## 3 Results
### CH\({}_{4}\) and CO\({}_{2}\) Gas Flux  
CH\({}_{4}\) flux was searched for in parallel with all CO\({}_{2}\)-flux measurements, but none exceeded the instrumental detection limit (0.01 cm\({}^{3}\cdot\)min\({}^{-1}\cdot\)m\({}^{-2}\)). This agrees with similar measurements in normally drained European soils [51]. CH\({}_{4}\) flux was thus considered as insignificant.  
CO\({}_{2}\) flux changes over time are shown on Figure 3. A total of 717 single measurements are available for the whole monitoring period. CO\({}_{2}\) flux varied from 0 up to 26.4 cm\({}^{3}\cdot\)min\({}^{-1}\cdot\)m\({}^{-2}\) (Point 4-B,June 2011). Mean and median values for the whole dataset were 5.8 \(\pm\) 4.2 cm\({}^{3}\cdot\)min\({}^{-1}\cdot\)m\({}^{-2}\) and 4.4 cm\({}^{3}\cdot\)min\({}^{-1}\cdot\)m\({}^{-2}\), respectively. These values are in the upper range of bibliographic values for biological flux in the European temperate climate zone, outside any known geochemical anomaly and on a normally drained soil (flux less than 4 cm\({}^{3}\cdot\)min\({}^{-1}\cdot\)m\({}^{-2}\)[51]; flux up to 14 cm\({}^{3}\cdot\)min\({}^{-1}\cdot\)m\({}^{-2}\)[61]).  
However, the recent analysis of the biogenic CO\({}_{2}\) and CH\({}_{4}\) emissions data in France (24 various natural sites, more than 2600 measurements) shows that the most probable CO\({}_{2}\) flux ranges are respectively from 0 to 6 cm\({}^{3}\cdot\)min\({}^{-1}\cdot\)m\({}^{-2}\) in winter and 3 to 20 cm\({}^{3}\cdot\)min\({}^{-1}\cdot\)m\({}^{-2}\) in summer. The average values are respectively 2.7 cm\({}^{3}\cdot\)min\({}^{-1}\cdot\)m\({}^{-2}\) in winter, and 9.3 cm\({}^{3}\cdot\)min\({}^{-1}\cdot\)m\({}^{-2}\) in summer and 5.8 cm\({}^{3}\cdot\)min\({}^{-1}\cdot\)m\({}^{-2}\) for all seasons' data [62].  
CO\({}_{2}\) gas flux measured between 2008 and 2015 on the surface of the Rousse site are mostly in the high range of this all reference flux, because they were measured on very fertile land in the oceanic climate of SW France that is characterized by hot and wet summers and mild winters.  
Regardless of the measurement period, the ratio between highest and lowest measured values is always highly significant. Here it was 4 to 60, without considering zero-flux measurements, but the same observation is made on reference sites and is due to the heterogeneity of biological activity and to the permeability of soil to gas.  
Quarterly measurements from September 2008 until March 2012 highlighted a clear seasonal variation of the gas flux between summer and winter conditions (Figure 3). Mean CO\({}_{2}\) flux values in June or September (6 to 11.8 cm\({}^{3}\cdot\)min\({}^{-1}\cdot\)m\({}^{-2}\)) were always higher than those in December or March (1.4 to 5.9 cm\({}^{3}\cdot\)min\({}^{-1}\cdot\)m\({}^{-2}\)). The same trend was observed for maximum and minimum values, except in one case: zero-flux and values close to zero occurred only in winter, when soil biological activity is low, or rain can saturate soil porosity with meteoric water, causing reduced permeability. The observed seasonal variation of the gas flux is well-known when measuring CO\({}_{2}\) flux from biological origin; it is due to natural changes throughout the year of bacterial and root activity in the soil.  
No significant difference of gas flux was observed between the baseline study from September 2008 to December 2010, and the injection period from January 2010 to March 2013. During these two periods, CO\({}_{2}\) flux followed the expected seasonal pattern, without any significant deviation. The mean values measured during the baseline-study and the injection periods were almost the same, about 6 cm\({}^{3}\cdot\)min\({}^{-1}\cdot\)m\({}^{-2}\). During the post-injection period, three field campaigns took place in winter; the mean value was about 4 cm\({}^{3}\cdot\)min\({}^{-1}\cdot\)m\({}^{-2}\), which was very similar to the flux measured in winter during the baseline and the injection periods.