Carbon balance of a rain-fed maize field

This thesis analyses micrometeorological measurements
carried out from June 2004 until October
2006 in the framework of the INTERREG IIIa
Project Nr. 3c.10 ”Impacts of climate change on vegetation
in the Upper Rhine Valley”.
The study addresses the exchange processes of
carbon, water and energy of a rain-fed field under
maize-fallow rotation. Measurements with an
ultrasonic anemometer-thermometer, an open-path
CO2/H2O infra-red gas analyser and of the meteorological
drivers such as photosynthetic photon flux
density (PPFD), temperature and precipitation give
insight in the interaction between atmosphere, soil
and vegetation.
Energy balance considerations show similar patterns
of the energy flux densities for vegetation periods
and bare field conditions. Energy balance closure is
80 % and 52 %, respectively. A closer look shows a
clear diurnal pattern with bad closure during nighttime
and an increasing closure fraction during daytime,
in fact resulting in an overshooting in late afternoon.
Evapotranspiration shows a clear seasonal pattern
with maximum values of ~3.5 mm d-1 reached in
mid-July. The total water need for the three subsequent
years is 321, 397, and 422 mm per kg kernels
(yield). The water use efficiency shows a strong relationship
with PPFD and the amount of biomass.
The focus of the study is on carbon balance. During
the three subsequent vegetation periods 930, 785,
and 841 g C m-2 are sequestered, respectively. The
yield is 455, 417, and 340 g C m-2. About 40 %
of the biomass remaining on the field at harvest are
decomposed during the dormant season. The resulting
numbers for the carbon balance show a ”yearly”
sink of this agroecosystem of ~250 g C m-2. Besides
unlimited photosynthetic active radiation the
combination of the optimal temperature range with
the needed precipitation amount corresponding to
the need of the actual growth stage are essential for
optimal maize growth.