Paysages virtuels et analyse de scénarios pour évaluer les impacts ...

Fertilization practices
We considered fertilization practices based on organic manures, which are characterized by a given balance
of Nitrogen (N), P, and Potassium (K). The type of manure spread each year on a given field was randomly
set to a given type with an overall proportion of 30%, 55% and 15%, respectively for cattle, pig and poultry
slurries. Two fertilization scenarios were successively implemented:
− N-based fertilization during the first 20 years: the amount of organic manure spread on a given field
is derived from crop needs in N, with an upper limit of 170 kg N/ha fixed by regulation. P loads were
then estimated using the P/N ratios of the different slurries (Table A.I.12.a);
− P-based fertilization during the following 30 years: the amounts of organic manure spread on a given
field is derived from crop needs in P, soil P content and fertilization background (Schvartz et al.,
2005). Preinforced is the soil P content below which it is recommended to increase the P load spread on
the field. Pskip is the soil P content above which it is reasonable not to spread manure on the field,
although the method forbids skipping fertilization two years in a row. We used the numerical values
recommended on the actual region (Brittany) of the studied site (Table A.I.12).
Both fertilization scenarios were applied at field-scale to reflect the management units of the farmers, where
the soil P contents used were calculated from the mean value of the pixels within each field. The switch of
fertilization practices accounted for the implementation of a new legislation concerning best management
practices and by a change of perception in the agricultural community towards environmental issues.
Soil P budget
Inputs of the soil P budget model were P loads from fertilizers, outputs are crop P offtakes and a fraction of
the soil extractable P which annually turns into a non-extractable form. The soil P stock of a given pixel i at
the end of year y can be calculated as follows:
Pi,y=(1-k)Pi,y-1 +Pfi,y-PEi,y
where Pi,y is the extractable soil P content at year y of pixel i, k is the proportion of extractable soil P which
turns into a non-extractable form, PFi,y is the P load input by P fertilization during year y on pixel i, and PEi,y
is the extractable P exported from pixel i by the crop harvested on year y. Phosphorous exports were
calculated at the pixel scale following the estimation of crop P needs modified with a stochastic factor:
Annexes I : Virtual landscapes to assess monitoring strategies – p. 177
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