Agriculture Reference
In-Depth Information
high rates of global N fertilizer inputs, coupled with the inefficient recovery of N by crops,
have increased biologically reactive soil N at farm, regional, and global scales (Hoang
and Alauddin, 2010), with far-reaching environmental consequences, including declines
in water quality and biological diversity and increases in atmospheric trace gas concentra-
tions. Developing integrated agricultural systems that efficiently cycle N is essential to
maintaining and increasing agricultural productivity and minimizing declines in envi-
ronmental quality and ecosystem services. To do this, we need to improve our manage-
ment of inorganic fertilizer inputs and our understanding of the soil biological processes
that regulate N transformations.
6.2 On-farm consequences of nitrogen management
Increases in both biological and inorganic nitrogen use in agricultural systems have sub-
stantially improved crop yields but not without unintended environmental trade-offs.
Excess N in agricultural soils can lead to increases in soil acidification, accelerated soil
organic matter (SOM) turnover, and altered microbial community structure and function.
Soil acidification is a potentially negative side effect of nitrification and other N trans-
formations and can be exacerbated by the coupling of base cations to NO
3
-
leaching. Soil
acidification is a widespread problem in areas that frequently experience high fertilizer
application rates, including the U.S. Great Plains, parts of Eastern Europe, and interior
China (Tarkalson et al., 2006; Guo et al., 2010).
The soil biota, which promote soil aggregation and regulate nutrient mineralization and
trace gas emissions, are also strongly influenced by N fertilizer applications, although the
direction of change for particular taxa is often difficult to predict. For example, increases,
decreases, and no change in microbial biomass following fertilizer applications have been
detected (O'Donnell et al., 2001; Ramirez et al., 2010), and the duration of reported changes
varies (Ryan et al., 2009; Singh and Ghoshal, 2010). Similarly, synthetic N application in
agricultural systems can increase soil CO
2
emissions and soil C loss (Al-Kaisi et al., 2008;
Russell et al., 2009;
Kwon and Hudson, 2010), but responses remain difficult to predict
(Taylor and Townsend, 2010), and reports showed accumulation (Al-Kaisi et al., 2008; Reay
et al., 2008; Poirier et al., 2009), loss (Hofmann et al., 2009; Khan et al., 2007; Mulvaney et al.,
2009), or no change in soil C (Halvorson et al., 2002).
Agricultural systems are typically managed independently of their surrounding ecosys-
tems, yet N originating in agricultural soils can have far-reaching effects on environmental
quality. Excess N that escapes agricultural fields can have a negative impact on drinking
water and air quality; increase atmospheric greenhouse gas concentrations; alter coastal,
marine, and forest ecosystem productivity and biodiversity; and change SOM decomposi-
tion rates. Galloway et al. (2003) described this as the “cascading effects” of N: One N atom
can be transformed multiple times into a variety of molecular forms (NH
3
, NH
4
+
, NO
3
-
,
NO
x
, N
2
, N
2
O) as it travels through the environment, creating numerous opportunities for
environmental interactions.
The predominant pathways of N loss are through leaching, denitrification, and under
some circumstances, ammonia volatilization (Bouwman et al., 2009). Many of the major
losses of N involve NO
3
-
and are thus dependent on nitrification, the microbially medi-
ated process that oxidizes NH
4
+
to NO
3
-
. Excessive NO
3
-
concentrations in freshwater and
coastal marine ecosystems can trigger rapid growth of heterotrophic and phototrophic
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