Biogeochemical Impacts of Dust on the Global Carbon Cycle - Mineral Dust: A Key Player in the Earth System - page 358

they are readily available from the atmosphere. The plant requirement for S can be
met in part from atmospheric deposition of S derived from marine or combustion
sources. This argument therefore leads to the identification of a critical role for
phosphorus supply in limiting terrestrial plant growth, as also discussed by Vitousek
et al. ( 2010 ).
Sterner and Elser ( 2002 ) note the potential importance of a wide range of trace
elements in growth but focus on C, N and P as the “three of the main constituents
of biological structural material” which are also not particularly abundant in the
Earth's crust. Similarly Aerts and Chapin ( 2000 ) argue that N and P are the main
growth-limiting nutrients in natural environments, suggesting that generally N is
limiting because of the high energy demands of nitrogen fixation along with the
requirements for P, Mo and Fe for nitrogen fixation, but with P limitation frequently
evident. Chapin and Eviner ( 2003 ) suggest that N is the most common limiting
nutrient, although P may commonly limit productivity in wetlands and old soils,
with K or other trace element limitation rare. Hedin ( 2004 ) suggests that there is a
systematic global gradient in terrestrial ecosystems from P limitation at low latitudes
to N limitation at high latitudes. Limitations by N and P are linked both through
their common role in primary production and through the potential for P limitation
of nitrogen fixation (e.g. Wang et al. 2010 and references therein) which may be
common (Augusto et al. 2013 ).
Since nitrogen can be derived from atmospheric sources (nitrogen fixation and
atmospheric deposition), it is clear that in the context of impacts of dust on
terrestrial ecosystems, dust supply of phosphorus is potentially an important issue,
even though the phosphorus content of dust (which is similar to that of average
crustal rock) is low. The P supply from dust will influence both photosynthesis
directly and nitrogen fixation, which can then relieve nitrogen limitation and thereby
further influence photosynthesis. There is much less published information on the
impacts of other trace nutrients although Kaspari et al. ( 2008 ) and Sayer et al.
( 2012 ) have demonstrated multiple nutrient limitation of different components of
the productivity of a system (litter production, cellulose decomposition, leaf litter
decomposition), including N, P, K and a mix of micronutrients (B, Ca, Cu, Fe, Mg,
Mn, Mo, S and Zn). The subsequent discussion here therefore focusses on P and
to a lesser extent other macronutrient cations (Ca, Mg, K, Na) because it is clear
that the supply of these elements has been implicated in the limitation of primary
productivity in at least some terrestrial systems. In the future we may discover that
other metals supplied by dust are also important nutrients in regulating productivity.
We will see later that iron plays an important role in limiting ocean primary
production. However, in soils iron is always relatively abundant, although its low
solubility means that specialist biogeochemical mechanisms, usually involving
organic chelation, have developed to facilitate this access. Iron limitation can still
occur in calcareous soils where high pH reduces iron solubility (e.g. Stevenson and
Cole 1999 ).
In most environments, the soil is formed in situ from the degradation of bedrock
on timescales of hundreds to millions of years (e.g. Okin et al. 2004 ). Vitousek
et al. ( 2010 ), for instance, identify soils ranging in age from <40,000 years to four