Agriculture Reference
In-Depth Information
negatively influenced ecosystem gross primary productivity, while the presence of
termitaria and
Acacia
trees facilitated soil water, nitrogen availability, and ecosys-
tem productivity (Otieno et al. 2011). The authors found that heterogeneity related to
topographic variations and disturbances critically influenced ecosystem functioning,
productivity, and carbon storage.
In long-term land management sites, Li et al. (2007) found that fenced-grazing
management was a better option for sustaining SOC and water-stable aggregates
than cropping or nonfenced extensive grazing in arid grassland. The effects of culti-
vation and overgrazing on soil quality in arid regions have been rarely addressed. In
a Mongolian meadow steppe, Han et al. (2008) found SOC, total soil nitrogen, and
coarse root biomass decreased with grazing intensity. For these grasslands, using
judicious herding to distribute livestock might be needed to sustain light to moderate
grazing levels. Xiao-Gang et al. (2007) reported that SOC, organic nitrogen, and soil
microbial respiration were lower under annual oats and perennial pasture compared
with native alpine pasture. The significant decreases in many of the SOC pools in
agricultural systems compared with native alpine pasturelands raise concerns about
the long-term sustainability of annual pasture involving intensive soil disturbances
in this environment.
4.2.2 S
oIl
b
Iota
and
b
IoloGIcal
a
ctIvIty
A strong, functioning soil food web is largely dependent on soil organic matter and
the continual cycling of plant litter, roots, and animal feces and microbial biomass
back to the land (Figure 4.11). Creating this strong food web in pastoral systems is
dependent on a balance between plant production and herbivore harvesting of for-
age. An optimum level of grazing can be hypothesized for each particular soil and
ecoregion (Figure 4.10). Lavelle and Spain (2001) described in detail the microbial
communities and food chains whereby the litter system is the primary food source
for the biota and accessed, redistributed, and incorporated within the soil matrix
by roots, worms, and termites. These are the “the ecosystem engineers,” and fungi
are the primary heterotrophic decomposers that make resources available to them
(Colloff et al. 2010). Worms can be of several types, inhabiting only the litter layer,
the litter soil interface, and the soil profile to varying depths. Worms can favor a
range of diets and can be of “compacting” types, which produce resilient casts, or
“decompacting” types, which feed on the casts of others. Together, the various types
of earthworms heavily influence nutrient cycling and soil macrostructure (Lavelle et
al. 1999). The longevity of earthworm casts and channels can be up to several years.
In tropical savannahs, termites play the role of earthworms in concert with other
mesofauna (Holt et al. 1996).
Biological crusts (Belnap 2006) form on soil surfaces and are hugely diverse, com-
prising combinations of algae, cyanobacteria, fungi, mosses, and lichen (Figure 4.12).
They can play a huge role in the fine-scale hydrology and stability of soil surfaces.
This is especially true in semiarid environments without complete vegetation
cover. Organic compounds are also important and must remain protected within
microaggregates—bacteria can break down these compounds and lead to structural
decay. If organic matter is protected within (hydrophobic) microaggregates, this
