Chemistry Reference
In-Depth Information
one-third of the current croplands were converted to forests, the US carbon
emissions could be reduced by as much as 10%. Carbon uptake depends on
climate, disturbance and management legacy, age and type of the forests.
114
While water availability limits productivity in semi-arid grass, shrub, wood-
land and dry forest ecosystems,
115
at high latitudes where low temperatures
limit water availability, early snowmelt and soil thaw initiate photosynthetic
carbon uptake, and warming induces a longer growing season where radiation
is the limiting factor.
116
Soil properties limit the availability of both water and
nutrients, potentially limiting plant growth. Soil texture, depth, salinity and
topography have strong local influences on forest growth.
117-121
Soil organic carbon (SOC) pools may store as much as 90% of the carbon in
terrestrial ecosystems
122
but can also vary widely in response to woody plant
encroachment (from 26200 g C m
22
to +2700 g C m
22
).
121,123-131
The increase
in woody encroachment of grasslands and deserts during the 20th century has
been attributed to a variety of factors, such as increased atmospheric CO
2
concentration, land-use change (grazing), climate patterns and fire suppres-
sion.
132-134
Woody plants influence SOC pool sizes, particularly beneath their
canopies,
135
through litter accumulation; they can affect soil respiration and
leaching (roots and microbes) and reduce erosion.
121
Because of the greater
rooting depths and higher root lignin content of woody plant species, soil
carbon is generally higher in shrublands than grasslands
122,136,137
and is
accompanied by more resistant organic matter in deeper soil layers.
138-141
Barger et al.
121
showed that bulk density and clay content mediate the
magnitude and direction of SOC changes with woody encroachment. Increases
in bulk density are linked to low SOC, and carbon losses are associated with
soils of bulk densities greater than 1.6 g m
23
. In the southern Great Plains,
USA, SOC accumulation rates are three times greater in fine soils than in
adjacent coarse soils,
136
and woody encroachment with higher SOC contents
has been associated with a clay gradient.
142-145
d
n
1
r
2
n
g
|
8
3.4 Nitrogen Responses
Nitrogen availability is closely tied to the water cycle
146,147
and it controls
photosynthetic rates and thus forest productivity, as well as carbon allocation
and resulting canopy development.
148-156
Nitrogen can limit carbon uptake
even when water is readily available, but when water is limiting, plants cannot
take up available nitrogen unless they develop a symbiotic relationships with a
nitrogen fixer.
157-159
For this reason, the sizes of the soil carbon and nitrogen
pools are good indicators of any change in the local soil nitrogen-supplying
capacity.
160-162
Nitrogen dynamics are very much driven by the constant feedbacks from
plant, soil and microbial interactions.
163
Tree species influence nitrogen cycles
in different ways through root uptake, mycorrhizal associations, exudation
and the chemical quality of plant litter,
164
and trees in the same climate with
