Geoscience Reference
In-Depth Information
water into biomass via plant roots. As a nutrient ion is
absorbed by the plant, cation exchange releases an ion
from colloid exchange sites, to maintain the concentra-
tion. Nutrients in plants are returned to the soil via litter
into the soil organic matter, to be released again into the
soil solution by mineralization. Leaching causes a loss of
nutrients from the ecosystem into streams.
Precipitation
input
from the atmosphere provides an import from
outside the ecosystem, and dry deposition from dust can
also take place. Some of the precipitation input can be
absorbed by plants through
leaf uptake
; rainwater running
across leaf surfaces can also leach ions back to the soil
solution by
leaf leaching
. If the nutrient element forms
insoluble and unavailable chemical compounds, it is being
removed, even if only temporarily, from the cycle by
fixation
. The base cations held on the cation exchange
sites of the soil's clay mineral and humic colloids are
available to plants, and whilst held as adsorbed ions are
not subject to leaching. Thus the soil colloids assume
pivotal importance in biogeochemical cycles. In turn the
ability of the colloids is determined by soil pH. Acid
soils have colloids with hydrogen ions occupying many
of the exchange sites on the soil colloids. These soils are
associated with low base saturation, and the availability
and cobalt (Co
2+
). The term 'biogeochemical' reminds us
that vegetation, soil, rocks, atmosphere and wildlife must
never be considered separately, in isolation; each is part
of a continuously interacting ecosystem. The bio-
geochemical cycle is a good example of a process-response
system discussed in
Chapter 1.
In some respects the
biogeochemical cycles of the base cations are the simplest
cycles, as they do not have a gaseous component, although
there are atmospheric inputs of precipitation, dust and
aerosols. A general model of nutrient cycles for cations is
shown in
Figure 21.10
.
Considerable attention was paid to the study of
biogeochemical cycles by the Hubbard Brook ecosystem
study in New Hampshire, which started in 1963 as a major
experiment studying the biogeochemistry of a forest
ecosystem, under the direction of the US ecologists F. H.
Bormann and G. Likens. The ability of the watershed to
retain nutrients was monitored, and entire watersheds
were deforested in order to measure the effects on the
export of nutrients. No fewer than nine fluxes can be
identified in any biogeochemical cycle. Weathering
releases the element from rock minerals and the ion
becomes adsorbed by cation exchange on to clay minerals
or humic colloids in the soil.
Plant uptake
is from soil
Atmosphere
Dust, ions
3
1
Land,
ocean
1 Precipitation
2 Leaf leaching
3 Leaf uptake
4 Weathering
5 Mineralisation
6 Cation exchange
7 Fixation
8 Plant uptake
9 Leaching
2
Biomass
Soil H
2
O
8
5
7
5
Litter
Soil organic matter
6
Soil forming
minerals
Colloids
4
9
Groundwater
streams
