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The contrast between the results of these two studies is likely due to (1) soil mineral-plant
litter physical interactions (clay-loam vs. sandy-loam) and (2) microsite O
2
consumption and
diffusion rates. Loecke and Robertson (2009b) tested the second part of this hypothesis by
incubating uniform and clumped distributions of clover litter under 50% and 80% WFPS to
alter the diffusion of soil O
2
. Under the more O
2
-restrictive environment (80% WFPS), the uni-
formly distributed litter continued to decompose more rapidly than the clumped distribution,
whereas at 50% WFPS decomposition of the clumped litter was only temporarily dampened
and exceeded the uniform decomposition rate by the end of the first week of the incuba-
tion. Further support for the hypothesis is provided by the sevenfold greater N
2
O emissions
from the clumped relative to the uniformly distributed litter at 50% WFPS, presumably from
denitrification. This largely confirmed Parkin's assertion that if the pigweed leaf had been
uniformly distributed in the soil denitrification would have been dramatically lowered.
The
Trifolium
litter used in the two examples (Breland, 1994; Loecke and Robertson,
2009b) is known to decompose rapidly and is relatively rich in N. In contrast, N-poor plant
litter (high C:N ratio,
Brassica napus
L.) has been found to decompose more rapidly when
aggregated in a sandy-loam soil (Magid et al., 2006). This suggests that heterogeneity
affects soil processes independently of mean resource levels; however, the exact processes
that are affected will vary with the characteristics of the resource. For example, an N-poor
litter may decompose more rapidly within a clump, yet it may be less likely to promote
denitrification due to greater microbial assimilatory demand for N in the clump versus
uniform distribution. Largely, these ideas are yet to be tested under field conditions.
Given the supposition that heterogeneity is the norm, the utility of including a uni-
form distribution in comparison to heterogeneous resource distributions is questionable.
I am aware of only a single study that has included multiple heterogeneous distributions
(Loecke and Robertson, 2009b), so the influence of the spatial distribution of soil resources
on biogeochemical processes is still limited. Using a gradient approach to spatial hetero-
geneity, Loecke and Robertson found large changes in decomposition and N
2
O emission
between a uniform distribution and 0.5-g clumps of red clover (as mentioned) but few
differences between 0.5-, 1.5-, and 4.5-g clumps. At least for red clover litter (either ground
or whole leaf), this suggests a threshold between tiny patches (<1 mg) composed of indi-
vidual ground red clover particles (i.e., the uniform distribution) and the 0.5-g clumps.
3.4 Opportunities to advance the understanding and
management of heterogeneity in agroecosystems
I see two general areas in which explicit consideration of heterogeneity in agroecosys-
tems can improve management and understanding. First, manipulating the spatial het-
erogeneity of organic matter (e.g., crop residue or organic amendments) has the potential
to improve agronomic efficiency and environmental protection. Second, conceptual and
process-based modeling may benefit from considering the role of heterogeneity in nutrient
turnover, C sequestration, and processes rate estimation. These two general opportunity
areas are synergistic as the former will help test and parameterize the latter.
The interface of hydrologic flow paths from terrestrial and aquatic ecosystems is a
well-known area of intense biogeochemical transformations; in particular, this habitat
interface (i.e., riparian zone) is a hot spot for denitrification of nitrate-rich agricultural
drainage waters (Hedin et al., 1998; McClain et al., 2003; David et al., 2010). The thermody-
namic conditions are not always favorable for riparian zone denitrification to deplete NO
3
−
before it enters surface waters (Hedin et al., 1998). This has prompted several researchers
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