Partenaires et organisations internationales
(Anglais)
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BE, CH, CZ, DE, EE, ES, FI, FR, GR, IE, IL, IT, LV, NL, NO, PL, PT, SE, SI, SK, TR, UK
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Résumé des résultats (Abstract)
(Anglais)
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Below-ground respiration is often referred to as soil respiration, but in order to understand the processes that govern this important respiration flux, a detailed quantification of the two main flux components (1) microbial and (2) root-rhizosphere respiration is needed. Our aim was to quantify the contribution of root-rhizosphere respiration to net ecosystem CO2 fluxes of a mountain forest, the CarboEuropeIP site Laegeren, located at about 700 m in the Swiss Jura mountains. 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 (R2=0.82-0.92), but less so in 2006 (R2=0.56-0.76) when SR was water limited during a summer drought. Including soil moisture improved the fit of the 2006 model significantly (R2=0.78-0.97). Total annual SR for the study site was estimated as 869 g C m?2y?1 for 2006 and as 907 g C m?2y?1 15 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 20 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 (R2=0.86-0.90) of SR to soil temperature with a time-lag of 2 to 4 hours. Furthermore, SR was partitioned into microbial (MR) and root-rhizosphere (RR) respiration using small root exclusion treatments (mesh bags with 35 µm). Fine-root respiration (FRR) was assessed using the excised root approach. RR and FRR were closely related (r2 = 0.94), with RR contributing about 46% and FRR about 32% to total soil respiration. The temperature sensitivities differed sig-nificantly, with RR increasing more strongly with soil temperature than MR. The contribution of RR to total SR was higher during the growing season (50%) than during the dormant periods (40%) and also showed a higher (and much stronger) temperature sensitivity during the growing season com-pared to dormant periods. Since MR also responded quite pronouncedly to raising temperatures during dormant periods, we conclude that the partitioning between RR and MR is strongly related to phenology and the greening-up of the canopy in this mixed mountain beech forest.
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