Agriculture Reference
In-Depth Information
Sub: Subsidy effect
N: Normal operating range
St: Stress effect
R: Replacement
L: Lethal
Management (as a form
of perturbation) can
enrich or degrade the
environment
Usable input
Sub
Relative variance
Perturbations may be
•
Energy (harvest by
machine or grazing,
tillage inputs, chemical
control, etc.)
•
C source (type,
frequency, placement,
and quality of crop
residues)
•
Nutrients (N, P,
microelements, etc.)
Subsidy/stress gradient
N
Toxic input
St
R
L
Increasing perturbation
Ungrazed
Overgrazed
Moderately grazed
FIGURE 4.10
Effect of grazing as a form of ecosystem perturbation that results in either
a subsidy to increase ecosystem productivity (e.g., at moderate grazing intensity) or a stress
to decrease ecosystem productivity (e.g., when overgrazed). (Adapted from Odum, E.P., J.T.
Finn, and E.H. Franz,
BioScience
, 29, 349, 1979.)
sampling depth. Data from the limited number of studies presented showed that
nitrogen fertilization increases SOC storage and emissions, tall fescue pastures
store more SOC than Bermuda grass pastures, and grazing returns more carbon to
soil than haying or unharvested management. Additionally, SOC can also be spa-
tially affected by animal behavior and soil depth. SOC storage under pastures is
not only important for mitigating GHG emissions, but more important on the farm
level for improving water relations, fertility, and soil quality (Franzluebbers 2005;
Franzluebbers and Doraiswamy 2007).
There have been assessments of land use effects on SOC in many different parts
of the world. Cardoso et al. (2009) reported that continuous grazing on native pasture
reduced total SOC and microbial carbon contents compared with ungrazed pasture
in Brazil. In Inner Mongolia, heavy grazing and continuous grazing increased soil
labile organic carbon significantly compared with ungrazed sites (Wu et al. 2012);
the authors recommended a grazing system with moderate intensity for improved
sustainability. In semiarid grasslands in Inner Mongolia, Zheng et al. (2010) reported
that species diversity and ecosystem productivity were related with abiotic fac-
tors, such as soil nutrition and precipitation, as well as anthropogenic activities,
such as grazing and agriculture. Diversity and community composition provided
the best predictors of system productivity in this study. In a study of a degraded
steppe
Stipa
-grassland community on the Loess Plateau of China, Cheng et al.
(2012) recommended rotational grazing as one of the methods of choice to promote
gradual recovery of species diversity. In Kenyan savanna, removal of cattle grazing
