Spatial distribution of soil microbial biomass and respiration (H1)
Soil microbial biomass decreased significantly with increasing distance to the tree (estimate ± SE = -1.60 ± 0.57, p = 0.006) and with soil depth (-44.96 ± 8.20, p < 0.001, Fig. 2). The interaction of distance and depth was not significant (p = 0.064). Likewise, soil microbial respiration decreased with increasing depth (-0.14 ± 0.06, p = 0.01), but distance to the tree and the interaction of distance and depth had no significant effects (p = 0.08, p = 0.315, respectively, Fig. 2).
Spatial distribution of belowground overyielding (H2 - H3)
Our analyses showed no general belowground overyielding for microbial biomass or respiration between the mixed tree species pairs (Fig. 3A, D). An increasing depth reduced soil microbial biomass overyielding (-150.67 ± 42.17, p < 0.001) but increased microbial respiration overyielding (0.71 ± 0.21, p = 0.001, Fig. 3B and E, respectively).
We studied the spatial distribution of microbial properties overyielding by testing for the interacting effects of increasing distance toL. formosana (tree 1, i.e., closeness to tree 2) and depth. We found that an increased distance (from L. formosana ) had a significant negative effect on the microbial biomass overyielding (-452.51 ± 34.90, p < 0.001). The analysis showed overyielding close to L. formosana (i.e. tree 1), turning into underyielding close to S. saponaria (i.e. tree 2, Fig. 3C). This pattern was even stronger in the shallower soil (0-5 cm) compared to the deeper soil layer (5-10 cm), as indicated by a significant interaction effect between distance and depth (30.01 ± 10.07, p = 0.004). We found similar results for the overyielding of microbial respiration: increased distance to L. formosana (tree 1) decreased the microbial respiration overyielding significantly (-0.77 ± 0.19, p< 0.001) with overyielding close to L. formosana , and underyielding close to S. saponaria (Fig. 3F). Contrary to microbial biomass, this pattern was stronger in the deeper soil compared to the shallower soil layer (interaction estimate ± SE = -0.38 ± 0.05,p < 0.001).