Biogeochemical Impacts of Dust on the Global Carbon Cycle - Mineral Dust: A Key Player in the Earth System - page 356

Table 14.1 Essential nutrient elements for plant growth
Macronutrient elements C, O, N, Ca, Mg, K, P, S
Micronutrient elements Co, Fe, Mn, Cu, Si, Zn, Mo, B, Cl, Ni and Na
Based on Ashman and Puri ( 2002 ) and Mengel and Kirkby ( 1978 )

underlying rock provides very little soil components, such as on some oceanic
limestone islands (e.g. Caribbean, Bahamas, Bermuda). In other areas where soils
have developed under arid conditions and with very little available calcium from the
underlying bedrocks, calcium delivered in association with atmospheric dust can
lead to the formation of petrocalcic soil horizons, which can alter the soil structure.
In all these cases where dust provides a major component of the soil itself, dust
transport can be said to have a very important biogeochemical role (Muhs 2012 ;
Muhs et al. 2012 ). While dust deposition is potentially a source of soil in these
areas, it necessarily also must represent a loss of soil and associated carbon in dust
source areas (Webb et al. 2012 ).

14.2.2 Nutrient Supply

In addition to the role of dust in providing the soil fabric, dust can potentially be
an important source of nutrients for plants growing in soils. Mengel and Kirkby
( 1978 ) list 20 elements as essential nutrients for plant growth, although they suggest
four of these (Co, Si, Ni and Na) are only essential for certain plants and hence
will only limit species composition and not overall productivity. Fraústo da Silva
and Williams ( 2001 ) produce a similar list although they also include V and
Cr. Ashman and Puri ( 2002 ) further divide the nutrients into macronutrients and
micronutrients as shown in Table 14.1 . The C, N, S and H requirements of plants
are readily met from the atmosphere as carbon (CO 2 ), oxygen (O 2 and H 2 O),
sulphur (sulphate aerosol) and hydrogen (H 2 O) and hence are not relevant to the
discussion of dust here. However, the atmospheric transport and transformation of
atmospheric S (as sulphate and sulphuric acid) and N compounds (as nitric acid,
nitrate, ammonia and ammonium) may be modified by reactions with dust in the
atmosphere (e.g. Usher et al. 2003 ), and hence, the delivery of these nutrients may
be coupled to that of dust. Atmospheric N deposition influences primary production
and contributes significantly to the indirect biogeochemical impact discussed by
Mahowald ( 2011 ). The nitrogen requirement of plant ecosystems can also be met, at
least in part from the biological fixation of atmospheric nitrogen, and supplemented
by atmospheric deposition. However, biological nitrogen fixation does require
trace elements for the relevant enzymes and hence may be indirectly coupled to
atmospheric dust supply of these trace elements as discussed later (Vitousek et al.
2010 ).
Epstein ( 1972 ) lists the relative nutrient requirements for various elements as
ratios to the requirement for Mo (Fig. 14.1 a). Although such estimates are of course