Integrating plant-plant competition for nitrogen in a 3d individual-based model simulating the effects of cropping systems on weed dynamics
Résumé
Promoting biological weed regulation by shifting resource
availability and use from weed to crop may provide
an option for a more sustainable weed management.
Light is generally the main resource for which
crops and weeds compete in conventional cropping
systems. But, with the necessity to reduce mineral nitrogen
fertilizer use, better managing crop-weed competition
for nitrogen may become crucial. However, it
requires better understanding the functioning of heterogeneous
canopies in nitrogen-deficient situations. Simulation
models are powerful tools to reach this goal.
Our objective was to integrate plant-plant competition
for nitrogen into the FlorSys model, already simulating
competition for light. The final aim was to provide the
first mechanistic and 3D individual-based model simulating
the effects of cropping system and pedoclimate
on weed dynamics, integrating competition for nitrogen.
The new formalisms were mostly inspired from pre-existing
models and adapted to make them compatible
with the individual-based representation of FlorSys.
Soil-nitrogen concentration is predicted by the STICS
soil submodel linked to FlorSys. Plant nitrogen uptake
was simulated by confronting plant nitrogen demand
(driven by shoot growth) to plant nitrogen supply (depending
on root characteristics, soil-nitrogen availability
and the presence of neighboring plants with roots in
the same soil zone). Competition for nitrogen occurred
when the amount of nitrogen available in a soil voxel
(i.e. 3D soil pixel) was lower than the requirements of
all the plants with roots in this voxel. A nitrogen stress
index allowed to account for the impact of plant nitrogen
nutrition on plant photosynthesis, biomass allocation
and morphology. To reflect the plant adaptation to
the spatial heterogeneity in soil-nitrogen availability,
we introduced ‘compensation’. For a given plant, if nitrogen
uptake in one soil voxel is insufficient to fulfil
plant nitrogen requirements in this voxel, this local nitrogen-
deficiency could be compensated by increasing
nitrogen uptake in other nitrogen-richer voxels. The
new formalisms needed only seven plant parameters
which we measured for several crop and weed species.
Simulations showed that, despite simplifying hypotheses
in formalisms, predictions were consistent
with knowledge on canopy functioning and crop-weed
interactions.
The nitrogen version of FlorSys will be useful to understand
the role of nitrogen in crop-weed interactions
and to identify sustainable management strategies
promoting weed regulation by competition (see Perthame
et al., this congress). Due to its process-based
representation and genericity (it can simulate diverse
crop species), it will also be useful to better understand
crop-crop interactions in intercropping.