What is the contribution of the residual weed floras to reduce nitrate leaching?
Résumé
Integrated crop protection tolerates residual weed floras if they are not harmful for crop production.
Weeds are often solely considered with a negative viewpoint, but they may also provide beneficial
services for agroecosystems, such as plant biodiversity promotion and trophic resources for other
organisms such as pollinators. The role of residual weed flora as nitrate catchers, during the summer
fallow, has seldomly been assessed, in spite of the high potential of some weed species to take up
nitrogen. The present study aimed to (1) develop an indicator to account for the potential role of
weed floras as nitrate catchers at the field scale, and (2) calculate this indicator from the outputs of
a weed dynamics model (FlorSys) in order to analyse, with simulations, whether weed floras may,
in some situations (to be identified), significantly contribute to reduce potential nitrate leaching,
while limiting negative impacts on crop production. When developing the indicator, we considered
that the potential of weed floras as nitrate catchers depended on both the ‘nitrophilic’ level and
the plant leaf area of the species present in the weed floras, during the period running from the
harvest of the preceding crop to the beginning of water drainage. This indicator was used to predict
the potential of nitrate catching by weed floras in cropping systems from six regions from France
and one from Spain which were simulated with the FlorSys model. Data analysis (under progress)
will determine trade-offs between this indicator and other indicators of weed harmfulness for crop
production and weed contribution to plant and functional biodiversity. It will help to identify in
which production situations residual weed floras can play a significant role in preventing nitrate
leaching, while minimising negative weed impacts, such as crop yield loss and harvest pollution by
weed seeds.