Hatfield J.L., Gandini A. (eds.) Nitrogen in the Environment
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levels show correlations between rates of atmospheric N deposition and nitrate
leaching ( Johnson and Lindberg, 1992 ; Tietema et al., 1997 ). Other factors such as
forest floor C:N ratios ( Emmett et al., 1998 ), forest stand age and successional sta-
tus ( Vitousek and Reiners, 1975 ), and land-use history ( Aber et al., 1997 ; Magill
et al., 1997 ) explain much of the variation in nitrate leaching not attributable to N-
deposition rate. In general, late-stage N saturation with open N cycles is more likely
when N deposition is high; forest stands are either old or late-successional; soils are
poorly buffered (low levels of exchangeable Ca
2
, Mg
2
, K
); and previous man-
agement practices have not removed large amounts of soil N from ecosystems. High
elevation, steep topography, short growing seasons, and exposure to other stresses
(e.g., elevated ozone) can also contribute to nitrate losses ( Fenn et al., 1998 ). A
major question is whether forests receiving low to intermediate levels of N deposi-
tion that are not old or late-successional and are not characterized by low forest floor
C:N ratios or poorly buffered soils will eventually enter the later stages of N satura-
tion as have some forests with high rates of N input ( ⬃ 20 to 60 kg N/ha/year).
A major question regarding the long-term effects of elevated N deposition on
forests is whether forests receiving low to intermediate levels of N deposition will
eventually become N saturated with N cycles that undergo transition from closed
to more open states such as has occurred in some forests subject to high levels of
N inputs ( ⬃ 20 to 60 kg N/ha/year). Answering this question will require con-
tinued long-term study of forest responses to N deposition and improvements in
understanding of controls on N movements between plant, microbial, and nonliving
components of forest ecosystems.
Although anthropogenic N deposition has influenced temperate and high-
latitude forest thus far, social and economic factors are expanding the global distri-
bution of elevated N deposition to subtropical and tropical forests occupying lower
latitudes ( Galloway, 1998 ). As tropical forest growth is often limited by elements
other than N, increased N inputs could lead to more rapid increases in nitrate leach-
ing and denitrification in these systems than have been observed in higher-latitude
forests ( Matson et al., 1999 ). Recent work by Hall and Matson (1999) has suggests
that P-limited forests in tropical regions have much higher potentials to release
N oxides following the onset of elevated inputs than to N-limited forests. The shift
in distribution of N deposition to lower latitudes where forest growth is not typi-
cally N-limited, together with a paucity of studies on the effects of N deposition on
biogeochemical and ecological processes, limits our ability to predict the long-term
effects of increased N deposition on tropical forests.
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483
Chapter 15. Proven Practices and Innovative Technologies for
On-Farm Crop Nitrogen Management
N.R. Kitchen
a
, K.W.T. Goulding
b
, and J.F. Shanahan
c
a
USDA Agricultural Research Service, University of Missouri, Columbia,
MO, USA
b
Agriculture and the Environment Division, Rothamsted Research, Harpenden,
Herts AL5 2JQ, UK
c
USDA Agricultural Research Service, University of Nebraska, Lincoln, NE, USA
Nitrogen (N) from soil, fertilizer, and manure sources is generally ineffi-
ciently used (30–60%) in most crop production systems. As a consequence, unused
inorganic N can move off crop fields and contaminate surface and groundwater
resources. Local and national governments have responded with guidelines, stand-
ards, regulations, and in some cases fines when off-field losses of N have not been
reduced. Along with these environmental pressures, soaring energy costs have
resulted in commensurate increased costs for N fertilizers. These factors are real
for crop producers and are compelling them to scrutinize their crop N management
more closely than in previous decades. Numerous time-proven practices, estab-
lished by research and in crop production settings, are available that will result in
improved crop N use efficiency. More emphasis should be given to these practices
on farms throughout the world. Additionally, recent advances in sensor technolo-
gies are playing an increasing role in shaping the future of crop N management.
We highlight some of these technologies available to help producers make better N
management decisions. Both soil and crop measurements are considered and com-
pared. Nonetheless, “ on-farm ” implies that producers will be at the center of imple-
menting change, and change means N management options will motivate producers
to action. Prerequisites for grower adoption require that technologies and practices
be reliable, incur minimal additional expense (time and equipment), and integrate
with ease into current operations. When these criteria cannot be met, external incen-
tives (e.g., regulation or cost sharing) may be necessary.
1 . INTRODUCTION
Modern agriculture has come to embrace the concepts of environmental steward-
ship as a necessary component of crop production. The stories and studies that have
documented agricultural nutrients moving into and impairing ground and surface
Nitrogen in the Environment: Sources, Problems, and Management
J.L. Hatfield and R.F. Follett (Eds)
© 2008 Elsevier Inc. All rights reserved
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484 Nitrogen in the Environment
waters and the environment in general ( Vitousek, 1994 ; Delgado, 2002 ; Rabalais
et al., 2002 ), an outcome which has in some cases been followed by stepped-up
governmental regulation, have compelled producers to consider nutrient manage-
ment as more than an production decision. And then recently, starting in about
2002, steep increases in worldwide energy costs, along with stagnant grain prices,
have greatly altered how many farmers view N management. Today throughout
much of the world, farmers are paying 2–4 times more for N fertilizer than they did
15 years ago, and yet grain prices are similar. Nitrogen will continue to be given
special attention because of both environmental and economic pressures ( Mosier
et al., 2004 ).
As a nutrient, N is the main fertilizer with global environmental effects. In most
agricultural settings, soil N is insufficient for healthy nonleguminous crop growth;
consequently yield enhancement with N fertilizer typically ranges from 10% to
200%. The visual and subsequent yield response to historically inexpensive N fertili-
zer reinforces growers ’ reliance on it for profitable production. However, because
of the inherent chemical properties of N, it plays a major role in dynamic, climate-
mediated biological processes, all of which have the potential for adverse environ-
mental outcomes. Nitrogen transformation and transport in soil and water along
with plant N uptake is complex, making efficient management of N in the food,
forage, and fiber production system difficult to achieve. Nevertheless, our hope is to
do better. Developing more efficient N management systems for agriculture should
be a quest pursued by producers, agribusiness, and researchers around the globe.
Crop N management – including crop need, N source, amount, placement,
and timing issues – is difficult to anticipate because of spatial (within and between
fields) and temporal (within and between growing seasons) variability. Because of
this variability, N-management strategies have shown different levels of effective-
ness in meeting crop needs while minimizing environmental losses. Seldom will a
single N management plan used over multiple years result in optimal crop N use
and protection against off-field N losses for each of those years. Nitrogen fertilizer
use efficiency of crops varies greatly both between years and between different
crops. It rarely exceeds 70% ( Pierce and Rice, 1988 ) and more often ranges from
30% to 60% ( Bock, 1984 ) for many crops. Globally, N fertilizer use efficiency is
estimated to be closer to 30% ( Raun and Johnson, 1999 ; Cassman et al., 2002 ). To
improve N use efficiency, management needs to be time- and space-specific.
In essence, the N cycle is leaky ( Figure 1 ). Losses to water and the atmosphere
are part of the natural global N cycle. However, the conversion of stable atmospheric
and organic N into reactive forms by energy production, fertilizer production, cul-
tivation of legumes, plowing old grasslands, forest burning and land clearance, and
the drainage of wetlands is reckoned to have doubled the amount of reactive N in the
environment ( Goulding et al., 1998 ). New reactive N when mobilized can be readily
transported in solution or via the atmosphere so that local increases spread region-
ally and globally. The ultimate fate of this extra reactive N is uncertain. Much of it,
as with much of the extra carbon dioxide, is “ missing ” (i.e., current measurements
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Proven Practices and Innovative Technologies for On-Farm Crop 485
and calculations cannot account for missing N). It could be denitrified to N
2
or be
accumulating in the atmosphere, soils, groundwater, land vegetation, oceans and
marine sediments, changing ecosystems through eutrophication and acidification.
The International Nitrogen Initiative addresses these problems at the broadest scale:
http://www.initrogen.org/ .
Given the instability of N and leakiness in the soil–plant system, many have
asked, “ Can we really do better? ” We believe we can. Attention by producers, agri-
businesses, and researchers should be heightened to develop and employ management
practices proven to optimize N use efficiency. Some of the greatest improvements
are likely to be found as new innovative technologies, and sensors are integrated into
nutrient management plans. Of note are those technologies enabling timely and spa-
tially accurate assessments of crop N need. Future management systems will rely
upon a combination of these new technological tools along with time-proven practices
that together are jointly responsive to the N dynamics in the crop–soil environment.
This chapter describes practices and technologies that have either helped pro-
ducers use N more efficiently or shown promise in doing so. The phrase “ nitro-
gen use efficiency ” (NUE) is widely used in agricultural and ecological studies.
However, it connotes various explicit meanings, depending on what measurements
and calculations are made ( Bock, 1984 ; Pierce and Rice, 1988 ). Since no standard
Atmosphere
Crops Animals
Deposition
Fertilizers
N
2
fixation
Soil
Water
Organic N
NH
4
fixed in
clays
Denitrification
NH
3
volatilization
NO, N
2
O, N
2
NH
3
volatilization
Mineralization
Immobilization
Manures
Offtake Feeds
Milk
meat
NO
3
leaching
Surface
runoff N
Inorganic N
NH
4
NO
3
Figure 1 . A simplified nitrogen cycle.
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