1. Do urban and rural soils differ in their abiotic properties and
in R. nudiflora plant traits (e.g., biomass, root shape)?
The literature indicates that urban soils are importantly affected by
anthropogenic activity (Wiseman & Wells, 2005; Harris, 1991; Kayeet al. , 2006; Irwin et al., 2020). Nutrient enrichment,
such as P and N (Kaye et al. , 2006), high temperatures (Menberget al. , 2013), compacted soil, and the presence of impervious
surfaces (Day et al. , 2010) can lead to a reduction in plant size
in urban environments (Yakub & Tiffin, 2017), as well as to a reduction
of soil mutualistic interactions (Newbound et al., 2010; Irwinet al. , 2020). Our results show that DUS tends to have less
nutrient concentrations than RS and OUS, except for P. These findings do
not match with the general expectation that urban soils should be
nutrient enriched (Wiseman & Wells, 2005; Kaye et al., 2006;
Irwin et al., 2020). Our results highlight the high
microenvironmental heterogeneity within cities that are under different
anthropogenic management, such as parks and road median strips which
have higher organic matter that could explain the enriched soil in OUS
(Irwin et al. , 2020).
The difference between the two urban environments (DUS and OUS) can be
attributed to DUS impermeable surfaces (Day et al. , 2010) that
reduce P runoff and lixiviation, which is expected to be higher in OUS
as well as in RS (Estrada-Medina, 2016). Meanwhile, the increase in P
concentration in DUS may be caused by combustion and atmospheric
deposition (Kaye et al., 2006). A previous study along a urban-rural
transect in New York city, also found higher soil nutrient enrichment in
rural than in urban soils (Zhu & Carreiro, 2004), concluding that the
low N enrichment in cities was due to reduced nitrification from
bacteria due to greater concentrations of heavy metals. In the same
study, they explained the reduction in P in cities might be due to
reduced organic mass inputs; however, this does not explain the higher P
concentrations we found in DUS in this study. Soils in the region of
Mérida city are poor in nutrients due to high soil permeability and
lixiviation (Estrada-Medina, 2016); however, because Fabaceae species
are dominant in the Yucatan forests, the presence of N-fixing bacteria
on legumes root nodules could explain higher N concentration in rural
areas (Rivero-Villar et al. , 2018). Nevertheless, these arguments
cannot explain the park and road medians nutrient enrichment since
Fabaceae species are missing or in a lower abundance than in rural areas
(pers. obs.). High organic matter and bacteria in the rural soils
(Estrada-Medina, 2016) could also explain nitrogen enrichment for both
RS and OUS, although when considering the higher heavy metal
concentrations in OUS we would not expect an increase in bacterial
nitrification (Zhu & Carreiro, 2004).
Environmental contrasts between urban and rural areas are predicted to
shape the phenotypic expression of functional traits (Cheptou &
Lambrect, 2020). In plants, contrasting differences in several plant
morphological attributes, have been reported (see Yakub & Tiffin,
2017), but no root morphology attributes have been explored. It is
expected that urban conditions, such as higher temperature, soil
compaction, and impervious surfaces, would promote reduced root growth
due to reduced root elongation and fine root production (Day et
al. , 2010). In this study, we confirmed this prediction by finding
smaller and more compact roots in both urban environments, which can be
explained by more compact and shallower soils (pers. obs.). However,
given the connection between root functionality and nutrient and water
intake, it would have been useful to evaluate secondary root properties
such as diameter, length, density, and biomass. (Karliński et
al. , 2014; Freschet et al., 2017), which have been found to be
different between deep urban and rural trees (Karliński et al. ,
2014). A previous study in R. nudiflora found that this
long-lived herb reallocates carbon reserves into roots, as a
compensatory mechanism, and that higher investment in root biomass
favored reproductive compensation to herbivory (Rivera-Solís et
al., 2012). Interestingly, for R. nudiflora , individuals living
in the city, such compensatory mechanisms might not be relevant against
folivory since the incidence of foliar damage is lower than in rural
areas (pers. obs.), but it can play an even more important role to
invade the city as a strategy to deal with strong and regular mowing due
to street maintenance (around every month in the rainy season). This and
other ecological contrasts (e.g. soil compaction) set the basis to
explore for root phenotypic divergence and their implications on
above-ground interactions (Bardgett et al. , 1998); however, there
is currently no experimental research that has explored root phenotypic
divergence driven by urbanization and their physiological and ecological
implications. In this context, the use of common gardens or reciprocal
transplant experiments will be key to explore such divergence and the
importance of phenotypic plasticity (Nuismer & Gandon, 2008).
2. Do R. nudiflora’s AMF-colonization rates, spore density, and
diversity vary between rural and urban environments?
In general, urban soil conditions have been shown to negatively affect
fungal communities including ectomycorrhizal fungi (ECM) and AMF;
however, there are still few studies focusing exclusively on AMF
(Newbound et al., 2010). Most studies support our finding that
plant populations growing in urban areas have lower rates of
AMF-colonization than rural populations (see Egerton-Warburton & Allen,
2000; Wiseman & Wells, 2005; Tyburska et al. , 2013), while only
one study found similar colonization rates between urban and rural
environments in AMF (Karliński et al. , 2014). Interestingly, all
these previous studies were conducted in tree species, and despite some
of these studies recording multiple soil attributes (e.g. Tyburskaet al., 2013), only few have explored which soil chemical
component drives the intensity of the interaction (Egerton-Warburton &
Allen, 2000; Wiseman & Wells, 2005; Buil et al. , 2021).
In this way, our study is the first to focus on an invasive perennial
herb, and to explicitly disentangle the implication of urbanization on
the functional role of macronutrients affecting this mutualistic
interaction (see next section).
In relation to mycorrhizal communities, previous studies have assessed
changes due to urbanization in ECM communities (Ochimaru & Fukuda,
2007) and in AMF communities (Egerton-Warburton & Allen, 2000; Cousinset al., 2003; Buil et al., 2021). In general, the trend is
a reduction in AMF richness, diversity, and density of propagules in
urban environments (Egerton-Warburton & Allen, 2000; Cousins et
al., 2003; Ochimaru & Fukuda, 2007; Lin et al., 2021; Builet al., 2021). In contrast, we did not find differences in
diversity or community composition between the urban and rural
environments. Similarly, to the case of AMF-colonization rate, there are
few studies that have evaluated the relevance of soil or other
properties to explain changes in urban AMF communities
(Egerton-Warburton & Allen, 2000; Buil et al., 2021). While Builet al., (2021) recorded multiple soil chemical attributes they
did not statistically tested for any associations with AMF community
parameters (e.g. diversity); however, they did find lower AMF richness
and diversity in the more extreme urban soils, and that both were
affected by vegetation coverage. On the other hand, Egerton-Warburton &
Allen (2000) found, in an anthropogenic N deposition gradient, that
enrichment in N reduced AMF spore richness and diversity. In our study,
despite the detected contrast in soil chemistry between urban and rural
soils, we did not find differences in either richness or diversity of
AMF spores, and neither in community composition, but we did find a
negative association between P concentrations and spore diversity. The
lack of differences in spore density between urban and rural
environments could be due to spore dispersion by vectors, such as wind,
insects, birds, and pedestrians (Buil et al. , 2021).
Contrastingly, Egerton-Warburton & Allen (2000) found that N enrichment
due to urbanization reduced spore density. Further experiments should be
designed to explore differences in AMF soil inoculum potential (Wiseman
& Wells, 2005; Ramos-Zapata et al., 2011) between urban and
rural soils to rule out the potential of low soil inoculum as a possible
explanation for the decreased AMF-colonization rate shown in both urban
environments (Buil et al. , 2021). The use of metagenomic
approaches will be key to gain knowledge on how soil microbiome
communities are affected by urbanization and to understand the role that
particular taxonomic AMF groups might play in plant colonization and
adaptation to urban environments (Unterseher et al. , 2011).