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 .