Agriculture Reference
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of knowledge that will serve as a compass in our efforts to manage the priming phenom-
enon to improve the synchrony between N mineralization and plant N uptake.
Developing integrated soil-crop management systems that diversify the forms and
sources of soil N may be another way to improve N synchrony (Chen et al., 2011). Mineral
fertilizer N and urea, which is rapidly converted to mineral forms in most cropping sys-
tems, are highly susceptible to environmental losses. These fertilizers are often applied
at times when crop N demand is low, and they are much more mobile and reactive than
organic N. In contrast, N that is derived from organic fertilizers or from the turnover of
native SOM generally becomes available more slowly than N derived from mineral fertil-
izers. These fertilizers may limit environmental N losses by reducing the pool size of soil
inorganic N at times when plants do not need it and also provide opportunities for plants
to prime the release of N in the rhizosphere when they most need it. However, N  min-
eralization, like other biological processes, is also strongly dependent on soil moisture
and temperature, microbial communities, and trophic-level interactions (Robertson and
Groffman, 2007; Wickings and Grandy, 2011). Given the range of controls over N mineral-
ization and the different temporal and spatial scales at which they operate, predicting N
release from organic sources—and synchronizing N mineralization with plant N needs
over space and time—is difficult.
An alternative approach to relying on either mineral or organic forms of N is to use
both and to do so with specific knowledge of plant N requirements and soil N availabil-
ity determined through soil tests. Recent reports have shown that mixing organic and
inorganic forms of N can have positive interactive effects on yields and NUE (Vanlauwe
et al., 2001; Chivenge et al., 2011). Further, organic forms of N can be diversified to include
legumes, animal manures, or a range of different crop residues. There are a number of
reasons why a mixed fertility strategy may increase yields and N uptake. First, it may be
a way to supply adequate N throughout the season without ever having to saturate the
soil with inorganic N. Inorganic N fertilizers are used to meet crop N demands early in
the season and during peak crop growth; organic fertilizers provide a longer-term, slower
release throughout the season and may immobilize excess inorganic N. Second, the use of
organic fertilizers increases SOM N pools, which increases the potential long-term supply
of N. Third, a diverse pool of organic and inorganic forms of N may provide opportunities
for crops to partition N, thereby reducing competition for N among crops and weeds or
intercrops (Smith et al., 2010). Finally, micronutrients may be made available through the
use of organic fertilizers (Zingore et al., 2008; Chivenge et al., 2011).
One of the most effective practices to diversifying and building soil N pools is to inte-
grate legumes into a rotation sequence (Carlsson and Huss-Danell, 2003; Snapp et al., 2005;
Herridge et al., 2008; Mazzoncini et al., 2011). However, the net biological nitrogen fixation
benefits from a given species will depend on the extent of growth and N fixation, as well
as on the extent of N removal in harvested product. Grain harvest in a crop with a high
harvest index such as soybean will remove over half of the aboveground N, which in some
cases may be more than the N that was fixed. A review of elite soybean lines showed that
70% of the genotypes removed as much N as they fixed (Sanginga, 2003). Although there
are clear economic reasons why farmers choose legumes with a high harvest index, there
is a trade-off in that little N remains in the soil system after harvest.
Crop diversification can address multiple objectives related to improving soil quality
and increasing the diversity of N inputs. For example, Smith et al. (2008) examined the
effects of rotational diversity on soil inorganic N pools. Cropping system diversity ranged
from one crop species grown over a 3-y period up to six species over the same period.
The most diverse treatments included legume/small-grain cover crops and increased
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