3. How are AMF-colonization rates on R. nudiflora associated with
soil properties, and to what extent are these associations affected by
urbanization?
In our study, the recording of several soil chemical attributes and the
combined use of different statistical approximations allowed us to
detect association patterns between soil chemicals and the plant-AMF
interaction. First, a PCA detected a positive pattern of association
between a set of soil chemical properties (pH, N, K, Na, Ca) that were
negatively associated with P concentrations (PC1soil).
In DUS, where P concentrations were the highest, we detected a reduction
in AMF-colonization as P concentration increased and N, K, Ca, Na, and
pH were reduced (PC1soil). This result suggests that P
plays a key role in reducing plant-AMF interaction as previously
reported (Smith & Smith, 2011; Chen et al., 2020), or that the
reduction on the other soil properties were mediating the interaction.
Also, this result indicates that it would be crucial to distinguish the
indirect and direct effects of each variable influencing the R.
nudiflora -AMF interaction, using a SEM approach and considering the
patterns of covariation of the chemical properties.
With a SEM approach we were able to detect and confirm that P
concentration played a key role in reducing the AMF-colonization rates
in R. nudiflora , as it was expected (Salvioli di Fossalunga &
Novero, 2019; Chen et al., 2020; but see Treseder & Allen,
2002). Our results indicate that a negative effect of P on
AMF-colonization rates was observed in DUS and RS, but while in DUS the
higher P concentrations result in a reduction of the AMF-colonization
rates, the lower concentrations of P promotes higher AMF-colonization
rates in RS. While this explains the results found in DUS and RS, for
OUS we found low P concentrations and low AMF-colonization rates,
reflected on the positive effect of P concentrations on AMF-colonization
in the SEM. Chen et al. (2020) found a reduction in the magnitude
of the effect of P on AMF-colonization rates when native plants were
competing with invasive plant species, which have a greater negative
effect. It is possible that, in OUS, R. nudiflora encountered
greater competition, which changed the effect that P had on the
plant-AMF interaction. Furthermore, contrary to previous evidence
(Egerton-Warburton & Allen, 2000; Karliński et al., 2014), we
found a consistent positive effect of N concentrations on
AMF-colonization rates, which can be expected in N-limited soils.
Moreover, similar positive effects of P on AMF can also occur in
P-limited soils (Treseder & Allen, 2002). Such positive effect of P and
N on AMF-colonization can suggest that AMF are not only constrained by
carbon supplied by plants but also by soil nutrient availability (e.g. N
and P), and that below a certain threshold the addition of P and N can
have positive effects on AMF (Treseder & Allen, 2002).
While the above studies can explain the positive effect in N among the
sampled environments (i.e. DUS, OUS, RS), there is still limited
knowledge to help us to understand the observed associations between K
and AMF-colonization in our study (but see Benito & González-Guerrero,
2014; Garcia & Zimmermann, 2014). Even though K is a very abundant
element of soil composition, it has a low availability due to strong
mineral adsorption. In this way, plant-AMF association can improve K
uptake, favoring salt and drought stress tolerance in host plants
(Benito & González-Guerrero, 2014). To our knowledge, the functional
shift in K detected in our study has not been previously observed;
however, such change can be associated with K concentration (see
Schreiner & Linderman 2005). Accordingly, in DUS, where K was found
more limited, we detected the strongest positive effect favoringR. nudiflora -AMF association, potentially increasing stress
tolerance in R. nudiflora individuals from the most extreme urban
environment. Considering K in studies focusing in plant-AMF can be
fundamental to understand this interaction due its direct and indirect
implications mediating plant nutrition (Dibb & Thompson, 1985).
Finally, the negative effects of pH detected on P, K, and N are
consistent with previous reports (Wiseman & Wells, 2005; Osman, 2013),
supporting the idea that the indirect effect of pH can drive, through
changes in macro-nutrient availability, the plant-AMF mutualistic
interaction.
Despite the fact that several studies have evaluated the effect of
urbanization on the plant-AMF association (Egerton-Warburton & Allen,
2000; Wiseman & Wells, 2005; Karliński et al. , 2014; Wiseman &
Wells, 2005; Buil et al. , 2021), few of them have assessed which
soil chemical properties predict the intensity of this mutualistic
association (Egerton-Warburton & Allen, 2000; Wiseman & Wells,2005; Buil et al. , 2021). Egerton-Warburton & Allen (2000) found
that vehicular deposition of nitrogen oxides created a eutrophication
gradient in which N enrichment reduced AMF root infection in a plant
community in Riverside-Perris Plain, Southern California. In the same
way, in disturbed landscape soils, Wiseman & Wells (2005) found that
AMF-colonization in red maple roots was higher on more acidic soils in
Piedmont, South Carolina. They suggest that such increment in AMF
infectivity might be due to an indirect effect of acidification,
reducing P availability and increasing the benefit of the association
with AMF to increase P uptake; however, they did not find statistical
differences in P concentrations between developed and undeveloped sites,
nor a direct association between P and AMF-colonization. Finally, Builet al. (2021) found that AMF infectivity was negatively
associated with subsoil compaction in a gradient of four urban sites
with different disturbance levels in Córdoba, Argentina. We believe that
recording several soil chemical properties is the key to disentangle
complex patterns of association that would help to understand the
ecology and evolution of plant-AMF mutualistic interactions. In
particular, our preliminary analyses using univariate analyses only
detected positive effect of K on AMF-colonization, so the use of SEM was
fundamental to disentangle the hidden effects by other macronutrient.