Agriculture Reference
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Fig. 4.2 Global map of agronomic P imbalances for the year 2000 per unit of cropland area in
each 5 grid cell. The surpluses and deficits are each classified according to quartiles globally (0-
25th, 25-50th, 50-75th, and 75-100th percentiles) (Data taken from MacDonald et al. ( 2011 ) and
used with permission)
are unevenly distributed, resulting in massive agronomic imbalances and spatial
surplus or deficit patterns across regions and countries (Tiessen 2008 ; MacDonald
et al. 2011 ; Fig. 4.2 ). Additionally, large amounts of applied P are removed in
harvested products from the fields and across the globe (Lott et al. 2009 ), and
nutrient recycling, especially organic P sources, from urban areas or returning
biomass is rare (Cordell et al. 2009 ). Rock P fertilisers are extracted and exported
from only a few countries worldwide, which are geopolitically controversial and
potentially unstable (Cordell et al. 2009 ; Tiessen 2008 ; FAO 2011 ). Demand is still
increasing and estimated to be 1.9 % yearly from 2011 to 2015 (FAO 2011 ). In
order to meet the requirement for the future demands of an increasing world
population, bearing in mind that P reserves are limited and deposits cannot be
exploited infinitely (Kirkby and Johnston 2008 ), it is a necessity to enhance P
fertiliser use efficiency (Gregory and George 2011 ).
There is an essential need for agronomic P efficiency criteria to be defined in
order to be able to exploit and screen genetic variation in grain crops (Rose and
Wissuwa 2012 ), one of the key strategies for breeding more P efficient crops
(Gregory and George 2011 ). Phosphorus efficiency is a complex trait and the
genetic determinants, which are involved in enhanced low P tolerance or P effi-
ciency are still not clearly understood. As previously described, the focus of
molecular research lies mainly on P stress responses of individual genotypes or
model plants and it is questionable if these findings are generally applicable for
crops in an agricultural context. Genotypic phosphate efficiency differences that
exist might only rarely be conserved between and within species (Hammond
et al. 2009 ; Alexova and Millar 2013 ; Calder ´ n-V ´ zquez et al. 2008 , 2011 ; Niu
et al. 2012 ) and need to be identified before they can be exploited for breeding. Due
to the previously described interactions of P accessibility in soils (Holford 1997 ),
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