A field experiment using mesocosms has been set-up at each climosequence site, as shown in Fig. 8, with the purpose of evaluating the deadwood decay mechanisms over time in function of the slope exposure (N vs S), altitude (1000 – 2400 m a.s.l) and soil depth (0-5, 5-10, 10-15 cm). 

 

The mesocosms (10.2 cm, 25 cm long PVC tubes) have been inserted into the soil prior to the addition of CWD and they have been placed >1m from large trees and >0.5 m from the adjacent mesocosms (Fig. 8). To allow fine roots, fungi hyphae, and earthworms to penetrate into the core, the mesocosms have two 5 cm diameter windows. From the mesocosm set-up (August 2012) to the placement of the CWD blocks (June 2013) one year passed in order to permit normal conditions to be restored and as such, the decay monitoring study would be performed under undisturbed conditions (Fig. 9).

 

CWD blocks (Picea abies) with an uniform size (5 cm x 5 cm x 2 cm) and geometry have been placed in each of the mesocosms (June 2013), as depicted in Fig. 10. At each plot, chemical and molecular analyses on the wood debris and soils are planned after 12, 25, 52 and 104 weeks (Fig. 11); each having three replicates. Therefore, a total of 120 mesocosms (10 sites * 4 sampling times * 3 replicates) have been set-up. Soil samples have been taken from each mesocosm at 5 cm depth intervals (0-5; 5-10 and 10-15 cm) and placed in polyethylene bags and transported on ice to the laboratory for their characterisation (Fig. 12a,b). The same procedure has been done for the wood blocks (Fig. 12b). Microbial analyses at the level of functional key genes involved in the terrestrial C- and N-cycle are planned for either soil or wood debris samples after each sampling period (Fig. 11). 

 

Furthermore, highly 13C labelled CWD has been added to specific reserved mesocosm sites along the climosequences and it will be analysed after 52 weeks (Fig. 11). To trace the fate of the stable isotope added by CWD into the soil, the labelling of microbial biomass is measured. The determination of SOM quality (density fractionation, stable and labile fraction, radiocarbon dating, ∂13C, ∂15N, lignin components) and soil chemical analyses are also being assessed within the framework of this part. 

Fig. 8. Overview of the study sites showing the set-up of mesocosms along the altitudinal gradient at north (N1-N5) vs south (S6-S10) exposure, representing microclimosequences, located in Val di Rabbi (Trentino, Italy). The photos (J. Ascher) were assembled by Tommaso Bardelli on the modified scheme of Egli et al. (2006).

Fig. 10. Overview of the in field-mesocosm experiment to monitor microbial CWD-decay (Picea abies) over time (0-104 weeks) as a function of microclimate (exposure/altitude). The above photos correspond to the moment in which CWD blocks were placed into the mesocosms; and the photos below refer to 25 weeks later. Photos taken by J. Ascher.

 

Fig. 11. Design of the experimental mesocosms and the analyses planned for this part. Outline made by Markus Egli.

 

Fig. 12b. Overview of (a) the destructive sampling procedure of wood blocks and soil from the mesocosms; (b) the treatment of the wood blocks in the laboratory; and (c) including cut-milling (4 mm) prior to biochemical and microbiological analyses (enzymes vs DNA ). Photos taken by J. Ascher.

Fig. 12a. Overview of the mesocosm- soil sampling. Photos taken by J. Ascher.