LOCALIZING THE NITROGEN IMPRINT OF THE PARIS FOOD SUPPLY: THE POTENTIAL OF ORGANIC FARMING AND CHANGES IN HUMAN DIET - Organic Agricultural Practices - page 72

Yield = Ymax (1−exp(−fertilization/Ymax))
(with Ymax = 350 kgNha −1 yr −1 ) is used in the calculation, once the total
fertilisation (manure, legume N 2 fixation and atmospheric deposition) is
known, to calculate both the yield of arable land and the nitrogen surplus
defined as the difference between fertilisation and yield. In the construc-
tion of the OrgLoc scenario, the first step is to assess the size of the live-
stock required to sustain the needs of the local population. The current area
of permanent grassland in the Seine watershed is maintained unchanged.
The size of the nitrogen-fixing arable land (temporary grassland involved
in crop succession and legume fodder crops) is adjusted so as to meet
the feed requirements of livestock with (i) the production of the perma-
nent grassland, (ii) 70% of the production of nitrogen-fixing crops (30%
is considered to be used as green manure, thus not available for livestock
feeding) and (iii) an adjustable fraction of the production of non-legume
crops. The total fertilisation of arable land can thus be calculated as well
as the yield and the nitrogen surplus of arable land. The theoretical sub-
root nitrate concentration below arable land is evaluated by dividing the
surplus by the infiltrated water depth. Finally, the calculated production
of non-fixing cropland which has not been allocated to human or animal
feeding is considered available for export. Table 1 and Fig. 3 summarise
the calculations and compare the results with the current situation.
These calculations show that it is quite possible to conceive a scenario
of organic farming locally meeting the quantitative food requirements of
the current population of the Paris agglomeration and of the other cities of
the Seine watershed, totalling 16.9 million inhabitants, provided that the
livestock density is increased from the current value of 18 LU km −2 to 50
LU km −2 . The basin should still be able to export about 1630 kgNkm −2 yr −1 .
This should be compared to the current cereal export of 4820 kgNkm −2
yr −1 , an amount reduced to a net protein export of 800 kgNkm −2 yr −1 if
the import of feed and animal products, as well as the vegetal proteins
required for the production of the latter, are deducted (Fig. 3a). In this Or-
gLoc scenario, however, the nitrogen surplus, although lower than in the
current situation, would still represent a sub-root water nitrate concentra-
tion double the drinking water standard.