Global Change Abstracts The Swiss Contribution - SCNAT

154 Global Change Abstracts – The Swiss Contribution | Coupled Systems and Cycles
08.1-303
Effects of increased soil water availability on
grassland ecosystem carbon dioxide fluxes
Risch A C, Frank D A
USA, Switzerland
Agriculture, Soil Sciences , Ecology , Plant Sciences
There is considerable interest in how ecosystems
will respond to changes in precipitation. Alterations
in rain and snowfall are expected to influence
the spatio-temporal patterns of plant and soil
processes that are controlled by soil moisture, and
potentially, the amount of carbon (C) exchanged
between the atmosphere and ecosystems. Because
grasslands cover over one third of the terrestrial
landscape, understanding controls on grassland
C processes will be important to forecast how
changes in precipitation regimes will influence
the global C cycle. In this study we examined how
irrigation affects carbon dioxide (CO 2) fluxes in
five widely variable grasslands of Yellowstone National
Park during a year of approximately average
growing season precipitation. We irrigated plots
every 2 weeks with 25% of the monthly 30-year average
of precipitation resulting in plots receiving
approximately 150% of the usual growing season
water in the form of rain and supplemented irrigation.
Ecosystem CO 2 fluxes were measured with
a closed chamber-system once a month from May-
September on irrigated and unirrigated plots in
each grassland. Soil moisture was closely associated
with CO 2 fluxes and shoot biomass, and was
between 1.6% and 11.5% higher at the irrigated
plots (values from wettest to driest grassland) during
times of measurements. When examining the
effect of irrigation throughout the growing season
(May-September) across sites, we found that
water additions increased ecosystem CO 2 fluxes
at the two driest and the wettest sites, suggesting
that these sites were water-limited during the
climatically average precipitation conditions of
the 2005 growing season. In contrast, no consistent
responses to irrigation were detected at the
two sites with intermediate soil moisture. Thus,
the ecosystem CO 2 fluxes at those sites were not
water-limited, when considering their responses
to supplemental water throughout the whole season.
In contrast, when we explored how the effect
of irrigation varied temporally, we found that irrigation
increased ecosystem CO 2 fluxes at all the
sites late in the growing season (September). The
spatial differences in the response of ecosystem
CO 2 fluxes to irrigation likely can be explained
by site specific differences in soil and vegetation
properties. The temporal effects likely were due to
delayed plant senescence that promoted plant and
soil activity later into the year. Our results suggest
that in Yellowstone National Park, above-normal
amounts of soil moisture will only stimulate CO 2
fluxes across a portion of the ecosystem. Thus, depending
on the topographic location, grassland
CO 2 fluxes can be water-limited or not. Such information
is important to accurately predict how
changes in precipitation/soil moisture will affect
CO 2 dynamics and how they may feed back to the
global C cycle.
Biogeochemistry, 2007, V86, N1, OCT, pp 91-103.
08.1-304
Chemistry, transport and dry deposition of
trace gases in the boundary layer over the
tropical Atlantic Ocean and the Guyanas during
the GABRIEL field campaign
Stickler A, Fischer H, Bozem H, Gurk C, Schiller C,
Martinez Harder M, Kubistin D, Harder H, Williams
J, Eerdekens G, Yassaa N, Ganzeveld L, Sander R,
Lelieveld J
Switzerland, Germany, Canada
Meteorology & Atmospheric Sciences , Modelling
We present a comparison of different Lagrangian
and chemical box model calculations with measurement
data obtained during the GABRIEL campaign
over the tropical Atlantic Ocean and the
Amazon rainforest in the Guyanas, October 2005.
Lagrangian modelling of boundary layer (BL) air
constrained by measurements is used to derive a
horizontal gradient (approximate to 5.6 pmol/mol
km(-1)) of CO from the ocean to the rainforest (east
to west). This is significantly smaller than that derived
from the measurements (16-48 pmol/mol
km(-1)), indicating that photochemical production
from organic precursors alone cannot explain the
observed strong gradient. It appears that HCHO
is overestimated by the Lagrangian and chemical
box models, which include dry deposition but not
exchange with the free troposphere (FT). The relatively
short lifetime of HCHO implies substantial
BL-FT exchange. The mixing-in of FT air affected
by African and South American biomass burning
at an estimated rate of 0.12 h(-1) increases the CO
and decreases the HCHO mixing ratios, improving
agreement with measurements. A mean deposition
velocity of 1.35 cm/s for H 2O 2 over the ocean
as well as over the rainforest is deduced assuming
BL-FT exchange adequate to the results for CO.
The measured increase of the organic peroxides
from the ocean to the rainforest (approximate to
0.66 nmol/mol d(-1)) is significantly overestimated
by the Lagrangian model, even when using high
values for the deposition velocity and the entrainment
rate. Our results point at either heterogeneous
loss of organic peroxides and/or their radical
precursors, underestimated photodissociation