Agriculture Reference
In-Depth Information
[18]. This phenomenon is manifested by a proliferation of green algae,
reduced infi ltration of light, oxygen depletion in surface water, disappear-
ance of benthic invertebrates and the production of toxins harmful to fi sh,
livestock and humans. Soils are also at risk from eutrophication, as exces-
sive amounts of nutrients can cause oxygen depletion in the soil and thus
prevent the proper functioning of natural microorganisms. This, in turn,
affects soil fertility. Moreover, it has been reported that synthetic N fertil-
izers can promote microbial C utilization depleting both soil and sub-soil
organic N content [4]. Eutrophic soils are the source for the emission of
N
2
O (nitrous oxide), which can react with the stratospheric ozone [19],
thus increasing the greenhouse effect and also the emission of toxic am-
monia (NH
3
) into the atmosphere that can contribute to acidifi cation [20-
22]. The process of gaseous ammonia loss from plant foliage can range
from 2 to 15kg N/ha/year released, depending on the crop examined or the
location [23,24]. Additionally, when the plant does not take up urea fertil-
izers applied to the soil, up to 40% can also be lost in the form of ammonia
[25,26].
Mineral N fertilizers produced by the Haber-Bosch process are very
costly in energy production [1,27] and represent nowadays up to 50% of
the operational cost for the farmer depending on the cultivated crop [28].
Thus, NUE and energy input are seen as important indicators for the en-
vironmental impact of the production of conventional crops but also of
energy crops, since they have a large capacity to produce biomass with
the minimal amount of N fertilizer [29]. Comparatively, the net energy
cost of N
2
fi xation in leguminous species is lower than that necessary for
an equivalent production of synthetic N fertilizers [30,31]. Therefore, it
will be advantageous to the farmer to include more legumes both in crop
rotations and in cover crops, whether the main cultivated crop is grown for
grain or biomass.
Biological dinitrogen (N
2
) fi xation is one of the most important sources
of N in agricultural system, since it has been estimated to be around 122
Tg per year. The most important N-fi xing agents are the symbiotic associa-
tions between crop and forage/fodder legumes and bacteria of the genus
Rhizobia
[31,32]. There are accurate estimations of annual inputs of sym-
biotically fi xed N by legume crops. However, the amount of N fi xed by
other agricultural production systems involving non-symbiotic N
2
fi xing
