terrestrial and marine
ecosystems, but the mechanisms are different, as are the locations of impact. These
can simplified and summarised as follows:

￿ Low-latitude tropical forests respond to phosphorus inputs from dust with

increased rates of nitrogen fixation and primary production.

￿ Ocean gyre plankton communities respond to iron supply from dust with

increasing rates of nitrogen fixation leading to increased primary production.

￿ HNLC ocean regions (predominantly at high latitudes) respond directly to iron

inputs from dust via stimulation of primary productivity.

In the discussion above, we suggest the effects of dust on terrestrial productivity
are of the order of 0.06 GTC year 1 net atmospheric CO 2 uptake and the impact
on the oceans equivalent to 1 GTC year 1 . In terms of the global carbon cycle,
therefore, the impact on primary productivity in HNLC waters appears to dominate.
However, the ocean iron uptake represents a flux to the deep ocean where the
carbon will be trapped for hundreds or a few thousand years, before returning to
the atmosphere. Only a fraction of this carbon will be buried in ocean sediments
and stored for even loner timescales. The soil uptake is over long timescales
of thousands or even millions of years. Global and regional dust transport has
changed considerably over glacial, interglacial and shorter timescales and is likely to
change in the future. The timescales over which the terrestrial and marine systems
respond to changes in dust inputs are very different reflecting the very different
biogeochemical environments of soils and the oceans. We need to understand the
nature and scale of ecosystem responses to changes and how these will feed back
on global change pressures and processes, recognising that such changes will not
occur in isolation, but will be coupled to changes in global and regional temperature,
hydrology, CO 2 and nitrogen flows.

References

Aerts R, Chapin FS (2000) The mineral nutrition of wild plants revisited: a re-evaluation of
processes and patterns. Adv Ecol Res 30:1-67
Ashman MR, Puri G (2002) Essential soil science: a clear and concise introduction to soil science.
Blackwell, Oxford, 208pp