Agriculture Reference
In-Depth Information
systems without cover crops (Syswerda et al. 2012). This reduction could be linked
to increased evapotranspiration and soil nitrogen scavenging in cover crop systems.
When legumes are included as cover crops, as in the MCSE Reduced Input
and Biologically Based systems, there is the potential benefit of N input by BNF.
Although BNF will be low when adequate soil N is available, winter legumes can
provide the same degree of soil inorganic N scavenging as their nonleguminous
counterparts and have the additional advantage of a low C:N biomass that decom-
poses rapidly after spring killing, making more N available earlier for the growth
of the summer crop (Fig. 9.5; Corak et al. 1991, Crandall et al. 2005). Crop residue
quality (e.g., C:N ratio, lignin content) has also been shown to affect denitrifica-
tion rates and N
2
O emissions (Baggs et al. 2000b; Millar and Baggs 2004, 2005).
Cost savings from avoided N fertilizer use could be put toward cover crop seed and
planting expenses.
Fertilizer Formulation, Placement, and Timing
Applying an appropriate form of N when and where the crop can best use it can
readily improve ecosystem NUE. Fertilizer N should ideally be applied in sev-
eral doses to match the timing of crop N demand—this ensures the greatest syn-
chrony between fertilizer addition and crop need. However, except where N might
be applied continuously in irrigation water, weather and the availability of equip-
ment and labor typically limit N applications to no more than two per season—in
corn, a starter rate at planting and the remainder just before the rapid growth stage.
Worse, for about one-third of U.S. cropland, fertilizer N is applied once in the fall—
months before active crop growth (Randall and Sawyer 2008, Ribaudo et al. 2011).
Long-term paired comparisons show lower corn yields and 15-40% greater NO
3
−
losses with fall vs. spring fertilizer applications (e.g., Randall and Mulla 2001,
Randall and Vetsch 2005). Bundy (1986) concluded that fall-applied N is usually
10-15% less effective for crop utilization than spring-applied N.
Fertilizer placement also affects NUE. The spatial arrangement of available
N vis-à-vis the distribution of plants and their roots (e.g., Van Noordwijk et al.
1993) affects the likelihood of N uptake vs. N loss. Placing synthetic fertilizers in
a concentrated band within or very close to crop rows, rather than between them as
is more common, can increase NUE and reduce surface N loss (Malhi and Nyborg
1985, CAST 2011). Injecting anhydrous ammonia into soil near rows rather than
broadcasting over the soil surface can decrease N leaching and volatilization by as
much as 35% (Achorn and Broder 1984). And broadcasting it has been shown to
double N
2
O emissions (Venterea et al. 2010) when compared to broadcast urea.
The location and particle size of plant residue also have an impact on its N
release and uptake. Loecke and Robertson (2009) found that N in red clover residue
is more likely to be taken up by corn if the residue is sufficiently aggregated to con-
centrate mineralized N in small patches, but not so large as to inhibit decomposition
by creating anoxic microsites.
Fertilizer placement at the field scale can also greatly influence NUE. That soil
nitrogen availability is spatially variable in predictable patterns (Fig. 9.10) is well
known from studies at the KBS LTER (Robertson et al. 1993, 1997; Senthilkumar
