Agriculture Reference
In-Depth Information
amendments, leguminous cultures and fallow periods. The formation of
ammonia and thus synthetic N fertilizers by the Haber-Bosch process was
one of the most important inventions of the 20th century, thus allowing
the production of food for nearly half of the world population [1,2]. Con-
sequently, a dramatic escalation has occurred in global consumption of
synthetic N, from 11.6 million tonnes (Tg) in 1961 to 104 Tg in 2006 [3,4].
Over 40 years, the amount of mineral N fertilizers applied to agricultural
crops increased by 7.4 fold, whereas the overall yield increase was only 2.4
fold [5]. This means that N use efficiency, (NUE) which may be defined as
the yield obtained per unit of available N in the soil (supplied by the soil
+ N fertilizer) has declined sharply. This obviously implies that NUE is
higher at reduced levels of crop production when the use of N fertilization
is much lower. NUE is the product of absorption efficiency (amount of
absorbed N/quantity of available N) and the utilization efficiency (yield/
absorbed N). For a large number of crops, there is a genetic variability for
both N absorption efficiency and for N utilization efficiency [6]. More-
over, the occurrence of interactions between the genotype and the level of
N led to the conclusion that the best performing crop varieties at high N
fertilization input are not necessarily the best ones when the supply of N
is lower [7]. This is mainly because breeding for most crops has been con-
ducted over the last 50 years in the presence of high mineral fertilization
inputs, thus missing the opportunity to exploit genetic differences under a
low level of mineral or organic N fertilization conditions [8].
In most intensive agricultural production systems, over 50% and up
to 75% of the N applied to the fi eld is not used by the plant and is lost by
leaching into the soil [9-11]. Some microorganisms are able to improve
soil fertility by metabolizing the N that is not absorbed by plants. It is
however a lengthy process which involves a major risk because mineral
N, especially nitrate (NO
3
-
) and urea {CO(NH
2
)
2
} are very soluble and
can run off into the surface water or fl ow into the groundwater. Water
contaminated by nitrate is not potable and at high concentrations can be a
serious risk for human health [12,13]. Moreover, the water industry must
bear additional costs to remove nitrates from groundwater sources [14,15].
The detrimental impacts of nitrate loss from the soil have toxicological
implications for animals and humans [16] and also on the environment
leading to the eutrophication of freshwater [17] and marine ecosystems
