References

168 L. Philippot and J.C. Germon
losses of nitrogen compounds generating pollution in the environment
and more specifically NH + 4 ,NO−3 and NO−2 in surface water, and NH3,NOx
and N2O in the atmosphere.
ThefluxdataadaptedfromTiedje(1988)highlighttheuncertaintiesin
global estimations of N inputs and outputs and confirm that the questions
on a general equilibrium in global N cycle are unresolved (Table 1).
4.1
Biological Nitrogen Fixation
According to Newton (2000), 65% of total nitrogen fixed is provided by
biological nitrogen fixation (BNF), 25% by industrial fertilizer production
and 10% by abiotic, natural processes (lightning, combustion and
volcanic activity). BNF in the terrestrial ecosystem is in the order of 90–
130 Tg N year −1 (Galloway 1998). However, the quantities of biologically
fixed nitrogen vary with the microflora involved and the kind of association
with plants, and also with environmental conditions. BNF by free
bacteria in cultivated soil from temperate areas does not exceed several
kg Nha −1 year −1 . BNF by endophytic diazotroph bacteria colonizing sugarcane
rhizosphere can reach 170 kg Nha −1 year −1 in Brazil (Baldani et al.
2000). In water-submerged rice fields, BNF is due to free cyanobacteria
and diazotrophic bacteria colonizing rice roots. Fixed nitrogen can vary
from several tens to 150 kg Nha −1 year −1 (NRC, 1979, mentioned by Atlas
and Bartha 1993) and could provide the equivalent to 20% of the total nitrogen
incorporated into the plant (Baldani et al. 2000). Similar orders are
observed with the symbiotic association between a cyanobacterium (Anabeana)andaplant(Azolla)usedasgreenmanureintropicalareas(Vance
1998).
However, in the terrestrial habitat, the symbiotic fixation of nitrogen
by rhizobia accounts for the largest contribution of combined nitrogen.
The rates of nitrogen fixation by symbiotic rhizobia is often two or three
orders of magnitude higher than rates exhibited by free-living nitrogenfixing
bacteria in soil. Thus, BNF obtained with the symbiotic associations
between rhizobia and leguminous plants varies from several tens to
350 kg Nha −1 year −1 depending on the associations. BNF obtained with
nonleguminous angiosperms producing symbiosis with Frankia is between
15 and 77 kg Nha −1 year −1 for Casurarina equisetifolia (Dommergues
1997), 29 and 117 kg Nha −1 year −1 for Alnus nepalensis (Shrama 1993) and
18 kg Nha −1 year −1 for Myrica faya (Vitousek and Walker 1989).
An estimation of BNF by human-induced cultivation of rice and legumes
ranges between about 30 and 50 Tg Nyear −1 (Galloway 1998). Furthermore,
BNF efficiency depends on mineral nitrogen availability and is tremen-