Jökull - 01.12.2007, Síða 44

McPeek et al.
The mineral, organic, and occluded fractions of P
were compared among the soils of different ages in
southern Iceland to examine the dynamics of soil P at
different developmental stages. Occluded and organic
P increased over time (Figure 4). Occluded P was hi-
ghest in the oldest soil and the lowest in the youngest
soil, averaging 44.5, 9.4, 7.7, and 2.7 mg cm!2 for
the 934, 1300, 1554, and 1783AD flows, respectively.
The content of organic P showed the same trend, i.e.,
higher in the older soils and lower in the younger so-
ils. The organic P was 13.7, 8.6, 6.5 and 4.5 mg cm!2
for the 934, 1300, 1554, and 1783 AD flows, respec-
tively (Figure 4). This pattern of increasing content
of these P fractions with age supports previous mo-
dels of P transformations during weathering of rocks
and development of soils (e.g., Chadwick et al., 1999;
Walker and Syers, 1976). The contents of the three
P fractions along the trajectory of soil development
in southern Iceland therefore provide the critical time
components of soil P cycling that is often lacking in
general models (Figure 5).
Figure 4. Content of organic, mineral and occluded P
in soils derived from four lava flows in Iceland. n=1,
2, 2 and 3 for 1783, 1554, 1300 and 934 AD flows,
respectively. – Magn lífræns, ólífræns og ásogaðs
fosfórs í jarðvegi á Skaftáreldahrauni, Hekluhraunum
frá 1554 og 1300 og Eldgjárhrauni.
Figure 5. Modeled changes in soil P geochemistry
over time (based on Walker and Syers, 1976), sho-
wing transformation of mineral P into non-occluded
and organic forms before eventual dominance of occ-
luded (oxide-bound) and organic forms. The relative
bio-reactivity of P increases from mineral to occlu-
ded to organic forms of P. Note the continual loss of
total P from system. – Jarðefnafræðilíkan af hegðun
fosfórs (P) með tíma (byggt á Walker og Syers, 1976)
sem sýnir hvernig hlutfallslegt magn fosfórs breytist
með tíma frá ólífrænu formi yfir í ásogað og að lokum
á lífrænt form um leið og heildarmagn fosfórs lækkar.
In contrast to occluded and organic P, content of
mineral P in the basalt-derived soils in Iceland di-
splayed no pattern with age. Mineral P was 31, 9.4,
28, and 5.1 mg cm!2 for the 934, 1300, 1554, and
1783 AD flows, respectively. The lack of a pattern
in mineral P is presumably due to the varying mine-
ral P contents in the surface lava flows or varying in-
flux of eolian materials and tephra in different geogra-
phic locations thus impacting the net concentration of
this component from site to site. Although of rela-
tively homogenous compositions, lava flows may di-
splay some geochemical compositional variation with
time, especially for an element like P, which is relati-
vely low in concentration. Clearly, more needs to be
done, including comparing results of P using different
analytical methods (Blakemore et al., 1987), to better
42 JÖKULL No. 57