3. How are AMF-colonization rates on R. nudiflora associated with
soil properties, and to what extent are these associations affected by
urbanization?
Based on global pairwise correlations (npooled = 60), we
detected that the AMF-colonization rate was only positively associated
with N concentration (r = 0.37, P < 0.001)
(Table S3). Nevertheless, the patterns of associations between soil
properties and AMF-colonization rate changed when pairwise correlations
were estimated by environment (Table S4, S5 & S6). In the DUS
environment, the significant association between N and AMF-colonization
was lost and a positive effect of K and Ca was detected (r =
0.46, df = 18, P < 0.05; r = 0.90,df = 18, P < 0.001). Meanwhile, in the RS
environment only a negative association between P with AMF-colonization
was detected (-0.54, P < 0.05; Table S5). In contrast
to the other environments, OUS did not show any association between
AMF-colonization and soil characteristics (Table S6). Table S3 to S6
also show changes in patterns of associations between soil properties
which are summarized in PC1soil to explore what
variables may be predicting AMF-colonization rate in R. nudifloraunder the effect of urbanization.
We discovered that an increase in PC1soil reduced
AMF-colonization rates in plants growing in DUS, the environment with
higher P concentration(Fig. 4). This negative association between the
Z-scores from PC1soil suggest that the simultaneous
increment in P concentrations and the reduction on N, K, Ca, Na and pH,
decreases the AMF-colonization rates on R. nudiflora (β =
-0.232 ± 0.073, P = 0.017,R2adj = 0.238; Fig. 4). No
other association between PC1soil and AMF-colonization
rate was detected neither in OUS and RS environments (RS, β =
-0.026 ± 0.044, P = 0.555,R2adj = -0.035; OUS, β =
0.069 ± 0.051, P = 0.188,R2adj = -0.044; Fig. 4). No
statistical evaluation on the effect of the environment on the
association between PC1soil and AMF-colonization rates
(PC1soil × environment) was possible due to strong
collinearity between PC1soil Z-scores and the
categorical factor environment . Even though this approach
suggests the importance of P on AMF-colonization rates, such effect can
be confused with the reduction on other soil attributes (N, K, Ca, Na,
and pH) along the PC1soil axis. To disentangle the
specific contribution of each soil attribute we implemented SEM.
The SEM that supported the results below is a reasonable description of
the processes that generate the observed correlations among variables
(χ22 = 2.661, P =0.264, CFI = 0.99; RMSEA = 0.074; SRMR = 0.053; Fig. 5). Overall, our
findings showed that P concentration reduced AMF-colonization rates in
RS and that this effect was unaffected by variation in other soil
characteristics. (Fig. 5). Moreover, the negative effect of P on
AMF-colonization rate was different among environments
(LRTmultigroup test = 3.985, df = 1,P = 0.046; Fig. 5). Furthermore, regardless of soil origin, we
found that N concentration had a positive effect on AMF-colonization
rate (LRTmultigroup test = 0.26, df = 1,P = 0.011; Fig. 5). In the case of K, we detected that while it
increased AMF-colonization rate in DUS, it reduced colonization in RS
and OUS environments (LRTmultigroup test = 6.013,df = 1, P < 0.05; Fig. 5). Such shifts in
association between K and AMF-colonization altered the N indirect
effects that run through K due to an important association between N and
K (path coefficient: ρAMF-colonization rate.K = - 0.17 ±
0.06). For instance, in RS, N had a negative indirect effect that
reduced the total positive contribution of this macronutrient on
AMF-colonization rate (indirect effect ρK.N ×
ρAMF-colonization rate.N = 0.17 × -0.45 = - 0.078; N
total effectdirect + indirect effect = 0.18); a similar
pattern was observed in OUS (indirect effect ρK.N ×
ρAMF-colonization rate.N = -0.043; N total effect =
0.22). Contrary to such negative indirect effects, we found in DUS a N
positive indirect effect, increasing the total effect of N on the
plant-AMF interaction (indirect effect ρK.N ×
ρAMF-colonization rate.N = 0.225; N total effect =
0.515). Finally, we also detected that pH affects K concentration
negatively in RS and DUS, but positively in OUS environments
(LRTmultigroup test = 5.774, df = 1, P =
0.016), and that these changes can impact the indirect effect that pH
have via K in AMF-colonization rates (Fig. 5).