This soil has developed under a grassland community that is dominated by so-called C4-plants.
These plant species have a different ratio of two stable carbon isotopes, C-12 and C-13,
in their tissues (and therefore in their recalcitrant organic residues that slowly accumulate
in the soil over the millenia) than the so-called C3-plants.
The tree species in our experiment are C3-plants like all trees.
The change in the ratio of C-12 to C-13 in the soil during the experiment allows us to quantify
the carbon flux from these young trees into the soil even though the extra carbon input is minute
compared with the carbon content of the soil.
 

Months later ... The soil has been processed.
In other words, thousands of little pieces of paper from the disintegrated
paper lining of the plastic bags have been picked out.
Paper is made from C3 plant material and would therefore have interfered
with our measurements of carbon fluxes into the soil.
All that soil had to be thoroughly mixed
to make sure we were using the same substrate in the entire experiment.

Trees grow in associations with fungi, called mycorrhizas.
Before starting the experiment, we had to inoculate our soil with mycorriza
collected from stands of the different tree species.
 

Nothing much to see here, but then most of the interesting stuff lives in the soil.
As we wanted to follow the fate of plant material in the soil food web,
the soil used in the experiment was also inoculated with microfauna
(such as nematodes and microarthropods) and extracted from soil cores taken in the field.

Cores such as this one ...
 

... and the other 7499, waiting to be extracted (as seen here).

Late winter 2001. The first containers for our 504 plants are being filled
and put into the Solardomes, the special greenhouses used for this experiment.

The containers are home-made plastic sleeves, which allow the plants to put down deep roots.
If trees are grown in containers that are too shallow, root growth may be restricted
and this can have unforeseen consequences for the growth of the entire plant.
We are using six species, a relatively early-successional (pioneer) species
and a relatively late-successional (shade-tolerant) species each from three plant families:
oak (Quercus robur) and beech (Fagus sylvatica),
birch (Betula pendula) and hornbeam (Carpinus betulus), and
pine (Pinus sylvestris) and fir (Abies alba).
The species are expected to show different responses to the elevated CO2 concentrations.
 

Spring 2001. The experiment is carried out in an array of 12 Solardomes.
Four different CO2 concentrations are replicated in three Solardomes each:
ambient, ambient plus 100 ppm, ambient plus 200 ppm, and ambient plus 300 ppm CO2.
The plants are grown at two levels of nutrient availability to test
how this variable affects their responses to the elevated CO2 concentrations.
The green tank in the background contains CO2 that is added
to the air in the elevated-CO2 treatments.

The global CO2 concentration keeps rising, but nobody can be sure
which concentration the world will have reached by the end of this century.
It is, however, very likely that it will be somewhere in the range we are employing.

At intervals during the experiment and at the end, the carbon isotope ratios (C-12/C-13)
of the soil, the plants and the organisms in the soil are analysed
to find out how much carbon has gone into the soil,
whether the amount correlates with the growth rates of our different tree species
in the different CO2 concentrations and nutrient levels,
and what has happened to the carbon in the soil.
 

First, the shoots are detached from the roots, then roots are separated from the soil,
and finally the fauna is extracted from the soil.
The different fractions are dried, weighed and ground up for the analyses.

The carbon isotope ratio measurements are complemented by a range
of physiological investigations such as the determination of
photosynthesis and transpiration rates shown here.
 

Autumn 2001.
The first season is over, and the plants will be grown for a second one in 2002.