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).