Agriculture Reference
In-Depth Information
Ladha et al. (2005) estimated that synthetic fertilizers supplied approximately
84 Mt of N, or about 45% of the total N input for global food production in 2005. They
estimated other inputs to crop production as 33 Mt from biological nitrogen fixation,
16 Mt from recycling of N from crop residues, 20 Mt from atmospheric deposition,
and 24 Mt from irrigation water. However, it is important to remember that the N
from crop residues and much of that from irrigation water was not newly introduced
to the system, but recycled within the system. Ladha et al. (2005) estimated that
the amount of N removed by the harvested crop was almost equal to the amount
of N added as fertilizer, but that 37 Mt was lost to the environment by leaching,
runoff, and erosion; 21 Mt by ammonia volatilization from animal wastes, soil, and
vegetation; 14 Mt from denitrification; and 8 Mt from nitrous oxide and nitric oxide
emissions.
Although the losses discussed above are large and may be causing serious disrup-
tions in ecosystem functions, much research has been conducted in recent decades
to improve nitrogen use efficiency. There is also increasing awareness among pro-
ducers, as well as the public, that practices must be developed and used that better
synchronize the supply and requirement of N fertilizer for crop production. While
more improvement is needed, nitrogen use efficiency has improved significantly in
recent years. The use of synthetic N as fertilizer began in earnest following World
War II, and the results were astonishing. The positive results, coupled with the low
cost of the product, led farmers to apply more and more N. Although studies showed
that as the amount of N added increased there was less increase in yield for each
added increment, the guideline was to continue to add N until it was evident that
additional N would not lead to a further increase in yield. There was no thought
at the time among most scientists and producers that there were negative environ-
mental effects associated with excess N additions. However, things began to change
when scientists such as Rachel Carson (1962) in
Silent Spring
and Barry Commoner
(1971) in
The Closing Circle
began raising concerns about chemicals used in crop
production. Commoner was particularly concerned about nitrates in groundwater
and their potential for causing methemoglobinemia (commonly known as blue-baby
syndrome) in infants. Also, the US Environmental Protection Agency (EPA) was
created in 1970. These concerns became great enough that scientists began seri-
ously considering the effects of agricultural chemicals and practices on the environ-
ment, and particularly how they affected air and water quality and human health.
This change is clearly reflected with the data shown in
Figure 4.3
. Between 1964
and 1976, the rate of N fertilizer applied per unit area of corn in the United States
increased almost threefold while yield increased by about 50%. Since 1976, the aver-
age rate of N fertilizer has actually declined slightly, while the yield has increased
almost every year and is almost two times higher. These data show that nitrogen use
efficiency has greatly increased, and assuming the N content of corn grain is 1.6%,
the amount of N removed with the grain is essentially equal to the amount of N
applied as fertilizer. Therefore, the percentage of N fertilizer used for crop produc-
tion that is lost to the environment has been significantly reduced in recent years.
Europe has also made significant progress, but other areas such as China (Miao et al.
2010) have increased N fertilizer use so rapidly and to such high levels that serious
environmental concerns are being raised.
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