## 2 Materials and Methods
### Soil Gas Monitoring
#### 2.1.2 Gas Analysis  
Vials were stored in a fridge until they were analysed, usually within 24 h. Gas samples were analysed using gas chromatography. The gas chromatographs used have been especially configured to meet our specific requirements, using different combinations of columns and detectors to yield  
Figure 1: Set-up and instrumentation at the soil gas monitoring sites in Baden-Württemberg. Right: Enlarged view: cross section of a passive gas sampler used for the routine monitoring of soil gas concentrations. Left: The standard set-up included (**1**) arrays of passive soil gas samplers at different depths (**2**) a meteorological tower for reference measurements of temperature and precipitation above the canopy, (**3**) soil moisture sensors, (**4**) matrix potential and soil temperature sensors, (**5**) suction cups for collecting soil water.  
the best possible separation and detection of the specific gases. Over the last 25 years different gas chromatography systems have been used, but their general set-up remained the same even when the brand or product was updated. Here we report on the set-up that was used during the last five years (Table 1).  
We used two different GC systems that had an auto-sampler each. We used a Clarus 680 GC of Perkin Elmer (Waltham, MA, USA), which included 2 lines of gas chromatography that were supplied by the same autosampler. The first line included a CP-PoraBond Q precolumn (25 m, 0.53 mm inner diameter (ID), 10 \(\upmu\)m film) to eliminate air moisture. A CP-Molsieve 5A separation column (25 m 0.53 mm ID, 50 \(\upmu\)m film) and a thermal conductivity detector (TCD) were used to detect O\({}_{2}\), Ar, and N\({}_{2}\). A parallel CP-PoraBond Q, (30 m, 0.53 mm ID, 6 \(\upmu\)m film) after the precolumn allowed separation and detection of CO\({}_{2}\), with the same TCD. The same Clarus 680 GC had a second gas chromatography system which included a CP-SilicaPlot column (30 m, 0.53 mm ID, 6 \(\upmu\)m film) and a flame ionization detector (FID) to separate and detect CH\({}_{4}\) and C\({}_{2}\)H\({}_{4}\). Carrier gas was He. As second GC System, we used a Varian CP 3800 gas chromatograph (Varian, Inc., Palo Alto, CA, USA) with a CP-CarboWax 52 CB precolumn (25 m, 0.53 mm ID, 2 \(\upmu\)m film) and CP-PoraBond Q separation column (25 m, 0.53 mm ID, 10 \(\upmu\)m film and an electron capture detector (ECD) to separate and detect N\({}_{2}\)O at very low concentrations. Carrier gas was a mixture of Ar and CH\({}_{4}\).  
Tests showed that the detectors were highly linear within the normal range of soil gas concentrations so that we could run one-point calibration for the daily routine measurements. The precision of the concentration measurements (relative standard deviation % at atmospheric concentrations and calibration gas) was better than 1.5% for all gas species but C\({}_{2}\)H\({}_{4}\) and CH\({}_{4}\) (Table 1). This is of special importance because C\({}_{2}\)H\({}_{4}\) and CH\({}_{4}\) concentrations in soil air can be very low and fall below the limit of detection (LOD) and quantification (LOQ estimated as 10/3 \(\times\) LOD) of our systems, so that such data have to be treated accordingly. LOD was derived from repeated measurement of sub-ambient concentrations (1:4 and 1:10 dilution of calibration gas with Helium). Atmospheric C\({}_{2}\)H\({}_{4}\) concentrations are usually below LOD, and in soils only detectable in special situations.