| As two of the most demanding mineral elements for plants in terrestrial ecosystem,nitrogen(N)and phosphorus(P)play important roles on plant growth and physiological metabolism.Currently N deposition has been increasing and reducing its limitation on plant growth.How plant regulate biomass partitioning,arbuscular mycorrhizal(AM)colonization,root architecture and root turnover under different N or P levels to achieve favorable plant growth is still lack of comprehensive and synchronized research.The functional comparison between root system and mycorrhizal fungi on plant growth is largely unknown,especially for economic woody species.We planted two-year old woody seedlings,Sorbus pohuashanensis Hedl.and Acanthopanax sessiliflorus Seem.in the field,conducted N fertilization and P fertilization experiments,and followed them for three years.Both two experiments were split plot(“strip-plot”)design with two tree species and three N or P levels in four blocks.In each block,plant species was the main plot factor,and N or P fertilization was the subplot factor.In 2015,2016 and 2017,we examined the growth,biomass partitioning,AM colonization,root morphological and architectural traits,and root turnover traits of two plants.We aimed to explore the response of those traits and their relationship with plant nutrient uptake under N or P fertilization.Major results are as follows:⑴ With N addition,both plants increased shoot biomass by 20–45%,N and P content by 13–30%,while root biomass increased only by 2.1–5.4%.The slower increase in root growth relative to shoot growth resulted in lower root mass fraction.After plant size(ontogeny)was accounted for,root mass fraction still decreased significantly with high N fertilization in both species.Mycorrhizal colonization intensity and AM colonized root length decreased with an increase in N addition.In this P-rich site,the limited increase in root biomass and large decrease in AM colonization with N addition presumably promoted plant growth and nutrient uptake.The growth of these two species may be improved by increased carbon allocation to shoots,as N addition permitted sufficient nutrient uptake by roots and AM fungi to meet shoot nutrient demand without additional belowground carbon expenditure.⑵ After P fertilization,Sorbus and Acanthopanax increased shoot biomass by 20% and 40% respectively.P addition influenced root biomass partitioning and AM colonization differently between two plants: Acanthopanax increased shoot mass ratio by 10%,decreased root mass ratio by 16% after low P addition;Acanthopanax increased AM colonization by 45% under high P fertilization.P fertilization inhibited AM colonization,but did not affect plant growth or biomass partitioning of Sorbus.Compared with Sorbus,Acanthopanax had lower plant N:P ratio,and it was more strongly limited by soil N in this site.With P fertilization,Acanthopanax increased plant height,biomass and demand on nitrogen,then AM colonization was increased to improve nutrient uptake for host plants.Therefore,carbon investment on aboveground or belowground organs of plant is not sorely affected by soil nutrient level,but also depend on plants sensibility to soil nutrient environment.⑶ Root morphological traits of both plants were phylogenetically conserved and changed marginally with N or P fertilization.Comparatively,root branching traits were of higher plasticity,and easier to be affected by soil nutrient factor.With the reduction on soil N limitation,plants produced lower branched roots for lower carbon investment on new root system construction.Because Acanthopanax were greater limited by N than Sorbus,root branching intensity and branching ratio of Acanthopanax decreased by 14% with high N addition,and they increased by 10% and 18% respectively with low P addition.P addition made little effect on root branches of Sorbus.Moreover,root branching was positively correlated with mycorrhizal colonization,which indicated that the lower AM colonization would be another reason for the decrease on root branching under high N level.⑷ With N fertilization,Sorbus decreased root turnover rate and increased root lifespan,while no significant change on root lifespan of Acanthopanax.Compared with no fertilization treatment,root length production and mortality of Sorbus were lower in August–September and early September respectively,which led to lower turnover rate under N fertilization.After high N addition,root production of Acanthopanax increased in July,and decreased in September,which overall led to downward trend on turnover.With N limitation was weakening,plants extended their root lifespan to keep great benefit on nutrient uptake.Moreover,root turnover was positively correlated with AM colonization.With N limitation declined,the decrease in AM colonization would inhibit root turnover.The lower root branching usually represents the less first order(thin diameter and short lifespan)root relative to second order root.In this study,however,there was no significant relationship between root turnover and branching traits.⑸ With the N limitation in soil reduced,Sorbus and Acanthopanax weaken their root acquisitive traits,and improved root conservative traits.AM fungi was of higher plasticity than other root system traits,such as root mass fraction,root branching and turnover.We suspect that in nutrient-rich environments,plants can better control the tissue metabolism and function of their own roots than that of another species–mycorrhizal fungi.Therefore,under high fertility conditions,plants may preferentially decrease carbon expenditure on mycorrhizal fungi rather than their own roots.Both soil nutrient level and plant size can influence root traits.With the increase of plant size and ages,both two plants enhanced AM colonization and root branching ratio for adapting their fast growth and high nutrient demand.This study helps us to better understand root traits and nutrient uptake strategies for woody species under different nutrient levels: with the N limitation in soil reduced,plants reduced carbon investment on new root construction and mycorrhizal fungi,and then increased the growth and resource acquisition in aboveground part.Mycorrhizal colonization reduced more clearly than other root traits.Root morphology was the most conservative traits and did not change after fertilization.Moreover,with the increase of plant size in the first three years,plants enhanced AM colonization and root branching for benefit on nutrient uptake.In order to achieve the ideal allocating and resource uptake status on artificial cultivation of woody seedlings,fertilization or mycorrhizal inoculation should be carried out combining with specific soil type.Our results provided basic data and guidance for artificial cultivation on economic woody plants. |
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