Biogeochemical Impacts of Dust on the Global Carbon Cycle - Mineral Dust: A Key Player in the Earth System - page 359

million years. Many of the studies of the nutrient balance of soils in unperturbed
ecosystems have focussed on forested systems, which represent a major component
of the global carbon cycle, although the principles derived by such studies should be
more widely applicable. Over the long term, the nutrient balance of a forest depends
on the supply of nutrients from the atmosphere and from weathering and losses from
the soil by leaching and volatilisation. The measurement of none of these fluxes is
straightforward (Bruijnzeel 1991 ), particularly on the very long timescales that are
relevant to soil development. This makes it difficult to construct nutrient budgets and
evaluate the relative importance of atmospheric nutrient supply associated with dust.
Bruijnzeel ( 1991 ) reviewed a number of budget studies and suggested that overall P
is accumulating in forest soils, but that the balance of atmospheric supply to loss by
run-off varies systematically with soil type. The balance for other major nutrients
(Ca, Mg, K and N) between accumulation and loss was not clear. Sayer et al. ( 2012 )
and Vitousek et al. ( 2010 ) suggest that, on average, as soils age, they might be
expected to evolve from nitrogen to phosphorus limitation as the supply of P from
weathering declines, and nitrogen accumulates through nitrogen fixation, although
testing this paradigm at the ecosystem scale is challenging (Vitousek et al. 2010 ). In
an elegant study of soils developed on the Hawaiian Islands over different periods of
time ranging from 300 years to 4.1 million years, Chadwick et al. ( 1999 ) concluded
that over time, rock-derived nutrients are lost from soils during weathering. They
suggest that cations such as calcium are renewed by atmospheric deposition of
marine aerosols and that, over timescales of thousands of years, atmospheric dust
deposition becomes the main source of P inputs to soils to replace that lost by
leaching, despite the rather low dust deposition seen on these remote ocean islands.
Looking to the future, Sayer et al. ( 2012 ) note that changes in atmospheric CO 2
concentrations, N deposition and possibly P deposition may alter forest productivity
and the relative importance of different potential limiting nutrients, and Dezi et al.
( 2010 ) suggest that atmospheric N deposition has already significantly increased
forest carbon storage.
We now focus on the role of P supply from dust deposition on terrestrial
ecosystems. Mahowald et al. ( 2008 ) develop a global model of P deposition and
argue that dust deposition dominates the total P deposition, with biomass burning,
fossil fuel emissions, volcanoes and sea salt as secondary sources on a global basis,
although the relative importance of the sources will obviously vary regionally. The
readily soluble P component of atmospheric deposition is rather small, but on the
long timescales of soil P cycling (thousands of years), it seems likely that all forms
of P are potentially bioavailable. The global deposition of dust is illustrated in
Fig. 14.2 , which emphasises the importance of deserts as dust sources and the large
gradients in global dust deposition, reflecting the short lifetime of atmospheric dust
compared to global atmospheric mixing times (see Chap. 7 ) .
In regions of higher dust deposition, the atmospheric P supply will be relatively
large. Swap et al. ( 1992 ) have argued that dust deposition is an important source
of nutrient P and K to the Amazon Basin in regions of ancient and highly
weathered nutrient-poor soils. The atmospheric K supply will be partly directly
from atmospheric dust deposition and partly from deposition of sea-salt aerosols.