Agriculture Reference
In-Depth Information
regulate N availability. Many studies have focused on ways to improve fertilizer manage-
ment, but we also need to understand better the biological processes that account for much
of the N that is used by crops. In the sections that follow, we explore synchrony and how it
is influenced by spatial and temporal biological processes as well as different forms of N.
The interactions between these three factors—spatial and temporal N dynamics and the
form or chemical complexity of soil N—determine when and where N is available and the
degree to which plants will be able to access that N when it is most needed.
Nitrogen use efficiency broadly refers to the amount of N taken up by a crop relative to the
amount applied or available in the soil. While in the broadest sense there is widespread
agreement with this definition, there are disagreements over how NUE should be cal-
culated (Dawson et al., 2008). Ultimately, the correct approach depends on context, what
biomass components are targeted, and how a unit of N is defined. For example, Moll et
al. (1982) defined NUE as the ratio of grain yield to plant-available N in the soil, but many
studies have used applied fertilizer N uptake. Nitrogen fertilizer inputs are much easier to
quantify than total available soil N during the growing season. An additional advantage
of this approach is that N fertilizer uptake efficiency can be used to compare plant traits
and genetics that influence NUE, as well as plant responses to different rates and types of
fertilizer. The disadvantage of an approach that focuses exclusively on grain yield per unit
of N fertilizer input is that it largely ignores the substantial internal provision of N that
occurs in many soils and thus provides little information about N retention and conserva-
tion in cropping systems. A better understanding of long-term N mass balance, the fun-
damental controls over N mineralization-immobilization dynamics, and the regulators of
plant-soil-microbe interactions is needed to develop more resource-efficient systems that
conserve N and minimize environmental losses (Dawson et al., 2008). Further, measure-
ments of grain N alone—while useful for providing insights into yield components—do
not account for N accumulation in other biomass components. Total plant N uptake needs
to be figured into estimates of N balance and retention.
Although the concepts underlying NUE are relatively straightforward, the plant and
soil components related to NUE can be complex and vary across ecosystems. NUE is con-
trolled by multivariate interactions between plant traits, genetics, and environmental fac-
tors. From the plant perspective, there is a strong physiological and genetic component to
NUE (Dawson et al., 2008) that includes plant N utilization efficiency, N carryover between
seasons in perennials, and N uptake, retention, and conversion efficiency (Cassman et al.,
2002). Additional plant traits such as root architecture, responses to nutrient limitation,
and temporal patterns of demand also strongly influence NUE and point to the need for
including plant breeding in integrated efforts to increase NUE. Environmental conditions
and soil processes also play a key role in NUE. For example, the yield and N demand of a
crop are closely related to solar radiation, temperature, and moisture and their interaction
with other environment and management-related factors, including pest populations and
weed competition (Cassman et al., 2002).
6.5 Synchrony
At the core of NUE is the concept of synchrony. Synchrony refers to whether soil N avail-
ability coincides with plant N demand (Campbell et al., 1992); most agricultural systems
are highly susceptible to environmental N losses because N availability in space and time
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