Agriculture Reference
In-Depth Information
the initial parent material and additions
from plants (Amundson, 2005). To these
we must also include alteration of parent
material, as well as external inputs from
sources other than plants. The soil produc-
tion rate is the rate at which saprolite is con-
verted to soil. To expand upon these two
statements, it is first useful to recognize that
weathering and soil formation are coupled
through the 'state factors of soil formation',
originally described by Dokuchaev (1880;
cited by Fortescue, 1992). A description of
the factors of soil formation that is highly
relevant today was given by Jenny (1941).
These are: (i) nature of soil parent material;
(ii) regional climate; (iii) organisms includ-
ing humans; (iv) topography; and (v) time.
The combined influence of these soil-forming
factors determines the properties of a soil.
Understanding the complex interactions
among these factors requires an interdiscip-
linary scientific approach (Brantley et al .,
2007). Further, characterizing soil forma-
tion is complicated by the fact that the pro-
cesses involved occur over distance scales
from the submicroscopic to planetary (see
Fig. 6.1 ) and timescales from microseconds
to millions of years (Brantley et al ., 2007;
Brantley, 2008).
To appreciate the profound global role
that plants have played in weathering and
soil formation, it is useful to consider the
broad sweep of Earth's history. As expressed
by Amundson et al . (2007), 'The surface of
our planet is the result of billions of years
of feedback between biota and earth mater-
ials.' Paleosols, soils formed in Earth's past
and preserved in the rock record, document
the profound influence of biota on Earth
surface processes. Weathering processes
during Earth's early history differed from
modern ones as the result of the planetary-
scale evolution of Earth's biota and feed-
backs of this evolution to the atmosphere.
During the Archean (i.e. prior to 2.5 Ga), soil
formed under a reducing atmosphere, result-
ing in loss of iron from the profile (Rye and
Holland, 1998). None the less, these very
early soils are thick, clay-bearing, exhibit
loss of the base cations Ca 2+ , Mg 2+ and Na + ,
and based on carbon isotopic evidence, show
traces of life (see review by Retallack, 2005).
The intensity of Archean weathering may
reflect an elevated atmospheric CO 2 content
compared to today. Later, colonization of
land by mosses, fungi and liverworts by
700 Ma led to enhanced clay formation and
burial of organic C at continental margins.
Kennedy et al . (2006) proposed that the col-
onization of land surfaces by fungi and other
organisms stabilized soil cover, created longer
groundwater residence times, added organic
acids and chelating agents and drastically
enhanced secondary mineral formation, i.e.
produced a 'clay factory'.
The advent of land plants was a major
event in the Devonian that influenced weather-
ing and soil formation dramatically. Land
plants altered the balance of atmospheric
CO 2 by the removal of C to plants and soil
(Beerling and Berner, 2005). During the early
Devonian period (about 420 Ma), small, leaf-
less plants populated the landscape, but by
the end of the Devonian (about 359 Ma),
large trees had evolved. This late Devonian
flora was associated with complex and deep
rooting systems. Deep rooting likely led to
the enhanced dissolution of bedrock (Berner,
1997; Retallack, 1997).
Recently, the roles of mycorrhizal fungi in
weathering have received increased attention
Separation of the State Variables
Although the processes leading to soil for-
mation are interdependent, these can be
separated under some circumstances, thus
permitting clarification of the role of the in-
dividual variables.
The role of organisms
The recognition that vegetation plays an im-
portant role in weathering can be traced
back to Belt (1874; cited by White, 2003).
He wrote that 'the percolation through rocks
of rain water charged with a little acid from
decomposing vegetation' accelerated weather-
ing. Plants are the main source of carbon to
soils through tissue residues, root exudates
and symbiotic fungi.
 
Search WWH ::




Custom Search