The biological basis for nitrogen management in agroecosystems - Microbial Ecology in Sustainable Agroecosystems - page 115

Figure 6.1 Conceptual model of nitrogen use efficiency. Inequalities in either crop demand or nutri-
ent availability lead to reduced resource use efficiency, with concomitant negative impacts on crop
performance or environmental quality, respectively.
is not closely aligned with plant N needs ( Figure 6.1 ) . Synchrony is influenced by plant
traits, environmental factors, and their interactions. Crews and Peoples (2005) defined
two types of asynchrony that can occur in cropping systems. One type of asynchrony
is characterized by nutrient availability being insufficient to meet plant demand. Under
these conditions, environmental losses are often low, but yields are limited by N availabil-
ity. Most temperate, terrestrial ecosystems are N limited, and plant productivity is typi-
cally increased by increasing the supply of available N. However, the widespread use of
N fertilizers has led to another type of asynchrony in which N availability outpaces plant
demand. In these conditions, yield may be maximized but at the consequence of environ-
mental losses through leaching and denitrification.
For decades, scientists have experimented with ways to improve synchrony to enhance
N fertilizer use efficiency (Cassman et al., 1992; Crews and Peoples, 2005; Ross et al., 2008).
One line of research has focused on the development of fertilizers or chemical inhibitors
that slow the release of N, but their deployment has been limited because of cost and vari-
able yield responses. Another line of research has been focused on spatially and temporally
explicit fertilization practices. Fertilizer banding and split applications have both shown
promise in some studies, but their effects on yields and environmental N losses are often
inconsistent because of the overriding importance of interactions between soil type, tem-
perature, and moisture in regulating N availability (Fan and Li, 2010). Thus, despite years
of intensive research, N fertilizer use efficiency in modern, industrialized cropping systems
continues often to remain below 50% and in developing countries may be significantly less
(Cui et al., 2010).
While the timing of N fertilizer uptake is a key component of synchrony, we also have
to consider the additional challenge that soil-derived N presents to improving synchrony.
Nitrogen mineralization resulting from SOM turnover can generate large amounts of