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Fig. 10.3 Relationship
between cumulative C input
and stock, in two long-term
experiments in semiarid
environment
Indeed, contrasting observations came from two long-term experiments
in Sicily (Pietranera farm: 4155073 m N, 368943 m E; and Sparacia farm:
4164079 m N, 385269 m E), where no linear correlation between cumulative car-
bon input and soil carbon accumulation was found (Fig. 10.3 ). The annual SOC
sequestration increased with a cumulative C input up to 7.4 t C ha 1 , then showing
a saturation threshold with the actual cultivation practices and pedo-climatic con-
ditions at the investigated sites (Barbera et al. 2011 ).
In semiarid cropping systems, different steady state levels can be reached by
crop rotation management due to different mineralization rate of plant residue,
as emerged from these long-term trials. Surprisingly, the higher SOC content in
bulk soil was recorded in cereal monoculture in comparison to cereal-leguminous
rotations, while there were no significant differences between crop rotations in the
smallest soil aggregate fraction (<25 µm) (Barbera et al. 2011 ). These results are
in disagreement with other studies that reported the positive effects on enhancing
SOC in crop rotations including leguminous species (Upendra et al. 2005 ; Ashraf
et al. 2004 ). The lowest efficiency of C retained from crop residue with a low C/N
ratio and less rich in lignin (crop rotation with leguminous species) was likely due
to the higher labile C fraction than the sole wheat residues.
In all crop rotations analysed, the effect of soil management was no significant
different. Although many authors claim that conservation tillage (reduced and no-
till practices) increase SOC in the surface layer (Sainju et al. 2006 ; Melero et al.
2009 ; L￳pez-Bellido et al. 2010 ), improves soil aggregation and preserves soil
resources better than conventional tillage practices (Six et al. 2004 ), SOC values
did not significantly differ between conventional tillage (CT) and no-tillage (NT),
during observation carried out in these long-term experiments.
Despite the RMPs suggested by IPCC, the efficiency of carbon sequestration in
semiarid environment was apparently low, due to hot and drought climate which
facilitates the rapid oxidation of organic C. Conventional cropping management
did not result in SOC depletion, since the most of C was stored and stabilized in
the more stable silt-clay fraction. As highlighted from these long-term experiments
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