Dietzen, C.A., Larsen, K.S.,
Ambus, P.L., Michelsen, A.,
Ardnal, M.F., Beier, C.,
Reinsch, S. and Schmidt, I.K.
2019
Accumulation
of soil carbon
under elevated CO2
unaffected by warming
and drought.
Global Change Biology 25: 2970-2977.
NOTE:
One fear of
future CO2-induced
global warming
is rising temperatures
will stimulate microbial
decomposition
of soil carbon.
That would increase
rates of soil respiration
and release of CO2
into the atmosphere.
And that CO2 release would be
a positive (bad news) feedback
to rising global temperatures.
However several experiments
manipulating temperature and CO2
rejected the theory, and found
a negative (good news) feedback.
As does this study.
SUMMARY:
Dietzen et al. report that
"elevated CO2 had a positive effect
on soil carbon stocks in the studied
temperate heath-grassland,"
increasing soil carbon by 18.8%
between year 2 and year 8.
And that CO2-induced increase
could be a very conservative
estimate.
Soil carbon measurements
made before the start
of the experiment revealed
the elevated CO2 plots
initially contained
substantially less
carbon than the
ambient plots
(3.91 vs 5.06 kg C/m2).
Meaning the
actual increase
in soil carbon due to
elevated CO2 was
a much larger +50.1%,
when including
that difference.
Dietzen et al. also report
"observed increase in soil carbon
due to the stimulating effect
of elevated CO2 on below ground
plant growth was not diminished
by either drought or warming,
signifying that elevated CO2
had a stronger effect
on soil carbon
than either of these
climatic variables"
Because of this fact,
the authors conclude,
"given the lack of
significant interactions
between treatments,
our results suggest t
hat moderate changes
in warming and drought
are unlikely to modify
the rate of increase
in soil carbon stocks
under elevated CO2."
This site (and likely sites
elsewhere around the globe) is
"expected to store more carbon
under future climate conditions,
serving as a negative feedback
to elevated atmospheric CO2
concentrations."
DETAILS:
The eight scientists conducted
an eight year field study
at a site in
Brandbjerg, Denmark,
of a dry heath / grassland
ecosystem dominated by
two perennial species,
a grass
(Deschampsia flexuosa),
that dominates
with approximately
70% cover
and an
evergreen dwarf shrub
(Calluna vulgaris),
that accounts for
the remainder.
Dietzen et al. used a
multi-factor design approach
to investigate the effects
of elevated CO2, warming
and drought on soil carbon
stocks to a depth of 30 cm.
Atmospheric CO2 enrichment
was administered to a target
value of 510 ppm via a
free-air CO2 enrichment (FACE)
design from dusk until dawn,
but was not applied during
periods of complete
snow cover.
Warming was achieved
via the use of curtains
that reflected infrared
radiation back to the
soil surface, which
effectively warmed
the soil temperatures
by 0.4°C in the summer
and 0.2°C in the winter.
Drought periods
were introduced
one or two times a year
in the spring or summer
by excluding rainfall
via exclusion curtains
during rain events, which
exclusion was continued
until the soil water
content fell below 5%
in the upper 20 cm
of the soil profile,
at which point rewetting
was allowed so as to
maintain soil moisture
above the wilting point.
Drought periods typically
lasted from 1 and 5 weeks,
which corresponded to the
typical drought period
that occurs naturally
at the site.
