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).