An introduction to climate change - Climate Change: Biological and Human Aspects - page 25

or other of these two types of feedback dominates at any given time. The combined
global climate picture is one of stable (or semi-stable) states between which there
is occasional rapid flipping. One of the main pacemakers timing these flips is the
combination of Milankovitch's orbital parameters.
Two questions arise from all of this. The first is whether, with current global
warming, the Earth is now shifting towards a new feedback system that may encourage
further warming. One example of a mechanism that might drive this is soil carbons,
especially at high latitudes, and whether it may be released, through warming, into
the atmosphere. Such soils include peatlands that are at so high a latitude that they
are either frozen for part of the year or are permafrosts. Michelle Mack from the
University of Florida and colleagues (Mack et al., 2004), including those from the
University of Alaska Fairbanks, have looked at carbon storage in Alaskan tundra.
Carbon storage in tundra and boreal soils is thought to be constrained by carbon-
nutrient interactions because plant matter is the source of much (nearly all) soil
carbon and plant growth is usually nitrogen-limited. Should soils warm in response
to climate change, it is thought that nutrient mineralisation from soil organic matter
will increase. This should increase plant growth. However, total-ecosystem carbon
storage will depend on the balance between plant growth (primary productivity) and
decomposition. Experiments at lower latitudes (in temperate and tropical zones) have
in the past given variable results (although we will cite a major - albeit rough -
assessment of European soil carbon in Chapter 7), but high-latitude ecosystems,
because of the large amount of carbon in their soil, show a clearer relationship
between productivity and soil carbon storage. In 1981 Mack and colleagues began
one of the longest-running nutrient-addition experiments in Alaska by adding 10 g
of nitrogen and 5 g of phosphorus m − 2 year − 1 . This is about five to eight times the
natural deposition rate in moist acidic tundra soils. Two decades later poaceden or
the graminoid (grass) tundra - which is dominated by the tussock-forming sheathed
cottonsedge ( Eriophorum vaginatum ) - had changed to a shrub tundra dominated by
thedwarfbirch( Betula nana ). The carbon above ground (in the form of plants and
litter) had increased substantially; however, this was more than offset by a decrease
in carbon below ground in the soil. This decrease in below-ground carbon was so
great that the net result for the ecosystem was a loss of 2000 g of carbon m − 2 over
20 years. (The ecosystem's nitrogen did not change nearly so much, other than that
a greater proportion of it at the end of the experiment was found above ground in
the vegetation.) This loss of carbon was approximately 10% of the initial carbon in
the ecosystem, and much will have entered the atmosphere as the greenhouse gases
methane and carbon dioxide.
Another approach to carbon loss from soils is soil respiration, the flux of microbially
and plant-respired carbon dioxide from the soil to the atmosphere. In 2010 Ben Bond-
Lamberty and Allison Thomson of Maryland University in the USA conducted a
meta-analysis of the literature on soil respiration. They obtained 1434 data points
from 439 studies conducted worldwide and matched these with climate data. They
found that air temperature was positively correlated with soil respiration. Increasing
soil respiration globally with temperature does not necessarily constitute a positive
climate feedback as there could be higher inputs of carbon into soil (through plant