Agriculture Reference
In-Depth Information
and spread uniformly from where it came or deposited more narrowly directly behind the
harvester. The influence of this spatial distribution is often discussed in terms of trade-offs
for soil erosion and moisture management. Considerations into how this spatial distribu-
tion may affect soil C and N processes is less often mentioned. Few peer-reviewed articles
quantitatively described the spatial distribution of crop residue before or after soil tillage
(Staricka et al., 1991; Muller et al., 2009), so the influence of crop residue spatial heterogene-
ity on soil processes remains poorly known (see Grigera et al., 2007).
In humid regions where leaching of field-applied animal manure is of a concern for
ground- and surface-water quality, a relatively novel manure applicator (e.g., AerWay) has
been designed to apply liquid manure (e.g., swine manure) intentionally in a spatially
heterogeneous pattern. This manure applicator is designed to rotate a tool into the surface
8 cm of soil, which creates a cone-shaped void (~10 cm at maximum diameter and 8 cm in
depth), and then the resulting depression is filled with a similar volume of liquid manure.
The rotating tool is intended to shear and compress soil macropores. This minimizes mac-
ropore flow of liquid manure to below the root zone and results in the manure absorbing
into the soil directly surrounding the depression. The efficacy of this tool for minimizing
manure leaching and promoting crop growth is being studied; however, the spatially het-
erogeneous distribution of manure that this implement creates raises many questions. For
example, is this manure spreader simulating urine and feces patches?
Patches of livestock manure and urine are probably the most well-studied heteroge-
neous soil resource. At the pasture scale, the distribution of feces and urine patches is a
function of topography and animal species, size, behavior, and husbandry (Auerswald
et al., 2010). On the individual patch scale, N loss (via NH
4
+
volatilization, denitrification,
and nitrate leaching) increases with urine patch size, N concentration, and urine salt con-
centration (Orwin et al., 2009). Unique soil microbial communities form around newly
deposited urine (Orwin et al., 2009) and feces patches (Meyer et al., 2002), similar to that
surrounding decomposing plant litter (Hesselsoe et al., 2001). Specifically, the abundance
(or activity) of genes involved in nitrification (
AmoA
) and denitrification (
NirS
and
NirK
)
increases within days in the soil surrounding feces and urine patches (Orwin et al., 2010)
relative to background levels. This indicates that the soil microbial community is poised to
respond to resource additions regardless of the spatial distribution.
Naturally occurring heterogeneity of soil properties is often confounded with changes
in the mean resource level or environmental control. In that the two factors, heterogene-
ity and mean levels, often covary in soils, this leads to a difficult interpretation of soil
heterogeneity as a property of the system by itself (Benedetti-Cecchi, 2005a). Because of
this confounding, several investigators have been motivated to manipulate heterogeneity
independently of mean conditions (resources or environmental controls) in an attempt to
separate heterogeneity effects from mass effects.
Breland (1994) amended a clay-loam soil with the same mass of white clover (
Trifolium
repens
) leaves in a uniform distribution or as a single layer and found the layered clover
decomposed and mineralized N more rapidly than the uniform distribution. This layer-
ing effect also resulted in greater denitrification (assessed through the acetylene block tech-
nique). Breland attributed the process rate differences to a soil-to-litter contact effect. Because
the uniformly distributed litter had greater contact with soil particles, there was a greater
chance of the litter-soil interaction, leading to the physical protection of the litter C and mak-
ing it less available for enzymatic degradation. In contrast, uniformly distributed red clover
leaves initially decomposed more rapidly than when the litter was aggregated into clumps
varying in mass from 0.5 to 4.5 g in a sandy-loam soil (Loecke and Robertson, 2009b).
Search WWH ::
Custom Search