ENERGY USE EFFICIENCY IN BIOMASS PRODUCTION SYSTEMS - Economics of Sustainable Energy in Agriculture - page 73

differences are caused by differences in climate (air temperature and solar
radiation). The extremes found in Nonhebel (1997) are used here to
evaluate the differences in resource use efficiency between high and low-
input systems.
The following production systems were defined:
Potential production of poplar in North Western Europe,
with an annual yield of 12 ton/ha of stems and 5 ton/ha of leaves. In this
part of Europe, the annual precipitation is sufficient to prevent water
shortage and potential production can be achieved without irrigation.
Within this production system, the crop is fertilised and crop protection
measures are taken.
Potential production of poplar in Portugal may be 43
ton/ha/y for the stems and 18 ton/ha/y for the leaves. However, to obtain
this production level, irrigation is required.
Furthermore, two low-input systems in the same climatic regions
were recognised the yield of these systems is estimated at 5
ton/ha/y stems (and 2 ton/ha leaves). It is assumed that this yield can be
obtained without irrigation and use of pesticides.
2.2 Determination of the required inputs
All systems have to be initiated, which means that for all systems, energy
is required for soil cultivation, planting and weeding. Furthermore, after
each harvest (every four years) weeding has to be done in all systems. Data
required to quantify the energy costs of these actions were obtained from
Eriks et al. (1991), Lysen et al. (1992) and Hall et al. (1993).
With respect to fertilisation, only nitrogen is taken into account.
Production of this nutrient requires a lot of fossil energy. The amount of
nitrogen required is yield-level dependent: all nitrogen in the harvested
material (stems) is replaced and some losses are taken into account. The
nitrogen in the leaves is expected to remain in the system. Data from
Nilson and Eckerston (1983); Eckerston and Slapokas (1990); Eriks et al.
(1991) and Nonhebel (1997) were used. For the low-input systems, it is
only at the start of the plantation that some nitrogen is required (the
nitrogen in the leaves). Throughout the rest of the life span of the
plantation, the annual nitrogen deposition from the air (about 25 kgN/ha) is
sufficient to meet the requirements of the crop.
Furthermore, it is assumed that in high-input systems, crop
protection measures take place: the crops will be sprayed with pesticides
three times a year. Energy requirements for these measures were derived
from Pimentel (1980) and Eriks et al. (1991).