4. Discussion
In current LCA practice, crop-rotation effects are only partly included,
because it is difficult to quantify them, e.g. it is impractical to
measure each nutrient flow in the soil — these data are not easily
accessible, and they are not typically gathered in field experiments
over decades. LCAs today typically assess each crop independently of
its crop rotation and thus inadvertently ignore most of these croprotation
effects, even though they are crucial for maintaining soil fertility
and therefore are relevant for the sustainability of the agricultural
system. To help consider these difficult-to-measure nutrient flows and
difficult-to-quantify crop-sequence effects, we suggest this method.
This supplemental approach is theoretically suitable for all agricultural
LCAs. It takes into account all inputs and all outputs of the crop rotation
and thus includes inter-crop relations, as well. For instance, this
approach would make it possible to appreciate the benefit of legumes
in fixing N and providing it to other crops in the rotation.
Within recent LCA practice, it is not obligatory to consider nutrient
shifts from one crop to subsequent crops. Thus, fertilizing efforts are attributed
to one single crop. This leads to free-rider situations for crops
that consume nutrients left by preceding crops on the field (e.g. in
crop residues). In this sense, subsequent crops are at an advantage
and do not get charged for their true nutrient consumption because
they receive some of the fertilization of the previous crop; subsequently,
other crops within the crop rotation carry more environmental burden
than is physically true. In other words, if LCA is performed for one individual
crop, it does not consider that the crop may either enjoy the benefits
or suffer the burden of being part of a crop rotation. In this context,
it is worth mentioning that crops leaving high amounts of nutrients in
the soil are, in recent LCAs without nutrient-compensating measures,
systematically disadvantaged. This is because the full amount of fertilizers
applied during their vegetation period is allocated to them, even
if they do not consume all of it themselves, and some of their nutrients
are transferred to the subsequent crop via their crop residues. By
adapting system boundary to the crop-rotation level, free-rider phenomena
for nutrient-receiving crops and systematic disadvantage of
nutrient-lending crops can be avoided