Agriculture Reference
In-Depth Information
Inorganic fertilizers are also managed in ways that do not optimize the recovery of
N. Spring applications before or at planting are highly susceptible to environmental losses
through leaching, and maximum N 2 O emissions typically occur shortly after fertilizer
applications (McSwiney and Robertson, 2005). The use of two or more fertilizer applica-
tions, fertigation, and slow-release fertilizers have all shown potential to improve syn-
chrony in some systems but have not been widely adopted because of concerns about cost
and yield responses. Thus, whether N is derived from organic sources via mineralization
or from inorganic fertilizers, matching N availability with plant needs remains a major
challenge. Further, the problem of temporal synchrony is often intertwined with that of
spatial heterogeneity.
6.8 Spatial dynamics
There is increasing interest in the role of soil spatial heterogeneity in controlling crop
nutrient use efficiency, productivity, and off-site ecosystem degradation (Ettema and
Wardle, 2002; Patzold et al., 2008; Loecke and Robertson, 2009b). Spatial heterogeneity of
labile organic amendments such as crop residues, cover crops, manure, and compost can
influence temporal N dynamics in agroecosystems by several mechanisms and at various
spatial and temporal scales ( FigureĀ 6.3 ) . At the spatial scales at which organic amendments
tend to be aggregated or clumped (1 mm to 1 m), the degree of this aggregation (i.e., patch
size) influences the prevalence and rates of microbial N transformations (see Loecke's
Chapter 3 in this volume for further discussion). Overlapping spatial scales of organic
amendment patch sizes and plant root zones allows for interactions between spatially and
temporally heterogeneous microbial N transformations and plant root foraging behavior.
The potential role of these interactions in modifying soil inorganic N supply and crop N
uptake synchrony are severalfold.
Competition between soil microorganisms and plants for soil inorganic N has been a
topic of considerable debate (Kaye and Hart, 1997; Hodge et al., 2000; Schimel and Bennett,
2004) but is pivotal to understanding N synchrony (Korsaeth et al., 2002). Theory and a
small body of laboratory-based empirical work suggest that the interactions of spatially
heterogeneous soil resources, especially ephemeral patches of labile organic matter, and
spatially selective root proliferation (i.e., root foraging) may alter plant-soil microbe com-
petition for soil N (Hodge, 2006; Loecke and Robertson, 2009a). Wang and Bakken (1997)
and Korsaeth et al. (2001) tested this idea by varying the distance separating microsites
with N-rich clover leaves and N-poor oat straw to determine whether N derived from the
N-rich sites was immobilized in the microbial biomass of the N-poor sites, the timing of
this immobilization, and the quantity of soil N taken up by barley plants ( Hordeum vulgare ).
As the distance between N-rich and N-poor microsites increased from 0 to 9 mm, the plant
intercepted a greater quantity of N derived from the N-rich microsites; at the same time,
microbial biomass N in the N-poor microsite decreased. When the N-rich and N-poor
sites were in close proximity to each other (0 to 3 mm), plant N uptake was delayed. These
studies were carried out under controlled environments but may nonetheless reflect plant-
microbe interactions in agroecosystems.
We have taken the available data and developed a conceptual model highlighting the
relationships between patch size and distribution (referred to here as grain size) and plant-
microbe competition for resources. Our model uses a continuous grain size distribution
representing differences in patch size and distribution and breaks this continuum into
three zones. In zone A, corresponding to FigureĀ  6.3a, the N-rich and N-poor microsites
are too small and tightly packed together for the roots to proliferate selectively into N-rich
 
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