Agriculture Reference
In-Depth Information
use and maximize N conservation. Robertson and Vitousek (2009) organized prac-
tices to improve the N Use Efficiency (NUE) of high-productivity cropping systems
into four strategies: (1) provide farmers with decision support to better match fertil-
izer N rates to crop N requirements; (2) adjust the rotation to add crops and cover
crops that will improve uptake of added N; (3) better manage the timing, placement,
and formulation of N fertilizers—including organic—to better synchronize N addi-
tions with plant N needs; and (4) manage hydrologic flow paths to capture and
process the N leached from farm fields. Tillage management provides a fifth option
for some soils. KBS LTER results inform all strategies and as well the degree to
which farmers are likely to adopt various strategies (Swinton et al. 2015, Chapter
13 in this volume).
Nitrogen Fertilizer Rate
As noted earlier, a direct relationship exists between crop yield and N fertilizer
rate up to the point at which crop N needs are met. In recent years, hundreds of
N-response trials conducted throughout the U.S. Midwest (e.g., Vanotti and Bundy
1994) have contributed to a database sufficient to support the new MRTN approach
based on the EONR (ISU 2004, Sawyer et al. 2006).
At KBS LTER, the difference between the traditional Agronomic Optimum
N Rate (AONR) and the EONR approach illustrates potential fertilizer savings.
Results from the Resource Gradient Experiment (Fig. 9.2) suggest that applying N
fertilizer at economic optimum rates could result in a potential savings of 23% on
N fertilizer costs for corn and 36% for wheat, assuming a typical fertilizer to crop
price ratio of 0.10. That NO
3
−
leaching also increases substantially at N fertilizer
rates in excess of crop N demand (e.g., Andraski et al. 2000, Gehl et al. 2006) sug-
gests that the EONR approach could mitigate other forms of N in the environment
in addition to N
2
O.
Cover Crops
Winter cover crops can capture N that would otherwise be available for loss follow-
ing annual crop harvest. During active growth in the fall and spring, winter cover
crops take up mineralized N and/or residual N fertilizer, which can then become
available to the next crop upon remineralization (Rasse et al. 2000, Strock et al.
2004). Much of the N immobilized by the cover crop presumably would be other-
wise lost as leached NO
3
−
(Feyereisen et al. 2006) or as N
2
O and N
2
via denitrifica-
tion (Baggs et al. 2000a).
In the KBS LTER Living Field Lab Experiment (Snapp et al. 2015, Chapter 15
in this volume), McSwiney et al. (2010) found that the inclusion of cereal rye as a
winter cover crop in a conventional corn system maintained corn yield, while sig-
nificantly reducing N
2
O and NO
3
−
losses, particularly at N fertilizer rates in excess
of corn N requirements. They calculated apparent N recoveries of >80% for N
fertilizer rates up to 101 kg N ha
−1
when cover crops were included; typical esti-
mates for row crops without cover crops range from 30% to 60% (e.g., Cassman
et al. 2002). Cover crops also significantly reduced NO
3
−
leaching compared to
