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(a)
(b)
5mm
(c)
(d)
high
contrast
medium
contrast
low contrast
B
C
A
Grain-size gradient
Figure 6.3
A conceptual diagram of how grain size distribution of N-poor (light gray) and N-rich
(darker gray) microsites influences competition between plant roots and microorganisms in
N-poor microsites for inorganic N derived from N-rich microsites. Panels (a), (b), and (c) illustrate
a relative contrast in the size of N-rich and N-poor microsites compared to a maize root. Panel (d)
illustrates hypothesized relationships between grain size distribution and the outcome of plant-
microbe competition for N with zones corresponding to panels (a)-(c). The dark microsite interiors
in panel (c) indicate the potential for development of anaerobic centers in larger microsites. See text
for further details.
microsites; thus, the plants are ineffective at intercepting inorganic N flowing (by diffusion
and hydrologic transport) from N-rich to N-poor microsites. In zone B, the microsites are
separated at a sufficient scale for the plant roots to respond to the inorganic N gradients
and selectively proliferate into and around the N-rich microsites, thus increasing the prob-
ability that inorganic N will flow toward the root surface and not be intercepted by the
microbes in the N-poor microsites. In contrast, in zone C the plants are able to proliferate
into the N-rich microsites, but the size of the microsite promotes anaerobic processes in the
microsite centers and the conversion of NO
3
-
to N gas (NOx, N
2
O, or N
2
) via denitrification.
Alternatively, when moisture availability is limited, root proliferation into the large N-rich
microsites may limit plant water acquisition and microbial activity preferentially in the
large N-rich microsites. The influence of microsite grain size distribution on plant-microbe
competition for inorganic N will interact with the relative quality differences between
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