2.3. Addition of two 13C-labelled substrates (fructose and vanillin)
and incubation
In order to determine how the different soil structures affected
the mineralisation of soluble substrate, the mineralisation of two
different soluble, uniformly 13C-labelled substrates (fructose and
vanilline Cambridge Isotope Laboratories, USA)was monitored. 13Clabelled
substrates were used in order to differentiate the mineralisation
of the added substrate from that of native soil organic carbon.
After the two-month pre-incubation, samples were amended
with a solution of either 13C-labelled fructose or 13C-labelled
vanillin (d13C fructose ¼ 1722& and d13C vanillin ¼ 1272&) at a
concentration equivalent to 15 mg substrate C g1 soil C. The C
addition was sufficiently small to ensure that there was no N limitation
during the incubation. The substrate solutions (2.4 mL) were
applied to the surface of the samples using a multi-channel pipette.
This ensured an even distribution of solution across the samples.
Unlike the native soil organic matter, the added soluble substrates
were located in specific pore sizes in the pore network of the
samples: the substrate solutions (or water in the case of the control
samples) were added in order to bring the matric potential of the
samples to 31.5 kPa, meaning that the substrates were located in
pores with a maximal pore-neck diameter of 9.5 mm (the Jurine
Laplace equation). As the pre-incubation was also carried out
at 31.5 kPa, the samples were dried slightly prior to the substrate
addition by leaving the flasks open for several hours. Fructose and
vanillin were chosen because they are soluble, but differ in their