The results showing the EROEI can however be argued to be
subject to uncertainties related to the energy requirements for
transporting the biomass, harvest equipment and fertilizer application.
For habitats located at islands, near fjords or similar areas, the
transport distance and method are expected to vary significantly
from the assumptions in this study as it will require oversea transport
or large detours around the water territories. This will increase
the energy requirements, thus the EROEI must be expected to be
lower for biogasplants withsupply areas influencedby suchfactors.
In such cases it may be more feasible to transport the biomass to a
different biogas plant although it might not be the closest facility.
When estimating the EROEI, the energy inputs for fertilisation
with digestate were included. If assuming that only potassium fertilisation
combined with harvest is applied as an initial method of
nature conservation (for more intensive removal of soil nutrients),
the energy investments for transporting and applying the fertiliser
could be reduced. Sources of potassium could be animal manure,
chemical fertilisers or vinasse. Vinasse is a by-product from the
sugar industry and is approved by the Danish Plant Directorate to
be used for organic farming, and thus it is considered to be a suitable
for application on the meadow habitats. The fact that vinasse
can be acquired as a dry product is of significant importance for
the energy balance, whereas some of the other sources e.g. animal
manure contain high concentrations of water requiring much more
energy for transportation. Application of up to 115 kgK/ha has been
found to increase productivity of low nature quality meadow habitats
and yields of more than 9 t TS/ha were obtained (Nielsen and
Hald, 2008; Lærke et al., 2012). On longer terms as the concentration
of soil nutrients depletes and the productivity of the habitats
decreases, continued application of potassium is not considered
feasible for improving the nature quality.