Environmental variables controlling soil respiration on diurnal, seasonal and annual time-scales in a mixed mountain forest in Switzerland

Studies on soil respiration in mountain forests are rather scarce compared to their broad distribution. Therefore, we investigated daily, seasonal and annual soil respiration rates in a mixed forest (Lageren), located at about 700 m in the Swiss Jura mountains, during 2 years (2006 and 2007). Soil respiration (SR) was measured continuously with high temporal resolution (half-hourly) at one single point (SRautomated) and periodically with high spatial resolution (SRmanual) at 16 plots within the study site. Both, SRautomated and SRmanual showed a similar seasonal cycle. SR strongly depended on soil temperature in 2007 (R (2) = 0.82-0.92), but less so in 2006 (R (2) = 0.56-0.76) when SR was water limited during a summer drought. Including soil moisture improved the fit of the 2006 model significantly (R (2) = 0.78-0.97). Total annual SR for the study site was estimated as 869 g C m(-2) year(-1) for 2006 and as 907 g C m(-2) year(-1) for 2007 (uncertainty < 10% at the 95% confidence interval, determined by bootstrapping). Selected environmental conditions were assessed in more detail: (1) Rapid, but contrasting changes of SR were found after summer rainfall. Depending on soil moisture at pre-rain conditions, summer rain could either cause a pulse of CO2 from the soil or an abrupt decrease of SRautomated due to water logging of soil pores. (2) Two contrasting winter seasons resulted in SR being about 60-70% (31.2-44.6 g C m(-2)) higher during a mild winter (2007) compared to a harsh winter (2006). (3) Analysing SR for selected periods on a diurnal scale revealed a counter-clockwise hysteresis with soil surface temperatures. This indication of a time-lagged response of SR to temperature was further supported by a very strong relationship (R (2) = 0.86-0.90) of SR to soil temperature with a time-lag of 2-4 h.