Geoscience Reference
In-Depth Information
true that the only way humans derive food from semi-
arid rangelands is from livestock raised for meat and
milk—aside from hunting native animals and gather-
ing edible native plants. the alternatives are dryland or
irrigated agriculture, which may lead to soil erosion and
other environmental problems.
ing infiltration and the rate of plant growth. Fire, soil
disturbances, and grazing (above- and belowground) can
stimulate plant growth as well, because they increase the
rate of mineralization and improve nutrient availabil-
it y. 23 A simple mechanism by which mammals contrib-
ute to this process is through the deposition of urine and
feces, both of which are easily mineralized.
nutrient loss through leaching is an unlikely phe-
nomenon in most western grasslands, because, unlike
in forests, rarely is there enough precipitation to cause
water percolation below the rooting zone. Wind erosion
during droughts or episodes of heavy grazing can lead
to some nutrient loss, but only a very small proportion
of the nutrients in the soil are lost in this way. nitro-
gen can be lost during inevitable grassland fires, but
again, most of the nitrogen remains unaltered in the
soil. Some volatilization of nitrogen has been reported
from animal urine and feces, but overall, nitrogen tends
to accumulate in western grasslands (fig. 6.6). nitrogen
loss attributable to denitrification bacteria is very small
in mixed-grass prairie, if it occurs at all.
Because nutrients accumulate, grassland soils
can become quite fertile and are excellent for crop
production—assuming water is available (by irriga-
tion or otherwise) and temperatures are suitable for
the desired crop. of course, the harvesting of crops is
itself a nutrient drain; fertilization is required after a
few years of cultivation.
nitrogen fixation by bacteria in the nodules of
legumes and other plants is an important long-term
nitrogen source for many ecosystems. in shortgrass
and mixed-grass prairie, such legumes as locoweed,
lupine, milkvetch, scurfpea, and vetch are known to
have nitrogen-fixing bacteria, and there is evidence
that a few nonleguminous plants may have the same. 24
Lichens have been identified as nitrogen fixers in some
desert shrublands, 25 but so far there is no evidence that
it's done by the most common lichen in Wyoming
grasslands— Xanthoparmelia chlorochroa (fig. 6.7).
Symbiotic nitrogen fixation is commonly thought to
be the single most important mechanism that provides
nitrogen for plant growth, in the form of ammonium
and nitrate, but apparently this is not true in western
grasslands, where legume density is low and suitable
environmental conditions for nitrogen fixation occur
for only a short time each year. nitrogen fixation by
Nutrient Cycling in Grasslands
For their survival, organisms require nutrients as well as
suitable temperatures and adequate energy and water.
For example, calcium is required for plant cell walls;
phosphorus is important for the storage and release
of energy during metabolism; potassium is needed for
the regulation of cell water and manganese for the syn-
thesis of chlorophyll; and nitrogen is an important
component of amino acids, proteins, and chlorophyll.
All nutrients cycle through grassland ecosystems, with
losses occurring primarily during erosion events. those
losses that do occur are usually offset by nutrient inputs
from rock weathering, rain, snow, the deposition of dust
and finer aerosols, and such nutrient-specific processes
as nitrogen fixation. Studies on cycling must consider
the rates at which nutrients are being added and lost,
as well as the rates and pathways by which they move
through the ecosystem. All aspects of cycling in grass-
lands cannot be considered here, but a few processes
seem particularly interesting and relevant when think-
ing about the western Great Plains.
As noted, grassland plants typically have leaves and
stems high in lignin and cellulose, which are resistant to
decomposition. For this reason—and because the warm,
moist conditions required for decomposition last for
only a short period each year—dead plant material tends
to accumulate on the soil surface as well as in the soil.
Among other effects, this surface litter (often referred
to as mulch or detritus) can improve infiltration rates.
in addition, soil organic matter increases the water and
nutrient storage capacity of the soil while providing a
more erosion-resistant soil structure. Soil organic mat-
ter is resistant to decomposition, but nematodes, mites,
and other microbes are able to use it as a food source,
directly or indirectly, and in the process nutrients from
the organic matter are made available for plant growth.
Dead microbes also become part of soil organic matter.
in general, many processes create a friable soil, enhanc-
 
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