Agriculture Reference
In-Depth Information
energy. For this reason it was called diazo by Lavoisier, meaning “no life” (Mora
and Lara 2000 ). According to Voroney and Derry ( 2008 ), approximately 8.7 % of
the total nitrogen is present in the surface of the terrestrial system, being distributed
as follows: (1) about 26 % is geological N-N embedded into rocks; (2) approxi-
mately 74 % is N 2 , including over 99.9 % of atmospheric nitrogen; and (3) the
remaining, about 0.03 %, is reactive (or fixed) nitrogen in air, water, and earth. The
nitrogen in soil can be generally classified as organic or inorganic. The organic
nitrogen occurs as slowly labile organic nitrogen, immobilized in the microbial
biomass in residues of plants and animals or fixed in clays, and as a constituent of
free amino acids or proteins, amino sugars, and other complexes (Victoria
et al. 1992 ). With respect to the inorganic nitrogen in soil, it occurs as ammonium
(NH 4 + ), nitrite (NO 2 ), nitrate (NO 3 ), nitrous oxide (N 2 O), nitric oxide (NO), and
elemental nitrogen (N 2 ). The last form is inert, except when used by nitrogen fixer
microorganisms. From the soil fertility point of view, NH 4 + ,NO 2 , and NO 3 are
the most important forms, arising from the aerobic decomposition of organic matter
in soil or from the addition of various commercial fertilizers. Also, it is noteworthy
that N 2 O and NO are toxic gases for environment (Tisdale et al. 1985 ).
Chemical or synthetic fertilizers are the most important sources of nitrogen used
in large-scale cultivation of crops (Chien et al. 2009 ). Usually, nitrogen is used in
combined forms, for instance, ammonium nitrate or other ammonium salts or
nitrates (ammonium sulfate, calcium nitrate, potassium nitrate, etc.) and urea.
Urea ((NH 2 ) 2 CO) is the most common source of nitrogen, covering nearly half of
the worldwide need for nitrogen-based fertilizers. It has been widely used due to its
high nitrogen content and easy application, in dry granular form or as aqueous
solutions (Saggar et al. 2013 ). Also known as carbamide, urea is an endogenous
product from protein and amino acid catabolism. All industrial processes to produce
urea involve the reaction of ammonia (NH 3 ) and carbon dioxide (CO 2 ), at elevated
temperature and pressure to form ammonium carbamate, which is then dehydrated
to form urea (Lewis 1997 ). The concentration of biuret (an impurity usually found
in low concentrations) in urea deserves special attention due to its phytotoxicity
(Mikkelsen 1990 ). Biuret levels of about 20 % can be tolerated in most fertilization
programs. Citrus and other crops, including pineapple, are more sensitive to biuret
in urea, applied by foliar spraying. In this way, a product containing less than
0.25 % of biuret is recommended for those crops (Albrigo 2002 ; Achor and Albrigo
2005 ).
When urea is added to the soil, it is first hydrolyzed to NH 4 + by urease enzymes
(Eq. ( 11.1 )). This process depends on the number of active urease molecules and
factors that affect their activity, such as urea concentration, organic matter content,
temperature, humidity, and soil pH (Tabatabai and Bremner 1972 ; Wang
et al. 2004 ). In a second step, the NH 4 + is oxidized to NO 3 by the nitrification
process (Eqs. ( 11.2 ) and ( 11.3 )) (Cartes et al. 2009 ):
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