Agriculture Reference
In-Depth Information
Soil carbon stocks are reduced by 20 to 40 % following clearing and cultivation and
much of this is lost in the first two to five years‚ depending on climate (Davidson and
Ackerman‚ 1993). The stocks that remained after cultivation formed a constant fraction
of the pre-clearing amounts in the largely North American studies collated by these authors.
Major off-site exports also occur through biomass harvesting in cropped agricultural and
forest plantation systems.
Carbon is present within soils in all three phases and in the biota; its chemical composi-
tion‚ distributions‚ concentrations and movements within the soil are of great pedogenetic
and applied importance. Carbon in soils occurs in an enormous variety of compounds
ranging upwards in complexity from carbonates to simple gases to aliphatic acids to
large structured macromolecules. It also forms part of a range of poorly-defined humic
polymers‚ often colloidal in nature.
The carbon content is a major factor influencing such factors as the bulk density of
the soil mass‚ as illustrated by Walker and Adams (19S8) for 22 New Zealand soils
(Figure I.31). However‚ as shown by its generally favourable effects in promoting
soil structural development‚ this is much more than the simple dilution effect of higher
organic matter concentrations. The carbon concentration strongly influences nutrient
supply to plants and is important in determining soil stability.
In the solid phase‚ carbon occurs predominantly in the living biomass and dead
organic matter and in the soils of drier areas as calcium‚ magnesium and other carbon-
ates precipitated
in situ
and sometimes forming discrete layers or horizons. Soils formed
from calcareous parent materials such as certain limestones may also contain substan-
tial carbonates.
Considerable carbon may occur as charcoal in the upper profiles of soils whose
vegetation cover has been subject to burning‚ or be present as bands in soils subjected to
periodic ash showers. In addition to its presence as discrete bands in certain soils‚ finely-
divided charcoal occurs widely in almost all Australian soils (Skjemstad
et al
.‚ 1997)
and may be expected to be so in many tropical environments‚ and wherever biomass
burning has been common. Skjemstad
et al.
(1996) estimated that finely-divided charcoal
