Agriculture Reference
In-Depth Information
Interactions of Soils with Elevated Atmospheric [CO
2
]
In many agricultural systems, production is limited either by nutrients or other factors
such as water. As a consequence, plant growth is not restricted by C availability
(Poorter and Perez-Soba
2001
) until these other limitations are overcome. Nutrient
use efficiency (NutUE) reflects the ability of plants to access nutrients from soil
(and fertilisers) as well as the dynamics of the nutrient once it is absorbed.
In contrast to the rapid growth in knowledge of how high [CO
2
] alters the above
ground physiological responses of plants, much less is known about potential
interactions between high [CO
2
], different soil processes and plant responses. For
example, soil available N levels have been hypothesised to gradually decline under
high [CO
2
]. This so-called progressive nitrogen limitation (PNL, Luo et al.
2004
),
will result in available soil N levels becoming increasingly limiting as N and C are
sequestered in plant biomass and soil organic matter. As C/N ratios of plant residues
increase (as indicated in previous sections), soil N mineralisation rates will decrease
(Prior et al.
2008
). Consequently, an understanding of how high [CO
2
] will affect
NutUE of plants must account for the influence of different soil properties on both
the capacity to supply nutrients to plants as well as potentially influence the ability
of the plant to access available nutrients (and other critical resources such as water)
in the soil.
Strong interactions can exist between different mineral nutrients (especially N)
at both a physiological level (see above) as well as in a broader agronomic context.
For example, the old weathered soils that underpin many Australian grain produc-
tion systems are very deficient in P (Donald
1964
) and McDonald (
1989
) noted the
importance of N and P interactions in controlling grain yield in cereal crops. Only a
relatively small proportion of P in the soil is in a chemical form that is immediately
'
to plants. Consequently, farmers generally manage soil P deficiencies to
plants by applying P fertilisers. However, a relative large proportion - up to 95 %
(McBeath et al.
2012
) - of the fertiliser P becomes unavailable for the crops. For
example, P can be strongly adsorbed onto soil particles, precipitated as insoluble
forms (normally associated with iron, aluminium or calcium) or immobilised by
soil biota. Considerable research has been dedicated to investigating the potential of
plants (at both an intra and inter species level) to access these
available
'
forms of
P as a strategy to improve NutUE (e.g. Armstrong et al.
1993
; Osborne and Rengel
2002
; Wang et al.
2010
). Although high [CO
2
] can actually temporarily increase P
immobilisation (as organic P) in the rhizosphere of crops (Jin et al.
2013
), there is
no evidence that high [CO
2
] alters the ability of plants to acquire P from different
sparingly soluble sources of P (Hocking and Barret
2003
; Jin et al.
2014
).
Differences in NutUE between plants depend on the ability of roots to access
nutrients in the soil and fertiliser; especially for relatively immobile nutrients such
as P, most nutrient acquisition is via root interception (Barber
1995
). In many
environments, soils can exhibit a range of physicochemical properties such as high
aluminium, sodicity, salinity and high boron that can significantly restrict root
growth, and therefore uptake of soil nutrients and water (Adcock et al.
2007
). It
has been hypothesised that high [CO
2
] may ameliorate the negative effect of some
unavailable
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