4 Soil Biogeochemistry Reflects This Trajectory
Because of the large size of these element pools, C, N and P metabolism in cropland
soil is largely affected by the long-term structural changes in the agro-food system
and agricultural practices described in the previous section.
4.1 Soil Organic Carbon Storage
Organic C storage in agricultural soil is determined by the balance between
(1) humified organic C inputs from above- and below-ground crop residues and
manure application and (2) soil organic matter mineralisation depending on the
soil content of labile organic matter and pedoclimatic properties [30]. C sequestration in agricultural soil therefore always reflects a long-term temporary imbalance
between C inputs to soil, which are determined by agricultural practices and soil C
mineralisation [31]. The dynamics of organic C storage in crop and grassland soils of
the Seine basin over the 1850–2015 period was calculated through the application
of the AMG model [32, 33], using the GRAFS estimation of humified C inputs
[30] (Fig. 6a, b). A low rate of C sequestration occurred during the second half
of the nineteenth century, followed by a destocking period until 1950 (Fig. 6c).
Then a period of enhanced C sequestration occurred during the phase of rapid
modernisation of the basin’s agriculture, due to increased net primary production,
and associated underground residue inputs, counterbalancing the reduction in
manure inputs in the regions adopting stockless cropping systems. The mean rate
of C sequestration then gradually levelled off and is now close to 40 kgC/ha/year,
i.e. about 1‰ of the Corg stock in the top 30-cm top of soil. However, in the NorthWestern regions of France, destocking of organic C in cropland soil occurs (Fig. 6d).
These results are consistent with the predictions of the STICS model concerning the
variations of soil organic nitrogen (SON), which are strongly linked with those of
soil organic carbon (SOC) in arable crops (Fig. 6e). The current agriculture in France
is therefore far away from the objective assigned by the COP21 of an annual 4‰
increase in soil organic C content to counterbalance anthropogenic CO 2 emissions
and mitigate climate change (the so-called 4‰ initiative; http://www.4p1000.org/
[34]). This is reinforced by the fact that total agricultural area has significantly
decreased in the same period, at a rate comparable to that of organic C storage.
4.2 Agricultural Greenhouse Gas Emissions
Besides C emissions related to a possible negative C soil balance, greenhouse gas
(GHG) emissions by the agricultural sector include N 2 O emissions linked to nitrification and denitrification in cropland and grassland soils, CH 4 emissions mostly related
100
G. Billen et al.
Because of the large size of these element pools, C, N and P metabolism in cropland
soil is largely affected by the long-term structural changes in the agro-food system
and agricultural practices described in the previous section.
4.1 Soil Organic Carbon Storage
Organic C storage in agricultural soil is determined by the balance between
(1) humified organic C inputs from above- and below-ground crop residues and
manure application and (2) soil organic matter mineralisation depending on the
soil content of labile organic matter and pedoclimatic properties [30]. C sequestration in agricultural soil therefore always reflects a long-term temporary imbalance
between C inputs to soil, which are determined by agricultural practices and soil C
mineralisation [31]. The dynamics of organic C storage in crop and grassland soils of
the Seine basin over the 1850–2015 period was calculated through the application
of the AMG model [32, 33], using the GRAFS estimation of humified C inputs
[30] (Fig. 6a, b). A low rate of C sequestration occurred during the second half
of the nineteenth century, followed by a destocking period until 1950 (Fig. 6c).
Then a period of enhanced C sequestration occurred during the phase of rapid
modernisation of the basin’s agriculture, due to increased net primary production,
and associated underground residue inputs, counterbalancing the reduction in
manure inputs in the regions adopting stockless cropping systems. The mean rate
of C sequestration then gradually levelled off and is now close to 40 kgC/ha/year,
i.e. about 1‰ of the Corg stock in the top 30-cm top of soil. However, in the NorthWestern regions of France, destocking of organic C in cropland soil occurs (Fig. 6d).
These results are consistent with the predictions of the STICS model concerning the
variations of soil organic nitrogen (SON), which are strongly linked with those of
soil organic carbon (SOC) in arable crops (Fig. 6e). The current agriculture in France
is therefore far away from the objective assigned by the COP21 of an annual 4‰
increase in soil organic C content to counterbalance anthropogenic CO 2 emissions
and mitigate climate change (the so-called 4‰ initiative; http://www.4p1000.org/
[34]). This is reinforced by the fact that total agricultural area has significantly
decreased in the same period, at a rate comparable to that of organic C storage.
4.2 Agricultural Greenhouse Gas Emissions
Besides C emissions related to a possible negative C soil balance, greenhouse gas
(GHG) emissions by the agricultural sector include N 2 O emissions linked to nitrification and denitrification in cropland and grassland soils, CH 4 emissions mostly related
100
G. Billen et al.