Biogeochemical Impacts of Dust on the Global Carbon Cycle - Mineral Dust: A Key Player in the Earth System - page 362

Fig. 14.4 Estimates of the terrestrial regions where net primary productivity is N limited, P limited
or N and P co-limited (Wang et al. 2010 )
model are shown in Fig. 14.4 suggesting that P limitation is relatively widespread
particularly in the tropical forest and savannah regions, with other regions N
limited or NP co-limited. The approximate scale of the effect of dust deposition
on terrestrial ecosystems globally can be estimated from this model simulation.
Wang et al. ( 2010 ) have developed a global model of C, N and P cycling
in the terrestrial biosphere which includes a spatially explicit representation of
atmospheric P deposition. They suggest that P limitation of net primary productivity
is particularly prevalent in tropical forest and savannah systems and this reduces
overall productivity by about 20 %, an estimate consistent with a separate modelling
study of Goll et al. ( 2012 ). Wang et al. use their model to estimate that globally
atmospheric P inputs represent 4 % of total P supply to the terrestrial biosphere.
Malhietal.( 1999 ) estimate global C sequestration in terrestrial systems to be
1.6 GtC year 1 (Gt is equivalent to 10 15 g), and if 4 % is sustained by atmospheric
dust supply of P, then the direct effect of dust deposition is about 0.06 GtC year 1 .
This calculation is very crude and simplistic and probably an overestimate because
it implies all the ecosystems respond to the P deposition, whereas only P limited
areas will respond, although in some of these, the importance of P deposition may
be higher. In the Amazon, Mahowald et al. ( 2005a ) estimate that dust deposition
supplies about 7 % of total atmospheric deposition (the remainder being primarily
associated with deposition of biological material) but that the dust P input is only
sufficient to sustain <1 % of net primary production. Hence, this estimate of the
impact of dust on terrestrial C sequestration at 0.06 GtC year 1 seems likely to
be an upper limit and can be compared later to the oceanic impacts of dust on the
carbon cycle.
As noted earlier, P limitation appears to be particularly prevalent in tropical
regions. Vitousek et al. ( 2010 ) and Okin et al. ( 2004 ) note that in high-latitude
regions, glaciations have occurred repeatedly over the last few million years.