Jökull - 01.12.2007, Page 40

McPeek et al.
position of nutrients, particularly phosphorus (P), and
of tephra could have significant impact on soil forma-
tion (Arnalds, 2004; Gislason et al., 1996), a concur-
rent examination of changes in mineral nutrients and
carbon (C) will improve our understanding of the ef-
fects of biotic and abiotic factors on soil formation.
Humid climate and uniform parent materials therefore
make Iceland an ideal location for studying how we-
athering affects soil development and the rate of chan-
ges in soil nutrients, e.g., nitrogen (N) and P, in young
volcanic soils.
A terrestrial ecosystem depends on the availabi-
lity of numerous elements, including C and P, to func-
tion properly. Carbon dioxide (CO2) enters the bio-
sphere from the atmosphere mostly via photosynthe-
sis of green plants. Carbon in plants can cycle thro-
ugh other pathways (e.g., plant respiration) and return
to the atmosphere. Carbon in plants can also become
part of soil organic C pool. Carbon content in young
soils tends to increase as the soil develops. Thus, C
content in the soil is indicative of how plant activity
and other aspects of an ecosystem have changed over
a period of time.
Phosphorus, on the other hand, has no natural ga-
seous phase. The main source of P in terrestrial envi-
ronments is rocks and P is released into the ecosys-
tem by chemical weathering of these rocks, although
deposition of airborne P can also contribute to soil P
pool to some extent (Chadwick et al., 1999; Gislason
et al., 1996). The most significant source for P in soils
is apatite minerals. These minerals can be congruently
weathered as a result of reaction with dissolved CO2
in water:
Ca5(PO4)3OH+ 4CO2 + 3H2O!
5Ca2+ + 3HPO2!4 + 4HCO
1!
3
In soils, P is released from mineral grains by se-
veral processes. First, the reduced pH produced from
respiration-related CO2 in the vicinity of both de-
composing organic matter and root hairs dissolves P-
bearing minerals (mainly apatites) and releases P to
root pore spaces. Second, organic acids released by
plant roots can also dissolve apatite minerals and re-
lease P to soil pore spaces (Jurinak et al., 1986). Pho-
sphorus is immobile in most soils, and its slow rate of
diffusion from dissolved form in pore spaces strongly
limits its supply to root surfaces. Furthermore, much
of the available P in soils is in complex organicmatter,
which is not directly accessible for plant or microbial
uptake. As a consequence, plants and microbes have
developed two specific tactics to increase the supply
of P to roots. One is through phosphatase, an enzyme
often excreted by plants and soil microbes. Phospha-
tase can catalyze the release of bio-available inorganic
P from organic matter. Another is through a symbio-
tic mutualism between plants and mycorrhizae, which
not only increase the absorption surface of plant ro-
ots, but also excrete phosphatase and organic acids to
release P and provide an active uptake site for P that
is diffused from soil pore spaces to root surfaces (e.g.,
Schlesinger, 1997).
The distribution among different forms of soil P
changes greatly with time and soil development. The
forms of soil P can be grouped into refractory (not re-
adily bio-available) and labile (readily bio-available).
The refractory forms include P in apatite minerals
and P co-precipitated with and/or adsorbed onto iron
and manganese oxyhydroxides (termed “occluded”
P). The reducible oxyhydroxides have large binding
capacities for phosphate, due to their immense surface
area and numerous delocalized positively charged si-
tes (e.g., Froelich, 1988). The labile forms include P
in soil pore spaces (as dissolved phosphate ion) and
adsorbed onto soil particle surfaces (these forms are
termed “nonoccluded” P), as well as P incorporated
in soil organic matter. On a newly-exposed lithic sur-
face, nearly all of the P is present as P in apatite. With
time and soil development, however, P is increasingly
released from this form and incorporated in the others
(Crews et al., 1995; Filippelli, 2002; Filippelli and
Souch, 1999; Filippelli et al., 2006; Walker and Sy-
ers, 1976). Over time, the total amount of P available
in the soil profile decreases, as soil P is lost through
surface and subsurface runoff. Eventually, the soil re-
aches a terminal steady state, when soil P is heavily
recycled and any P lost through runoff is slowly re-
placed by new P weathered from apatite minerals at
the base of the soil column.
38 JÖKULL No. 57