Agriculture Reference
In-Depth Information
organisms, accelerating microbial respiration and depleting oxygen through eutrophica-
tion. Although eutrophication in freshwater lakes has declined slightly since the 1990s
(Conley et al., 2009), it remains a major global threat to coastal and some freshwater eco-
systems. For example, the Gulf of Mexico contains one of the world's largest areas affected
by eutrophication and has a resulting “dead zone” that can encompass as much as 18,000
km 2 (Turner and Rabalais, 2003). This problem is related to the release of large quantities
of U.S. Midwest-derived NO 3 - into the gulf via the Mississippi River basin, which drains
the heavily fertilized U.S. corn belt (David et al., 2010). NO 3 - -N fluxes from the Republic of
Korea, northeastern United States, and Mississippi River Basin are estimated at roughly
1,700, 1,200, and 600 kg N km -2 y -1 , respectively (Howarth et al., 2008), compared to 100 kg
N km -2 y -1 in ecosystems with fewer anthropogenic N inputs (Howarth et al., 2002).
The denitrification process releases N 2 O, NO x , and N 2 into the atmosphere. N 2 is the
unreactive end product of denitrification that completes the global N cycle. NO x contrib-
utes to the formation of tropospheric ozone pollution and the production of acid rain,
while N 2 O depletes the stratospheric ozone layer and is a potent greenhouse gas with a
global warming potential (GWP) nearly 300 times that of carbon dioxide. Atmospheric
N 2 O concentrations have increased from 270 ppb prior to industrialization to 320 ppb
today and continue to rise at an estimated rate of 0.3-0.6% y -1 . Nearly 70% of current N 2 O
emissions are linked to fertilizer use (Smil, 1999; Halvorson et al., 2008), and fourfold
increases in emissions due to expanded synthetic fertilizer use are projected to occur by
2100 (Intergovernmental Panel on Climate Change [IPCC], 2001). Currently, annual N 2 O
emissions from U.S. croplands are estimated to be upward of 500 Gg (U.S. Environmental
Protection Agency [EPA], 2009). Nitrogen losses to the atmosphere can also occur through
volatilization of NH 3 . Volatilization rates are dependent on NH 4 + and NH 3 concentrations
in the soil, which are a function of pH and can be high where fertilizers are broadcast on
the surface and soil pH is greater than 8. NH 3 emissions from animal wastes and fertilizers
increased from 6.6 Tg N y -1 in 1860 to 32 Tg N y -1 in 1993 and are projected to rise exponen-
tially to 77 Tg N y -1 by 2050 (Beusen et al., 2008; Bouwman et al., 2009).
Agricultural N that enters the atmosphere and freshwater and marine environments
can also have strong indirect effects on ecosystem structure and function. Chronic N
deposition is pervasive in many forest ecosystems and has led to excess N in these previ-
ously N-limited ecosystems. Northeastern U.S. forests and parts of Eastern Europe have
experienced some of the highest levels of N deposition and in some cases have begun
to show declines in forest productivity (Aber et al., 1998; Lamersdorf and Borken, 2004)
and plant diversity (Bobbink et al., 2010). Several studies have shown that N deposition
increases NO 3 - leaching and denitrification (Aber et al., 1998; Fang et al., 2011) and can have
varying effects on decomposition dynamics (Berg and Matzner, 1997; Waldrop et al., 2004;
Grandy et al., 2008).
Nitrogen fertilizer use has greatly increased agricultural productivity and played a
major part in sustaining current population levels. Synthetic N fertilizers will become
even more important in the decades ahead as the human population approaches 9 bil-
lion people. Sustaining agricultural productivity while maintaining or even enhancing
other ecosystem services depends on improving nitrogen use efficiency (NUE). NUE is
a function of complex interactions between plant-, soil-, and management-related factors,
but at the core of NUE is the concept of synchrony. Synchrony refers to whether N avail-
ability coincides with plant N demand: Most agricultural systems are highly susceptible
to environmental N losses because N availability in space and time is not closely aligned
with plant N needs. This is due not only to fertilizer mismanagement and variable envi-
ronmental conditions but also to our lack of understanding of the biological processes that
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