This approach changes the system boundary during the LCI. Consequently,
all crops and thus the effects between them are included in
the LCA. This happens because nutrient flows and crop-rotation effects
between crops do not cross a system boundary without consideration.
Both the subsequent crop and the studied crop belong to the same
crop rotation; thus, the nutrients and positive effects remain inside
the system boundary. Thus, the positive effects between crops are included
via improved yields in the LCA.
Even though modifications to the system boundary are well-known
in LCA practice, they are not widely used in LCAs for agricultural systems.
One reason for that is the vast number of different outputs from
one crop rotation and the enormous effort necessary to handle so
many outputs. This is particularly the case when product-specific LCAs
are performed. Integrating the entire crop rotation into the inventory
analysis certainly introduces higher effort for practitioners, as well
higher complexity to the assessment and – at a first glance – additional
allocation problems are caused. But the LCA gains a better understanding
of agricultural processes; the task of attributing environmental
burdens to the vast number of different agricultural products and coproducts
can be shouldered by an agriculture-specific allocation
approach, such as the Cereal Unit allocation. Of course, introducing
allocation is connected to assumptions and value choices; an extensive
discussion in the context of Cereal Unit allocation is given in
Brankatschk and Finkbeiner (2014). The Cereal Unit is suitable for
depicting all agricultural products and co-products. It organizes different
agricultural outputs, making them comparable to each other and
introducing computability. This serves as basis for an agricultural specific
allocation approach (Brankatschk and Finkbeiner, 2014). Within the
LCI, the Cereal-Unit-allocation approach is used to allocate all inputs
uniformly to the individual outputs. Thus, agricultural inputs that are
applied in one vegetation period but not used in the same period to
grow crops or contribute to some extent towards the growth of other
crops in the crop rotation can be more fairly attributed to the overall
crop rotation. Thus, the crops currently regarded as ‘single players’,
can be considered as ‘team players’ within the crop rotation, since
their interactions are taken into account.