Jökull - 01.12.2007, Blaðsíða 43

Rates of carbon ingrowth and nutrient release from young Icelandic basalts
found in our study site is nonetheless in agreement
with results from other studies, which showed a de-
crease of organic C with soil depth in most developed
soils (Schlesinger, 1997). On the more recent flows,
however, the soils were young and poorly developed.
As a result, a trend of organic C as a function of soil
depth at these sites could not be clearly established
(data not shown).
Figure 2. Organic C content in the 934 AD site, the
oldest of the four flows in this study. – Magn lífræns
kolefnis í jarðvegi á Eldgjárhrauninu.
Organic C content in the soils per unit ground area
was determined to examine the relationship between
soil development and accumulation of soil organic
C. We found an increase in the organic matter con-
tent in the total soil profile in older flows, i.e., older
soils had higher organic C. The organic C content
decreased from 3.3 kg m!2 for the 934 AD flow to
1.01, 0.80, and 0.21 kg m!2 for the 1300, 1554, and
1783 AD flows, respectively (Figure 3). This trend
demonstrates that the ingrowth of organic C into the
soil profiles was due to the gradual accumulation of
complex organic compounds in the soil over time.
The net rate of C accumulation was determined
for each site by dividing the total soil C content in the
whole soil profile by the age of the flow upon which
the soil developed. The net accumulation rate tota-
led 3.08, 1.44, 1.78, and 0.95 kg C m!2 yr!1 for the
934, 1300, 1554, and 1783 AD flows, respectively.
Interestingly, our findings reveal that the organic mat-
ter production in the Icelandic soils is continuing to
increase even after 1000 years of soil development.
As a matter of fact, rate of C accumulation seems to
be accelerating in older soils, presumably because of
their ability to support more and larger plants, which
contribute to higher rate of C ingrowth in the soil.
Figure 3. Organic C content in soils derived from four
lava flows in Iceland. n=1, 2, 2 and 3 for 1783, 1554,
1300 and 934 AD flows, respectively. – Lífrænt
kolefni í jarðvegi mynduðum á Skaftárelda- (1783),
Heklu- 1554 og 1300 og Eldgjárhrauni (934).
The values derived for the organic C accumula-
tion in southern Iceland are within the range of C con-
tent of 0.1–21.6 kg m!2 for rocky and boreal environ-
ments (Schlesinger, 1997). The average C uptake in
these Icelandic soils, 0.95–3.08 g m!2yr!1, is com-
parable to 2.2–11.7 g m!2yr!1 for boreal ecosystems
(Schlesinger, 1997), but much higher than the sequ-
estration rate in severely degraded soils in Iceland
(Arnalds et al., 2000). Additionally, the mean organic
C content of the four sites is closer to the values fo-
und in rocky environments (Schlesinger, 1997). These
results demonstrated that Icelandic Andosols have a
similar rate of increase of organic C and a similar ca-
pacity for C storage as in boreal ecosystems in rocky
environments.
JÖKULL No. 57 41