Soil Carbon Stocks and Fluxes - The Greenhouse Gas Balance of Italy - page 117

multiplying country specific C sequestration rates, estimated by the CESAR model
(Vleeshouwers and Verhagen 2002 ), with the average surface area reported by
Mucher ( 2000 ) and FAO ( http://www.fao.org/waicent/portal/statisticsen.asp ) . To
our knowledge, there are only two large-scale (national) and long-term invento-
ries of C in agricultural soils that can be used to compare these SOC loss rates. In
a study by Sleutel et al. ( 2006 ), repeated sampling of cropland soils in Belgium
(210,000 samples taken between 1989 and 1999) indicate an average annual soil C
loss of 0.8 Mg ha − 1 y − 1 , while Dersch and Boehm ( 1997 ) in a large-scale inven-
tory study estimate for Austrian croplands, a net C loss of about 0.2 Mg ha − 1 y − 1 .
Also at European level, a net loss of SOC is observed for cropland soils and can be
mainly related to the harvesting that reduces C returns to soil, and to agricultural
practices such as tillage (Janssens et al. 2005 ).
8.4 Increasing the Size of the Soc Stock for the Cropland
Category
To increase the size of the cropland SOC pool, and at the same time reducing the
emission from soil, it is important to take into account the different mitigation
options. In fact, trying to increase the size of the soil C stock means increasing
the C input, decreasing the output or a combination of the two through improved
management. Carbon sequestration can also occur through a reduction in soil
disturbance because more C is lost from tilled soils than from soils that are less
disturbed. Measures for reducing soil disturbance include reduced or zero tillage
systems, set-aside of lands and the growth of perennial crops. Measures for increas-
ing soil C inputs include the preferential use of animal manure, crop residues, sew-
age sludge, improved rotations with higher carbon inputs to the soil, and in some
cases fertilisation/irrigation management to increase productivity. According to
Freibauer et al. ( 2004 ) and to Smith et al. ( 2000a , b ), the impact of such practices
can be estimated: for example permanent set-aside or zero tillage might result in
a maximum C sequestration potential of 0.4 Mg C ha − 1 y − 1 , the use of perennial
crops or deep rooting crops in 0.6 Mg C ha − 1 y − 1 , while changing from conven-
tional to organic farming result in a sequestration potential of 0.5 Mg C ha − 1 y − 1 .
A practical example for Italy can be done considering the 2007, the more recent
year for which ISTAT report the areas occupied by the different cropland subcat-
egories (ISTAT 2010 ). The average SOC stock in 2007 can be obtained by apply-
ing the annual SOC net loss rate of 0.16 Mg C ha − 1 y − 1 estimated in this study
to the average SOC stock found in 2000 for each subcategory and multiplying the
resulting stocks for the area each subcategory occupy. This calculation results in a
total SOC stock for the whole cropland category of 489.7 148.2 Tg C (Fig. 8.3 ).
Consequently some future scenarios can be hypothesised (e.g. for 2020) assuming
no further variations in the area occupied by each subcategory.
In 2007, the ISTAT report that about half of the surface occupied by arable
lands (3.48 Mha) is currently ploughed. About 60 % of this area is ploughed