Agriculture Reference
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7%
84%
33%
85%
<.01%
84%
% of total soil mass
% of total denitrif.
100%
100%
33%
6%
33%
9%
Redrawn from Parkin 1987
Figure 3.4
A depiction of Parkin's soil core dissection that traced the origin of most of the denitri-
fication activity (84% of the total activity) within a single core (2-cm i.d. × 15 cm in length) back to a
single pigweed leaf. The three columns of the soil core represent the sequential process of dissection
and acetylene block assessment of denitrification activity in each divided core section. (Redrawn
from Parkin, T. B. 1987. Soil microsites as a source of denitrification variability.
Soil Sci. Soc. Am. J.
51:1194-1199.)
At even smaller scales (e.g., within soil pores, << mm), several efforts to characterize
the relationships between the spatial distribution of microbial communities, the processes
they perform, and the physical structure of soil habitats have yielded informative insights.
For instance, using hypodermic needles as soil corers, Grundmann et al. (2001) dissected a
soil aggregate and sampled for the spatial distribution of ammonia- and nitrite-oxidizing
bacteria (i.e., nitrifiers). They found the nitrifier communities were spatially clustered, and
this spatial pattern was strongly associated with soil micropore size. Through a three-
dimensional simulation of microhabitat, Grundmann et al. (2001) were able to conclude
that the diffusional properties of micropores likely controlled the spatial distribution of
nitrification. This provides evidence of soil spatial structure controlling the spatial dis-
tribution of a specific biogeochemical process, nitrification. At a slightly larger scale (2-5
cm), the soil surrounding decomposing plant litter (e.g.,
Trifolium repens
L.) can be a hot
spot for nitrification within a few weeks of soil incorporation given sufficient soil moisture
(Hesselsoe et al., 2001).
Many agronomic management practices intentionally and unintentionally manipulate
the spatial distribution of crop and cover crop residue. In temperate humid annual crop-
ping systems (e.g., the U.S. cornbelt), during or prior to planting the soil surface immedi-
ately above the seed is intentionally maintained free of crop residue, thereby decreasing
surface albedo to promote seedbed warming and drying (e.g., strip tillage and no-till
planting) (Licht and Al-Kaisi, 2005). During cool and wet springs, the influence on the
seed germination and seedling survival can be dramatic to nonexistent in mild springs (as
reviewed in Guerif et al., 2001). Knowledge of how seedbed preparation (apart from total
soil surface tillage) alters soil microbial processes appears limited (Guerif et al., 2001; Lee
et al., 2009); however, these spatially limited tillage systems (e.g., strip tillage and no till)
can substantially increase soil C and N content (e.g., Al-Kaisi et al., 2005), likely a product
of altered microbial processes.
Farmers have a range of choices for handling the crop residue that is processed through
a modern combination harvester. The residue can be chopped into a range of particle sizes
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