Principles of terrestrial ecosystem ecology.pdf

324 14. Landscape Heterogeneity and Ecosystem Dynamics
Soil C (kg m -2 )
7
6
5
4
Conversion to agriculture
from native vegetation
Historical management
53% of 1907
Conventional
tillage
Reduced
tillage
61% of 1907
3
1900 1920 1940
Year
1960 1980
Figure 14.11. Simulation of loss of SOM after conversion
of grassland to agriculture, followed by a
small increase with conversion to low-till agriculture.
Losses of soil carbon reduce the productive potential
of the soil and transfer carbon in the form of CO 2
from the soil to the atmosphere. New techniques of
low-till and no-till agriculture can reduce the magnitude
of soil carbon loss or, under some circumstances,
lead to a small net carbon accumulation.
(Redrawn with permission from Science, Vol. 277 ©
1997 American Association for the Advancement of
Science; Matson et al. 1997.)
cultural systems (Fig. 14.11). These soil carbon
losses often occur within 20 years in temperate
ecosystems or within 5 years in tropical systems,
depending on soil temperature and moisture.
The large regular inputs of nutrients required
to sustain intensive agriculture (see Fig. 8.1)
increases the emissions of nitrogen trace gases
that play a significant role in the global nitrogen
cycle and link these ecosystems with downwind
ecosystems (see Chapters 9 and 15).
Nutrient loading on land increases nonpoint
sources of pollution for neighboring aquatic
ecosystems (Carpenter et al. 1998). Phosphorus
additions on land have particular large effects
for at least two reasons. First, primary production
of most lakes is phosphorus limited and
therefore responds sensitively to even small
phosphorus additions. Second, much of the
phosphorus added to agricultural fields is
chemically fixed, so orders of magnitude more
phosphorus are generally added to fields than
is absorbed by crops. These large additions rep-
resent a massive reservoir of phosphorus that
will continue to enter aquatic ecosystems long
after farmers stop adding fertilizer. Phosphorus
inputs from human and livestock sewage have
similarly long-lasting effects.
Intensive agriculture for rice production in
flooded ecosystems produces a different type of
biogeochemical hot spot. Rice feeds half of the
world’s population, with 90% of the production
occurring in Asia. The most intensive rice cultivation
takes place in periodically flooded fields,
where reduced oxygen supply and decomposition
lead to greater accumulation of SOM than
in upland agricultural systems. Flooding creates
an ideal environment for methanogens, which
produce methane during decomposition of
organic matter under anaerobic conditions
(see Chapter 7). Paddy agricultural systems are
therefore important sources of atmospheric
methane and now produce about half as much
methane as all natural wetlands. Together with
cattle production, rice cultivation accounts for
much of the increase in atmospheric methane
(see Chapter 15).
Land use change caused greater ecological
impact during the twentieth century than any
other global change. Understanding and projecting
future changes in land use are therefore
critical to predicting and managing future
changes in the Earth System. Development of
plausible scenarios for the future requires close
collaboration among climatologists, ecologists,
agronomists, and social scientists. Optimistic
scenarios that assume that the growing human
population will be fed rather than die from
famines, wars, or disease epidemics project continued
large changes in land use, particularly
in developing countries (Alcamo 1994). What
actually occurs in the future is, of course, uncertain,
but these and other scenarios suggest that
land use change will continue to be the major
cause of global environmental change in the
coming decades. Ecologists working together
with policy makers, planners, and managers
have the opportunity to develop approaches
that will minimize the impact of future landscape
changes (see Chapter 16). This vision
must recognize the large effects of land use
change on landscape processes and their consequences
on local to global scales.