Environmental Engineering Reference
In-Depth Information
11.10 (
a
) Biospheric sulfur cycle. (
b
) Anthropogenic sulfur
emissions, 1850-2000. From Smil (2002).
Applications of N compounds amounted to about 85
Mt N in 2005, and it is unlikely that organic recycling
and legume crops added more than 35-40 Mt N/a
(Smil 1999b). The third most important human interfer-
ence in the global nitrogen cycle is the high-temperature
combustion of fossil fuels: it released more than 30 Mt
N, mostly as nitrogen oxides (NO
x
), in 2005. In aggre-
gate, anthropogenic mobilization of some 150 Mt N/a
is roughly equal to the annual rate of natural biofixation
by symbiotic and free-living bacteria (Galloway and
Cowling 2002). An excess of reactive N acidifies precipi-
tation, lowers soil pH, causes eutrophication of fresh and
coastal waters, and raises concerns about the long-term
effects on biodiversity and productivity of grasslands
and forests (Bergstr¨m and Jansson 2006; Phoenix et al.
2006). As already mentioned, NO
x
are key precursors of
photochemical smog, and N
2
O, released during denitrifi-
cation, is an important greenhouse gas.
Sulfur is an even rarer component of living molecules
than is nitrogen. Only 2 of the 20 amino acids that
make up proteins (methionine and cysteine) have the
element as a part of their molecules, but every protein
