drivers of soil respiration of root and microbial ... - Unitus DSpace

3.2.2. In situ pulse labeling procedure and gas sampling
Pulse labeling was performed in the first days of June 2008.
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1. Three labeling zones of 60cm 2 were established on the study site two months prior to the
labeling procedure. Inside each zone two PVC collars (Ø20cm) were inserted to the soil.
One of the collar was placed on the planted soil (total respiration, Rtot) and another one - on
the bare soil with previously clipped plants (belowground respiration, Rb).
2. A Plexiglas labeling chamber (LC) was designed and constructed (Fig. 1 and 2) in the
laboratory of forest ecology, University of Tuscia. LC was placed on soil to cover the
chosen labeling zones. Air was circulating between LC and an infrared gas analyzer
(CIRAS DC, PP-System, UK) using its internal pump at a constant rate. Some of the
circulating gas stream was by-passing the IRGA by the use of adjacent pump, connected to
the column with soda-lime for CO2 scrubbing.
3. A labeling solution (Na2 13 CO3) was placed in 100 ml jar and sealed with a lid containing
two valves and a septum port. Plastic tubes were connecting the jar to the IRGA-soda lime
system, in the entrance of it to LC. A 5M lactic acid was introduced to the jar through the
septum port with a syringe, after that the valve on the lid was opened letting the label to
enter the LC. To each LC was introduced 350 mg of the Na2 13 CO3.
4. To stimulate the photosynthesis when the 13 CO2 was assimilated an additional unlabeled
pure CO2 was released (by acidifying Na2CO3) into LC, keeping the level of CO2 in the
chamber close to the ambient by regulating the rate of the flow of gas through soda-lime.
The hole labeling procedure took 1 h.
5. After the opening of LC was started the trapping of recently assimilated CO2 from the PVC
collars: each collar was sealed with a lid for 1 hour (Fig. 3). Each chamber headspace was
connected through a valve to a 2L flask, which previously had been evacuated to a high
vacuum. A 2L vacuum flask were collected after 1 hour of CO2 accumulation in the
chamber space. After that the lid of the PVC chambers remained open, up to the next
accumulation. A 2L flask from bare plot was collected before the pulse labeling to estimate
the
13 C-CO2. background signal. Gas sampling was performed ten times during the chase
period which lasted ten days. The timing of the sampling was the following: 1h-2h-4h-10h-
16h-24h-35h-48h-96h-240h.
6. Along with the pulse labeling, measurements of soil respiration from mesh bags which were
installed previously to separate heterotrophic and autotrophic respiration were performed.
Both time lags, obtained with 13 C labeling and meshes experiment were confronted.