Agriculture Reference
In-Depth Information
readily account for the effects of spatial variation at realistic field scales (Robertson
et al. 2007).
Syswerda et al. (2012) made similar comparisons of no-till vs. conventional till-
age, but examined NO 3 losses over an 11-year period that began 6 years after no-
till establishment. On average, they found NO 3 leaching losses from the MCSE
No-till and Conventional systems represented 50 and 76%, respectively, of the total
N applied to these systems. NUE was higher in the No-till system despite 16%
higher drainage losses (388 vs. 334 mm H 2 O yr −1 , respectively), suggesting that
channelized flow in the better-structured no-till soils allows water to leave the pro-
file before it has equilibrated with NO 3 in small pores (Rasse and Smucker 1999).
Higher plant demand for NO 3 may also have contributed to lower no-till fluxes as
the No-till system had somewhat higher average yields (Grandy et al. 2006; Smith
et al. 2007; Robertson et al. 2015, Chapter 2 in this volume) and therefore more N
uptake.
No-till management can also affect N 2 O emissions from soil, although such effects
are not consistent. Comparisons of N 2 O emissions in the MCSE Conventional and
No-till systems have shown no consistently significant differences (e.g., Robertson
et al. 2000, Grandy et al. 2006, Gelfand et al. 2013). This is in agreement with other
(Parkin and Kaspar 2006, Dusenbury et al. 2008, Sey et al. 2008, Gregorich et al.
2008) but not all (e.g., Liu et al. 2006, Omonode et al. 2011) similar studies. Van
Kessel et al. (2013) concluded through meta-analysis that it typically takes at least
10 years before no-till soils exhibit lower N 2 O fluxes, in which case we can expect
the MCSE No-till system to emit less N 2 O in coming years.
Summary
Nitrogen is an essential nutrient for crop growth and the N demands of today's
intensive cropping systems are met primarily by synthetic N fertilizer application.
Direct consequences of over-applying N fertilizer are substantial losses of reactive
N to the environment in the form of NO 3 leached to ground and surface waters
and N 2 O emitted to the atmosphere. The environmental costs of excess N loading
include coastal zone eutrophication, compromised drinking water and air quality,
climate warming, stratospheric ozone depletion, and biodiversity loss. However,
cropping systems can acquire N through legume N fixation, manure addition, and
crop residue return—offering many options for N management at the farm scale.
Results from KBS LTER research underscore the value of practical agronomic
practices that improve N retention in row crops. Potential interventions include
increasing rotational complexity with different primary and cover crops; using
no-till management; and improving N synchrony with better rate, timing, place-
ment, and formulation of N fertilizers, so crop N needs are met more precisely.
Residue management can also contribute to N conservation. Improved landscape
management can partially mitigate N leaching losses from the farm field through
measures such as maintaining or planting riparian vegetation, restoring stream
channels and small wetlands, and the targeted planting of forage or other peren-
nial crops such as cellulosic biofuels. The most poorly known N cycle fluxes at
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