The biological basis for nitrogen management in agroecosystems - Microbial Ecology in Sustainable Agroecosystems - page 121

the microsites. As illustrated with the three separate lines (low, medium, and high) in
Figure 6.3d, the contrast between microsite quality (or N richness) is predicted to interact
with microsite grain size such that the larger the contrast in quality, the larger the potential
for plant roots to compete with microbes in the N-poor microsites is. We see in Figure 6.3
that the contrast in microsite quality can alter root proliferation, a critical component of
this conceptual hypothesis.
An example of a cropping system with both N-rich and N-poor residues is one with
small-grain cereals grown in conjunction with legume cover crops, which is a common
component of biologically managed crop rotations in the U.S. corn belt (e.g., Liebman et al.,
2008). The small-grain crops grow and senesce while the legume remains under the grass
canopy. Following cereal harvest, a mixture of N-poor straw and N-rich legume shoots is
incorporated into the soil prior to planting the subsequent crop. As of yet, the degree to
which the spatial heterogeneity of this mixture of straw and legume residue influences the
temporal dynamics of soil inorganic N availability during the growing season remains
unknown. What is known is that many crops are capable of displaying optimal root forag-
ing behavior (Drew, 1975). Crop roots have the potential to proliferate in N-rich microsites
and avoid N-poor microsites when plant growth is N limited ( Figure 6.4 ). Furthermore,
roots of maize and other crops are capable of foraging across neighboring plants' root
zones to acquire inorganic N from N-rich patches (Hodgen et al., 2009). However, we do
not have a good handle on the occurrence of root proliferation into N-rich microsites in
the field, and its importance in terms of plant N acquisition has not been well established.
The few field data reported on the location of crop plant roots relative to organic matter-
rich microsites indicate a strong spatial association (Van Noordwijk et al., 1993), but the
available data do not lend themselves to broad predictions about root foraging. We need to
understand better the factors that regulate root foraging given that it may be an important
mechanism crops use to acquire N.
6.10 Managing synchrony
It can be argued that managing nitrogen mineralization-immobilization dynamics to
improve synchrony is one of the great challenges in agroecosystem science. However,
given the complex array of processes on which mineralization-immobilization dynam-
ics depend, is it realistic to think that we will ever be able to predict the release of N
from organic sources? Soil biological communities, temperature and moisture, soil texture,
organic matter quality, and crop management all vie for their place in the hierarchy of
controls over nitrogen mineralization. Predicting the effects of these factors in isolation is
difficult; it seems almost Sisyphean to predict their interactive effects on N mineralization-
immobilization dynamics and synchrony.
However, despite the many challenges, there have been exciting new advances in
understanding decomposition and N dynamics that may ultimately improve agroeco-
system management. For example, there has been a major push to understand better the
priming phenomenon in soils. For decades, the priming phenomenon—the stimulation
of microbial activity and soil organic C or N turnover via the addition of relatively small
resource inputs—was considered an experimental artifact. However, it is now well estab-
lished that this is a real process caused by resource stimulation of microbial communi-
ties (Kuzyakov et al., 2000; Fontaine et al., 2004; Garcia-Pausas and Paterson, 2011), and
that it may influence C and N cycling in agricultural soils (Neill, 2011). We are still in
the earliest stages of understanding priming, but future advances in crop breeding and
genetics may lead to the development of new crop varieties that are capable of stimulating