Hechun Piao·Siliang Li·Shijie Wang

?

Nutrient uptake by mulberry and Chinese prickly ash associated with arbuscular mycorrhizal fungi

Hechun Piao1·Siliang Li1·Shijie Wang1

Understanding how nutrient absorption processes in plants are related to arbuscular mycorrhizal（AM）association is critical for predicting the effects of AM symbiosis on elemental cycling for plants.Both mulberry（Morus alba）and Chinese prickly ash（Zanthoxylum bungeanum）are AM-associated plants，widely distributed in southwest China.It was hypothesized that if the nutrient absorption processes were efficiently associated with AM symbiosis in both mulberry and Chinese prickly ash，foliar nutrient concentrations-especially calcium（Ca）-would be primarily determined by the soil conditions in different regions.To investigate this，AM colonization levels of soils，nutrient levels in soils and leaves，and δ13C values of leaves were analyzed for mulberry and Chinese prickly ash. In this study，spore density in soils with low pH was higher than that in soils with high pH.The average concentrations of sugar delivered to roots in both mulberry and Chinese prickly ash in soil with relatively low pH and soil extractable cations were higher than those in other areas. The values of foliar δ13C in both mulberry and Chinese prickly ash in low soil-pH and soil extractable cations were lower than those in contrast areas，indicating that water availability was impacted by soil characteristics.The efficiency in AM-mediated processes might play an important role in translocation between soil nutrients and plant tissue. The results suggest uptake and translocation of nutrients，especially Ca，in AM-associated plants may be affected by an efficiency of AM-mediated processes.Since Sr does not appear to be similarly affected，expressing Ca and other nutrient concentrations relative to Sr could be used to evaluate whether the uptake and translocation of Ca and other nutrients are affected by AM-mediated processes.

Arbuscular mycorrhizal fungi·Elemental

ratio·Carbon isotopes·Mulberry·Chinese prickly ash

上文已經(jīng)界定了定義，為什么還要談概念的現(xiàn)代詮釋呢？這是因?yàn)槟壳案黝悓W(xué)者及組織的定義和內(nèi)涵闡述多數(shù)體現(xiàn)在標(biāo)準(zhǔn)化對(duì)于商業(yè)貿(mào)易、產(chǎn)業(yè)發(fā)展、科技進(jìn)步、社會(huì)治理四個(gè)方面的作用，缺乏現(xiàn)代內(nèi)涵的延伸。而與此同時(shí)，標(biāo)準(zhǔn)化活動(dòng)的范圍和內(nèi)容正在不斷的豐富與擴(kuò)展，這也賦予了標(biāo)準(zhǔn)化基本概念的新內(nèi)涵，需要對(duì)標(biāo)準(zhǔn)化給出新的詮釋[6]。

1 Introduction

A number of laboratory and field studies indicated that arbuscular mycorrhizal（AM）fungi was important in the uptake of nutrients，such as calcium （Ca），magnesium（Mg），potassium（K），nitrogen（N），and phosphorus（P），resulting in high levels of foliar nutrients in AM-associated plants（George et al.1992；Taylor and Harrier 2001；Veresoglou et al.2011；Doubkova′et al.2012；Mu¨ller et al. 2013；Khabou et al.2014；Xiao et al.2014）.Mulberry（Morus alba）and Chinese prickly ash（Zanthoxylum bungeanum）both provide economic benefits and ecosystem services in southwest China.Mulberry is a fast-growing species，while Chinese prickly ash is a slow-growing species；both easily adapt to calcareous and some loam soils and are able to form associations with AM fungi.Both mulberry and Chinese prickly ash possess natural defenses and characteristics that enable them to survive and even flourish under systems with anthropogenic disturbance，such as karst areas that have experienced environmental degradation.AM fungi contribute to plant adaptation and ecosystem sustainability by promoting a closed nutrient cycle（Rillig 2004；van der Heijden et al.2008）.AM fungi can also improve the water absorption capacity of plants（Evelin et al.2009）.The importance of AM fungi inenhancing host plant growth is well known（Smith and Read 1997；Gosling et al.2006），and has been explained by the ability of the fungal extraradical hyphae to spread in soil and take up nutrients and water that are spatially unavailable to roots（Doubkova′et al.2012）.

? Siliang Li lisiliang@vip.skleg.cn

1The State Key Laboratory of Environmental Geochemistry，Institute of Geochemistry，Chinese Academy of Sciences，Guiyang 55002，People’s Republic of China

Soil conditions can affect the response of plants to inoculation with AM fungi and influence the development of mycorrhizal associations（Orozco-Patin?o and Medina-Sierra 2013）.Soil pH has a significant effect on organisms and processes in the soil（Olsson et al.2010），and plays an important role in AM growth；in turn，AM symbiosis regulates soil pH（Orozco-Patin?o and Medina-Sierra 2013）. Ma°rtensson et al.（2012）found AM fungal hyphae decrease in the soil with increasing soil pH.Furthermore，fungal growth is favored at low pH，while bacteria growth is favored at high pH（Ba′rcenas-Moreno et al.2011）.Soil acidity can directly affect AM fungal distribution，and in some cases soil acidification may restrict some fungal species in the soil（Guo et al.2012）.Base cation concentration and organic matter can also affect fungal distribution（Olsson et al.2010）.An et al.（2008）found that soil chemical properties，especially soil pH，are the major driving force for AM fungi communities in acid soils. Plants can acidify or alkalinize the soil medium（Moody and Aitken 1997）thus affecting the optimal pH needed by AM species for proliferation in the roots（Orozco-Patin?o and Medina-Sierra 2013）.Mycorrhizal colonies can form and excrete organic substances by the extraradical hyphae and/or the roots of host plants，which can solubilize nonavailable nutrients，resulting in a strong capacity to mobilize nutrients absorbed by the host plant（Theuerl and Buscot 2010；Piao and Liu 2011）.

Calcium and strontium （Sr）ions have many similar properties，including the same ionic charge，similar ionic radii，and the ability to form complexes and chelates of varying levels of solubility（Shtangeeva et al.2011）.Strontium istaken up by plants in similar ways toCa；however，Sr and Ca accumulations in plants cannot be predicted simply from the behavior of Ca because Ca and Sr can interact competitively for uptake into biological systems（Shtangeeva et al.2011）.Because of the marked immobility of Ca2+and Sr2+within plants（Caines and Shennan 1999；Sua′rez 2010），root morphological features should be responsible for variations in Ca2+and Sr2+acquisition. Since plant-mycorrhiza associations are common，AM fungi’seffectsonplantnutrientavailabilityshouldbetakeninto account in interpreting differences between Ca and Sr uptake.Mycorrhizal symbiosis does not modify Sr accumulation or transfer to host plants（Ladeyn et al.2008）.Van der Heijden et al.（2015）showed that the internal pool of Ca in trees is more active than previously thought based on Ca isotopic data.On acid soils a preferential uptake of Ca over Sr occurs，while on calcareous soils a preferential uptake of Sr over Ca occurs（Poszwa et al.2000），implying that the foliar Ca and Sr concentrations should differ in different regions for the same plant species.Soils with a high concentration of CaCO3significantly decrease root growth and alter the symbiosis development steps of the AM fungus（Labidietal.2012）.Therefore，arelativelyhighefficiencyin an AM-mediated process should be responsible for high foliarCaandothernutrientconcentrations，whilearelatively low efficiency should be responsible for low Ca and other foliar nutrient contents（Piao and Liu 2011）.The value of δ13C is often increased when water availability is limited，as a result of stomatal closure and hence reduced transpiration（Grant et al.2012）.Limited transpiration rates result in low Ca and Sr concentrations in plant tissues（Funk and Amatangelo 2013）.Therefore，Ca uptake and translocation byplantsisdrivenbythetranspirationstream（Clarholmand Skyllberg 2013）.The ratios of foliar Ca/Sr versus soil Ca/Sr can express the degree to which Ca concentrations in plants have been discriminated over Sr relative to their soil resources（Blum et al.2012）.It was hypothesized that the differences in ratios of foliar Ca/Sr versus soil Ca/Sr in differentregionsshouldbedeterminedbysoilconditions.Thus，the objective of this study was to use measurements of stable carbon isotopes，and nutrient and sugar concentrations，to identify the parameters causing different foliar Ca andothernutrientconcentrationsinAM-associatedplantsof different regions.

2 Materials and methods

2.1 Research site

The study area is located in a transitional zone between lowland and hills in the mountainous terrain of the Guizhou Province of southwest China.Soil samples were collected in karst areas，where the geological environment is extremely fragile，the area overpopulated，and the economy backward，leadingtoseriouslanddegradationintheformofkarstrocky desertification and extensive exposure of basement rocks in some regions（Wang et al.2004；Liu 2007；Li et al.2013）. Mulberry is a perennial tree，which is widely cultivated in China for leaves，fruit，medicine，and sericulture purposes. Chinese prickly ash is a shrub species that is also widely cultivatedinChinaasacondimentandasmedicine.Because of their high drought resistance，both mulberry and Chinese prickly ash have played an important role in economic development and adapting fragile ecosystem restoration.

The samples were divided into two groups for both mulberry and Chinese prickly ash.For mulberry，the first group included the samples collected from Libo soils（17 sites）and the second group samples collected from Huangpingsoils（16 sites）.For Chinese prickly ash，sampleswere collected from Zunyi soils（22 sites）and from Huajiang soils（22 sites）.The sites were located in areas where soils developed in mixed terrains，except for the soils at Huajiang where the parent material was primarily limestone.The soil type was yellow Ultisols in all sampling areas，havingalongland-usehistoryasmaizefields.Sample locations and environmental parameters for sampling sites are listed in Table 1.Soil samples were taken from 0 to 15 cm depth to provide an indication of available nutrients，with at least six replicated plots for each soil sample. Nurseries growing Chinese prickly ash seedlings varied in size from～10 to 100 m2.Under normal conditions，the farmers sow seeds in September and remove weeds at regular intervals.A chemical fertilizer was not added to any of the nursery soils after sowing seed.Belowground parts of mulberry trees grown on the sampling sites for less than 4 years were used in this study.All shoots（aboveground parts）were developed every year in the mulberry plantation for sericultural purposes.The leaf samples were collected in late May for mulberry，while the seedlings were collected in early September for Chinese prickly ash.Detailed samples of leaves and roots were taken.

隨著手術(shù)例數(shù)的增加，手術(shù)熟練程度的提高，手術(shù)時(shí)間、術(shù)中出血量、并發(fā)癥達(dá)到一個(gè)相對(duì)穩(wěn)定的狀態(tài)。研究結(jié)果表明，通道下經(jīng)肌間隙入路聯(lián)合固定并椎間融合術(shù)的學(xué)習(xí)曲線為30例左右，學(xué)習(xí)曲線短、平緩，適合推廣運(yùn)用。

2.2 Laboratory analysis

The average concentrations of extractable Ca in Libo soils growing mulberry was lower than that in Huangping soils，but the difference was not significant.The concentration of extractable Sr in Libo soils was slightly higher（not significantly）than that in Huangping soils（Table 2）.

2.3 Statistical analyses

Statistical analysis was conducted using SPSS 12.0 software（SPSS Science，Chicago，USA）.Differences between mean values of nutrient concentrations in samples between Libo and Huangping soils for mulberry，and between Zunyi and Huajiang soils for Chinese prickly ash were determined by a t test.Pearson correlation coefficients were performed to assess relationships between each plant tissue and soil parameters，and linear regression was used to assess relationships between foliar Ca/Sr ratios versus soil Ca/Sr ratios.For all statistical tests，differences were considered significant at P＜0.05.

Table 1 Sampling locations and environmental conditions

3 Results

3.1 AM colonization level and number of spores

In this study，six samples were used for determining AM colonization levels for each plant species.Colonization levelsofAMfungiformulberrywere16，13，and12%inLibosoils；and16，32，and9%inHuangpingsoils.Colonizationlevelsof AM fungi for Chinese prickly ash were 50，82，and 86%in Zunyi soils；and 17，20，and 31%in Huajiang soils.Six samples were used for determining AM spores for mulberry soils.Thenumbersofsporesin20 gdrysoilswere33，11，and 19 in Libo soils；and 16，15，and 13 in Huangping soils.

3.2 Total carbon and sugar concentrations in plant tissues

本實(shí)驗(yàn)選用的移動(dòng)終端APP軟件統(tǒng)一為滬江開心詞場，因?yàn)楦鶕?jù)實(shí)驗(yàn)前對(duì)學(xué)生的課堂調(diào)查，大部分同學(xué)在平時(shí)的學(xué)習(xí)中經(jīng)常使用此款軟件，經(jīng)過指導(dǎo)教師和多位其他教師商討，此款軟件設(shè)計(jì)合理，趣味性強(qiáng)，收詞全面，師生應(yīng)用體驗(yàn)一致較好。此款軟件在應(yīng)用軟件下載排行上名列第一，用戶體驗(yàn)也是最高，5顆星。

The average concentration of foliar sugar in mulberry in Libo soils（340±138 mmol kg-1）was significantly lower than that in Huangping soils（530±285 mmol kg-1）（P＜0.05）.However，the average concentration of root sugar in mulberry in Libo soils（628±217 mmol kg-1）was significantly higher than that in Huangping soils（458±219 mmol kg-1）（P＜0.05）.For Chinese prickly ash，the average concentration of foliar sugar in Zunyi soils（226±39 mmol kg-1）was slightly lower than that in Huajiangsoils（242±90 mmol kg-1）.Theaverage concentrationofrootsugarinZunyisoils（262±115 mmol kg-1）was slightly higher than that in Huajiang soils（250±125 mmol kg-1）（Fig.1b）.

3.3 Soil pH and soil extractable nutrients

Table 2 shows that the mean soil pH in Libo soils was significantly lower than that in Huangping soils.In the Chinese prickly ash plots，pH ranged from 5.49 to 8.12 in Zunyi soils，and from 6.79 to 8.10 in Huajiang soils.The mean soil pH in Zunyi soils was significantly lower than that in Huajiang soils（Table 3）.

Plant samples for laboratory analysis were dried for 48 h at 60°C and ground with a mortar and pestle.Total carbon（C）and N were determined with a CHNS autoanalyzer（PE 2400-ll，Norwalk，CT，USA）.Plant P was digested using nitric-perchloric acid and analyzed by the vanadomolybdate colorimetric method.Olsen extractable P（Olsen P）was measured by 0.5 M NaHCO3extraction（adjusted to pH 8.5 with NaOH）（Olsen et al.1954）.Total soil P was determined after combustion of 1 g soil for 2 h at 550°C followed by digestion with 6 M HCl（Graetz et al.1999）.Standard soil analysis methods were used to measure soil extractable Ca，Mg，K，and Sr with 1 M NH4OAC（Thomas 1982）.The samples of both mulberry and Chinese prickly ash were sent to the Key Laboratory for Conservation and Utilization of Bio-resources of Yunnan University and examined for mycorrhizal colonization following the method of Li et al.（2005）.Briefly，afterclearingtherootswith10%KOH，they were acidified in lactic acid，stained with acid fuchsin，and subsequentlyexaminedfortheirlevelofcolonizationundera compound-light microscope（OLYMPUS-BX51，Japan）at magnification×200 according to the method of McGonigle et al.（1990）.For soluble sugar determination，0.25 g of airdriedmaterialwasextractedfourtimeswithdistilledwaterat 75°C，modified from the method of Chinnasamy and Bal（2003）wherebywater was usedinsteadof 80%ethanol，and the water temperate was 75°C rather than boiling.After each extraction，samples were filtered，and the filtrates were used to determine soluble sugar colorimetrically through anthrone reaction（Piao and Liu 2011）.The δ13C values of leaves were measured by combustion of～2 mg C mixed withCuOat850°Cina vacuum-combustionsystemfor2 h. Carbon dioxide generated in the combustion tubes was separated by cryogenic distillation，then collected in breakseals and analyzed on a mass spectrometer（MAT 252 Finnigan，Bremen，Germany）.Thedata are expressedrelative to the international standard PDB as‰.The precision for sample repeats was better than 0.2‰for δ13C.

On average，the concentrations of extractable Ca in Zunyi soils growing Chinese prickly ash were lower than that in Huajiang soils，and the concentration of extractable Sr in ZunyisoilswassignificantlyhigherthanthatinHuajiangsoils（Table 3）.

Fig.1 a Carbon concentrations in leaves and roots of mulberry and Chinese prickly ash，and b sugar concentrations in leaves and roots of mulberry and Chinese prickly ash.LB Libo，HP Huangping，ZY Zunyi，HJ Huajiang（*P＜0.05）

3.4 Tissue chemistry and carbon isotopic compositions of leaves

作為近代著名的文學(xué)大家，魯迅不僅對(duì)當(dāng)時(shí)的種種社會(huì)問題給予了高度關(guān)注，同時(shí)對(duì)女性問題作出了深刻的思考。在他的作品當(dāng)中，不乏女性人物形象，比如《明天》中的單四嫂、《子夜》中的子君等，但無論是什么樣的女性角色，都一定程度受到封建思想的侵害。任何人都不可能脫離社會(huì)而存在，個(gè)人的命運(yùn)很大程度上會(huì)受社會(huì)種種弊端影響，而《祝?！分械南榱稚﹦t是一個(gè)典型的代表。

4.3 Relationship between water availability and Ca uptake and translocation

Anti-corrosion Design of Concrete Structure in Sulfate Strong Corrosive Environment in Saline Soil in Plateau Area LI Yuting,ZHANG Xiaofeng,YE Sheng(102)

首先，中小企業(yè)管理制度內(nèi)容不能全面化地涵蓋企業(yè)管理中的日常工作。制定制度的管理者由于自身知識(shí)的缺乏，對(duì)崗位要求和內(nèi)容等認(rèn)識(shí)不清，因而不能很好地完善制度政策。如管理思想的偏差或管理細(xì)節(jié)的缺失導(dǎo)致企業(yè)管理效能不均衡。其次，管理制度內(nèi)容缺乏針對(duì)性，容易在執(zhí)行時(shí)產(chǎn)生監(jiān)管不力的情況，制度較難以落實(shí)，對(duì)管理形式和管理方式提出了很大的挑戰(zhàn)。最后，管理制度內(nèi)容不能及時(shí)更迭。在當(dāng)前經(jīng)濟(jì)時(shí)代發(fā)展迅速的時(shí)代，管理模式和管理方法及管理思想一定要緊跟時(shí)代潮流，但中小企業(yè)管理者在制定和修改制度的時(shí)候沒有很好地結(jié)合當(dāng)前的經(jīng)濟(jì)形勢對(duì)管理制度進(jìn)行修改。

The average concentration of foliar N in mulberry grown on Libo soils was significantly lower than that in Huangping soils，but little difference of foliar Mg，K，and P concentrations was found between Libo and Huangping soils（Table 4）.The concentrations of foliar N and K in Chinese prickly ash grown on Zunyi soils were significantly lower than those in Huajiang soils（Table 5）.Except for Ca，Mg，and Sr，the average concentrations of root nutrients in mulberry（K，N，and P）did not differ between Libo and Huangping soils（Table 4）.Except for N，theaverage concentrations of root nutrients in Chinese prickly ash（Ca，Mg，K，Sr，and P）returned little difference between Zunyi and Huajiang soils（Table 5）.

用矢量網(wǎng)絡(luò)分析儀分析超聲換能器阻抗匹配電路的阻抗曲線變化趨勢,并不斷地優(yōu)化修改高低頻段中心頻率處的匹配阻抗值及LC參數(shù)值,調(diào)整阻抗曲線的位置,使S11與S22的軌跡壓縮在Smith Chart中心點(diǎn)附近,如圖5所示,AOTF光譜衍射效率達(dá)到最高,此時(shí)高低超聲換能器優(yōu)化后的匹配阻抗值為15-j*10 Ω和20-j*15 Ω。在AOTF衍射效率最佳的情況下,優(yōu)化后的高低頻段換能器匹配電路的S11參數(shù)在-30 dB～-20 dB之間,滿足回波損耗小于-10 dB的參數(shù)指標(biāo)[7],雙路超聲換能器阻抗匹配電路輸入回波損耗測試結(jié)果如圖6所示。

Soil microorganisms，especially fungi，may contribute significantly to the long-term retention of elements in organic layers of forest soils，excepting Sr（Parekh et al. 2008）.If the amounts of nutrients acquired by AM fungi are low，AM fungi have only a relatively minor impact on reducing nutrient leaching（van der Heijden et al.2008）. Using two-chambered pots where only AM fungal hyphae connected the chambers，Meding and Zasoski（2008）examined nutrient transfer between native plant species of grasses and forbs，and suggest that there is no evidence of significant Sr transfer between experimental plants via extraradical hyphae.However their results could be attributed to immobilization of Sr within plant tissues.It seems that the more severe leaching losses of Sr relative to other nutrients should be attributed to Sr acquisition and retention and not related to AM symbiosis.

Table 2 Mean values for soil pH；total N；P and Olsen P；organic C；and soil extractable Ca，Mg，K，and Sr concentrations in mulberry soils

Table 3 Mean values for soil pH；total N；P and Olsen P；organic C；and soil extractable Ca，Mg，K and Sr concentrations in Chinese prickly ash soils

Table 4 Mean values for foliar and root Ca，Mg，K，Sr，P，and N in mulberry plants

3.5 The ratios of foliar nutrient/Sr versus soil nutrient/Sr

The ratios of foliar nutrient/Sr versus soil nutrient/Sr are listed for mulberry and Chinese prickly ash in Table 6. Significant differences were found in the ratios for foliar Ca/Sr，Mg/Sr，and K/Sr between Libo and Huangping soils and between Zunyi and Huajiang soils.The ratios of foliar P/Sr and foliar N/Sr were similar between Libo and Huangping soils and between Zunyi and Huajiang soils.The ratios of foliar and soil Ca/Sr，Mg/Sr，and K/Sr were generally higher than 1.The ratios for Ca/Sr of mulberry were higher in Libo soils（with mean soil pH value of 4.92）when compared to Huangping soils，which had a relatively high soil pH（P＜0.05）.Similarly，the ratios for Ca/Sr of Chinese prickly ash were somewhat higher in Zunyi soils（with relatively low soil pH）than in Huajiang soils with a mean soil pH 7.68（P＜0.05）.

Table 5 Mean values for foliar and root Ca，Mg，K，Sr，P，and N in Chinese prickly ash

4 Discussion

4.1 Ca behavior differs from Sr in soils

林區(qū)郁閉度低主要是指該區(qū)域中的林木郁閉度低于0.7的人工林或郁閉度低于0.6的天然林，管理人員也要分情況落實(shí)森林撫育管理工作[3]。如果郁閉度較低，幼齡木生長易受到其他植物的抑制，但情況較好的林區(qū)，管理人員需要重點(diǎn)調(diào)節(jié)幼齡木與其他植物的營養(yǎng)，幫助幼齡木獲取更多的生長營養(yǎng)元素，幫助其快速成長。具體可通過伐除其他植株，移栽生長密集的幼苗來實(shí)現(xiàn)。

The limestone of the study area has a high concentration of Sr relative to aluminosilicate minerals（Jiang and Ji 2011）. However，the concentrations of Sr in extractable fractions were lower in Huajiang soils developed from limestone than in the Zunyi soils developed on mixed terrains，indicating the intensive leaching of Sr relative to other nutrients during carbonate weathering and soil development in the karst area（Jiang and Ji 2011）.Sr concentration in plants is maximized in soils with a low pH（Karpova and Gomonova 2006）.Increasing soil pH with CaCO3，especially in combination with application of organic fertilizers，can lead to changes in the soil chemical properties controlling Sr mobility，which results in a lower uptake of Sr by plants（Karpova and Gomonova 2006）.Previous studies have indicated that soil nutrient availability is limited in high soil pH（Kerley 2000；Al-Karaki and Al-Omoush 2002）.However，Stark et al.（2011）found that high soil pH usually parallels high soil extractable nutrient concentration，with important consequences for plants and chemical quality of soil organic matter.This finding was supported，especially for Ca，by a study in southwest China（Piao and Liu 2012）.While an acid condition might produce more soil nutrient availability，it might also lead to more soil nutrient loss through leaching.This is likely to be the main reason why the Sr leaching loss was more severe than that of other nutrients in both Libo and Huangping soils.Most soil nutrients in this study were relatively high in Zunyi and Huajiang-both soils with higher pH values. Soil organic C did not show a difference between the two soil groups in this study.In ion exchange on clays，Ca and Sr behave almost identically owing to the divalent charge and similar hydrated radius（Blum et al.2008）.In ion exchange on soil organic matter，Ca may be retained preferentially over Sr to a small degree（Baes and Bloom 1988；Blum et al.2008），and the presence of soil microorganisms does not significantly enhance the retention of Sr in the organic system the way it does other elements（Parekh et al.2008）.

Fig.2 Average foliar δ13C in Chinese prickly ash grown on Huajiang and Zunyi soils，and in mulberry grown on Huangping and Libo soils（**P＜0.01）

4.2 Relationship between AM fungi and nutrients distribution

Foliar δ13C in mulberry in Libo soils（-30.0±0.5‰）was significantly lower than that in Huangping soils（-29.3±0.7‰）（P＜0.01）.For Chinese prickly ash，foliar δ13C in Zunyi soils（-28.5±1.4‰）was similar to that in Huajiang soils（-28.0±0.8‰）（Fig.2）.

Table 6 Ratios of foliar nutrient/Sr versus soil nutrient/Sr（mean±standard deviations）

There was little difference in AM colonization levels between mulberry and Chinese prickly ash.This supports previous research conclusions that there is no significant relationship between AM colonization levels and nutrient acquisition（Piao and Liu 2011；Piao et al.2012）.In this study，spore density in the Libo soils（with a relatively low soil pH and some soil nutrients）was higher than that in Huangping soils，which is consistent with the results of Aliasgharzadeh et al.（2001），who have reported a negative correlation between spore density and available soil Mg and Ca（Evelin et al.2009）.Guo et al.（2012）have found that high soil pH with CaCO3treatment significantly decreases root growth and alters the symbiosis development steps of the AM fungus Rhizophagus irregularis，such as germination，hypha elongation，root colonization rate，extraradical hyphal development，and sporulation.However the fungus is able to completely fulfill its life cycle（Labidi et al.2012），which might influence the distribution of nutrients between soil and plants.

The AM fungal community structure is significantly influenced by environmental factors，especially soil moisture，pH，available N，and P（Wang et al.2013；De Beenhouwer et al.2015）.The present data show that the average concentrations of sugar delivered to roots in both mulberry and Chinese prickly ash with relatively low soil pH and soil extractable N and P were higher than values in contrast areas.AM symbiosis depends on the host sugars for the formation，maintenance，and function of fungal structures（Zhu and Miller 2003）.Root turnover can account for up to 40%of the C input into soil and is clearly the major driver for soil microbiological processes（Richardson et al.2009）. The fungi are obligate symbionts and cannot survive without a photosynthate supply from the host plant（Smith et al.2010）.Therefore，acquisition of nutrients and growth responses depend on soluble photosynthate production and its concentration in the soil（Orozco-Patin?o and Medina-Sierra 2013）.Meanwhile，root exudates not only influence nutrient availability but also interact with soil microorganisms（Richardson et al.2009）.The amount of C allocated to AM processes is estimated to range from 4 to 20% of a plant’s total C budget（Smith and Read 1997）. M°rtensson et al.（2012）reported that the allocation of C to the extraradical AM hyphae decreases with increasing pH，ranging from not detectable to 1.2 μg13C g-1dry soil in the pH range of 5.5-8.5.CaCO3treatment of soil decreases chicory root growth（Labidi et al.2012）.This effect is more drastic in mycorrhizal roots than in non-mycorrhizal ones，which may be related to the allocation of a considerable portion of C by mycorrhizal roots to maintain fungal metabolism（Labidi et al.2012）.Mycorrhizal colonization can result in the excretion of organic substances by extraradical hyphae and/or roots of host plants，which can solubilize non-available forms of nutrients，resulting in a strong capacity to mobilize nutrients absorbed by the host plant（Theuerl and Buscot 2010；Piao and Liu 2011）.Differences in sugar levels of plant tissues were found between Libo and Huangping soils，and between Zunyi and Huajiang soils.A key feature of AM-mediated processes is the transfer of photosynthate from the host plant to the AM fungal hyphae（Zhu and Miller 2003；Richardson et al. 2009；Piao and Liu 2011）.For example，there is significantly less sucrose in both shoots and roots of summer wheat with AM fungus compared with summer wheat without AM fungus（Hawkins and George 2001）.The efficiency of the AM-mediated processes was higher in both Libo and Zunyi soils than those in Huangping and Huajiang soils.

The average concentration of foliar Ca in mulberry in Libo soils was relatively higher than that in Huangping soils（Table 4）.For Chinese prickly ash，the mean value of foliar Ca in plants grown in Zunyi soils was higher than for those in Huajiang soils（Table 5）.The average concentration of foliar Sr in mulberry in Libo soils was significantly higher compared to that found in Huangping soils（Table 4）.The average concentration of foliar Sr in Chinese prickly ash in Zunyi soils was significantly higher than concentrations found in Huajiang soils（Table 5）.

Arbuscular mycorrhizal symbiosis is beneficial in providing host plants with water，thus enhancing the tolerance of the host plants to water deficit（Ba′rzana et al.2012）.The value of δ13C is often increased when water availability is limited，as a result of stomatal closure and hence reduced transpiration（Grant et al.2012）.Previous results show that plants under water stress increase their foliar δ13C relative to those without water stress（Hubick et al.1986；Farquhar et al.1988）.Values of δ13C are not affected by irrigation or N treatments（Pascual et al.2013）.Both mulberry and Chinese prickly ash have the capacity to grow in a widerange of environments.Under drought stress，the growth of mulberry is inhibited，but the trees are known to respond to the stress by increasing the root absorptive area and enhancing capacity for water retention（Huang et al.2013）. Mulberry trees have a high water demand owing to their fast growth rate and high metabolism（Guha et al.2012），resulting in large biomass.The present results show that the values of foliar δ13C in both mulberry and Chinese prickly ash in Libo and Zunyi soils，respectively，were relatively lower than those in Huangping and Huajiang soils.This indicates that water availability in both mulberry on Libo soils and Chinese prickly ash on Zunyi soils was higher than on Huangping and Huajiang soils，which would attribute to the higher efficiency of AM-mediated processes in the Libo and Zunyi soils due to different environmental characteristics，such as pH and soil nutrient distribution.

During water transport from root to leaf，divalent cations such as Ca and Sr undergo a series of exchange reactions at negatively charged sites on cell walls（Funk and Amatangelo 2013）.Both Ca and Sr are immobile elements，meaning that they are not resorbed from a leaf once they have been assimilated（Karley et al.2000），and therefore foliar Ca and Sr concentrations should be correlated with how much water is taken up over a leaf’s lifetime（Funk and Amatangelo 2013）.Low transpiration rates result in a small quantity of Ca and Sr in plant tissues，with implications for the nutritional quality of plants and for rates of biochemical cycling（Funk and Amatangelo 2013）.Clarholm and Skyllberg（2013）reported that actual evapotranspiration is in control of the cycling of base cations，such as Ca and Mg，between soil and trees.Therefore，Ca uptake by tree roots is by passive movement in the mass flow of soil water driven by the transpiration stream.

4.4 The ratios of foliar nutrient/Sr versus soil nutrient/Sr

The ratios of foliar Ca/Sr versus soil Ca/Sr measure the degree to which Ca has been incorporated preferentially over Sr in plants relative to their soil presence（Blum et al. 2012）.However，the bias toward Ca over Sr depends on soil conditions.The current data show that both mulberry grown at Libo and Chinese prickly ash at Zunyi had a relatively preferential uptake of Ca over Sr compared with contrast areas.Differences between Ca and Sr behavior were found during uptake of nutrients from soils，and might be caused by ion exchange processes during ascent in the xylem （Clarkson 1984）.Our results show the ratios of foliar Ca/Sr versus soil Ca/Sr for both mulberry and Chinese prickly ash were higher on soils with relatively low pH and soil extractable cations，where the efficiency of AM-mediated processes would increase.This suggests that relatively high efficiencies in the AM-mediated processes might be responsible for the high ratios of foliar Ca/Sr versus soil Ca/Sr in both Libo and Zunyi soils relative to the Huangping and Huajiang soils.Meanwhile，Sr is retained by cation exchange sites in stems to a greater extent than Ca，resulting in increased translocation of Ca relative to Sr in the leaves of plants（Poszwa et al.2000；Dasch et al.2006；Funk and Amatangelo 2013），which might be another cause for the difference of translocation.

“互聯(lián)網(wǎng) +”時(shí)代帶給人們?nèi)轿坏挠绊?。利用互?lián)網(wǎng)的優(yōu)勢，教師和學(xué)生能有效將全面的信息數(shù)據(jù)為自己所利用，僅僅通過手機(jī)、電腦等終端設(shè)備就能完成相關(guān)的信息內(nèi)容的查找，避免了通過傳統(tǒng)的圖書館查閱等活動(dòng)。高校圖書館的影響力明顯不如從前，同時(shí)，其對(duì)于區(qū)域內(nèi)的號(hào)召力的影響也顯得越來越薄弱[1]。

The ratios of foliar Mg/Sr versus soil Mg/Sr and the ratios of foliar K/Sr versus soil K/Sr were also higher for soils with lower pH than for soils with higher pH for both mulberry and Chinese prickly ash（Tables 2，6），a pattern repeated with the ratios of foliar Ca/Sr versus soil Ca/Sr. These results also indicate that both mulberry at Libo and Chinese prickly ash at Zunyi had a preferential uptake of Mg and K over Sr compared with contrast areas.As for the ratios of foliar P/Sr versus soil P/Sr and the ratios of foliar N/Sr versus soil N/Sr，there were no differences among regions，so these nutrients may be not suitable for assessing the effects of AM-mediated processes on uptake by mulberry and Chinese prickly ash.

The average concentration of foliar C in mulberry on Libo soils（33.5±1.2 mol kg-1）wassimilartothaton Huangping soils（33.4±0.9 mol kg-1），but the average concentration of root C in mulberry of Libo soils（36.6±1.3 mol kg-1）was significantly higher than that of Huangping soils（35.0±2.7 mol kg-1）（P=0.038）.For Chinese prickly ash，the average concentration of foliar C in Zunyi soils（35.2±0.6 mol kg-1）was significantly lower than that in Huajiang soils（36.5±0.6 mol kg-1）（P=0.000），but the average concentration of root C in Zunyi soils（36.5±0.7 mol kg-1）was similar to that in Huajiang soils（36.7±0.7 mol kg-1）（Fig.1a）.

5 Conclusion

Mulberry（Morus alba）and Chinese prickly ash（Zanthoxylum bungeanum）are important economic plants widely distributed in southwest China.In this study，the nutrients and AM-colonization level of soils as well as carbon isotopic compositions of leaves were investigated for mulberry and Chinese prickly ash growing in varied environmental conditions.The results suggested that the high efficiency in AM-mediated processes was responsible for the relatively high levels of foliar Ca，Mg，and K relative to Sr absorbed by mulberry and Chinese prickly ash grown on Libo and Zunyi soils，respectively，while the relatively low efficiency in AM-mediated processes was responsible for the low amount of foliar Ca，Mg，and K relative to Sr in others areas.Water availability also affects ratios of foliar Ca/Sr versus soil Ca/Sr.Since uptake and translocation of Ca and Sr in AM plants might be influencedbyanefficiencyofAM-mediatedprocesses，expressing Ca concentration relative to Sr could be used to evaluate whether the nutrient could be affected by AM-mediated processes.However the ratios of foliar Ca/Sr versus soil Ca/Sr could vary with species and environmental conditions，including temperature and precipitation. More research on these processes in the future will improve the interpretation of foliar elemental ratios as an index for evaluating AM-mediated processes and elemental cycling on the Earth’s surface.

AcknowledgmentsThe authors wish to thank Dr.Z.W.Zhao，from the Key Laboratory for Conservation and Utilization of Bio-resources of Yunnan University for assistance in determining arbuscular mycorrhizal colonization levels.This study was financially supported by the National Natural Science Foundation of China（Grant no. 4121004），and water project of MEP（2012ZX07503003001）.

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14 October 2015/Revised：17 December 2015/Accepted：25 January 2016/Published online：23 March 2016?Science Press，Institute of Geochemistry，CAS and Springer-Verlag Berlin Heidelberg 2016