4.1 Differential effects of ectomycorrhizal strategies on the
below- and aboveground plant N and P nutrients
Concerning our first hypothesis, we confirmed the distinctive effects of
ECM strategies on plant N and P elemental stoichiometry in roots and
leaves across nature environmental gradients. In this study, the matureA. faxoniana trees were deficient in root P (Table 1, P
concentration: 0.72±0.13 mg g-1) as well as both root
and foliar N (values of N concentration <10 mg
g-1 and N:P ratio <14) as judged by the
normal standards (Güsewell & Koerselman, 2002; Güsewell, 2004; Tessier
& Raynal, 2003). P is generally more limiting than N in terrestrial
ecosystems as it is derived primarily from rock weathering and uniquely
depended on root systems (Walker & Syers, 1976; Vitousek et al. ,
2010). According to the results in this study, the variations of ECM
traits in A. faxoniana contributed more to root and foliar P
concentrations than N concentrations (Figure 2; Figure 3), suggesting
that ECM strategies are more functional on P uptakes than on N uptakes
under both N and P limitations. Basically, the resource allocation in
belowground by the mycorrhizal symbiosis is expected to abide by the
nutrition requires of plant species (Merrild et al. , 2013).
However, the priority in nutrient acquisition is frequently determined
by the strategic choices of plant species under several nutrient element
limitations. It has been recognized that the ECM symbiosis give priority
to the uptake of P but not N when in deficient supplies under different
experimental conditions (Smith et al ., 2011; Zavišić et
al ., 2016; Almeida et al. , 2019). Moreover, it has been reported
that the ECM symbioses sometimes do not largely alleviate N limitation
(Näsholm et al. , 2013; Franklin et al ., 2014), and that
plants could obtain N by the root pathway rather than the mycorrhizal
symbioses which would require extra C investment under N shortage (Jianget al. , 2017; Zhang et al. , 2019).
The way of nutrient acquisition might change the nutrition preferences
in plant species (Houlton et al. , 2007; Zhang et al. ,
2018). Apart from soil resources and climatic factors, our study shows
that the varied ECM traits greatly influenced N and P nutrients inA. faxoniana (Figure 2, Figure 3). Overall, the ECM traits
implying the uptake efficiency, such as the colonization ratio of ECM
root tips, the ratio of the living to dead root tips, the colonization
ratio of the contact exploration type, and the superficial area of ECM
root tips, were all positively related to the below- and aboveground N
and P concentrations in A. faxoniana (Figure 4). However, fine
root biomass and morphological diversity of ECM roots performed negative
influence on plant N and P concentration but positive influence on root
and foliar N:P ratio, suggesting that the trade-offs of A.
faxoniana between the improvement of ECM root proliferation and
morphology differentiation requiring more C invested and the nutrients
uptake. Accordingly, we could conclude that both N and P nutrients of
roots and leaves in A. faxoniana are primarily facilitated by the
nutrient uptake efficiency of ECM roots, while the N and P stoichiometry
is strongly related to the alteration of uptake or transportation
pathway of ECM roots. Researches show that the nutrient uptake
efficiency of the symbiotic fungi in plants might mediate the
concentration of the nutrients in roots and leaves, e.g. the ECM
colonization ratio, ECM absorption root vigor (Vandenkoornhuyse et
al ., 2003; Beltrano et al. , 2013; Li et al. , 2015), ECM
root tips biomass per stand basal area, and absorptive capacity of ECM
emanates (Ostonen et al. , 2011), etc.; whilst the N and P
stoichiometry in plant species could be affected by the function of
mycorrhizal symbionts, e.g. hyphae exploration ability and/or
extracellular enzyme secretion (Chen et al. , 2010), etc. Plant
nutrients uptake and balance exceedingly depends on the alternative
foraging strategies of the ECM root systems (e.g. foraging precision of
hyphae, morphology plasticity, and foraging scale) under different
environmental conditions (Wang et al ., 2006; Köhler et
al. , 2018; Einsmann et al. , 1999; Chen et al. , 2018).
While controlled experiments and isotope tracing studies have
demonstrated that ECM symbionts contribute to the improvements of plant
biomass, foliar N and P acquisition (Brandes et al., 1998; Hobbie &
Hobbie, 2006; Craine et al., 2009), such functional roles are not
readily observable in natural ecosystems due to the confounding effects
of biotic and biotic environments. In this study, under the natural
environmental gradients, we were able to reveal the differential roles
of ECM strategies in N and P uptake in A. faxoniana .