Environmental Engineering Reference
In-Depth Information
Figure 22.9
The calcium cycle in Hubbard Brook, New
Hampshire
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 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 concentration.
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
