施氮對(duì)空心蓮子草(Alternantheraphiloxeroides)和蓮子草(Alternantherasessilis)種間關(guān)系的影響

周 建，李紅麗，羅芳麗, *，黃文軍，張明祥，于飛海

1北京林業(yè)大學(xué) 自然保護(hù)區(qū)學(xué)院，北京 100083 2四川省林業(yè)科學(xué)研究院，成都 610081

施氮對(duì)空心蓮子草(Alternantheraphiloxeroides)和蓮子草(Alternantherasessilis)種間關(guān)系的影響

周 建1，李紅麗1，羅芳麗1, *，黃文軍2，張明祥1，于飛海1

1北京林業(yè)大學(xué) 自然保護(hù)區(qū)學(xué)院，北京 100083 2四川省林業(yè)科學(xué)研究院，成都 610081

自然界的氮素釋放總是呈現(xiàn)出空間和時(shí)間上的異質(zhì)性，但關(guān)于異質(zhì)性氮釋放對(duì)于入侵植物和本地植物種間關(guān)系影響的研究相對(duì)較少。將入侵植物空心蓮子草(Alternantheraphiloxeroides)和同屬本地植物蓮子草(Alternantherasessilis)分別進(jìn)行單種種植(12株，無(wú)種間競(jìng)爭(zhēng))和混種種植(每種6株，有種間競(jìng)爭(zhēng))，模擬大氣氮濕沉降設(shè)置由兩種不同施氮總量(15g N m-2a-1和30g N m-2a-1)和兩種不同施氮頻率(每5天1次和每15天1次)交叉組成的4種施氮處理，并以不施氮為對(duì)照。施氮總量的增加顯著促進(jìn)了兩種植物的生長(zhǎng)，但對(duì)兩種植物的種間競(jìng)爭(zhēng)關(guān)系沒(méi)有顯著影響。施氮頻率對(duì)兩種植物的生長(zhǎng)以及種間競(jìng)爭(zhēng)關(guān)系都沒(méi)有顯著影響。兩種植物在面對(duì)競(jìng)爭(zhēng)時(shí)表現(xiàn)出不同的生物量分配策略，空心蓮子草將更多的生物量分配到莖，而蓮子草將更多的生物量分配到根。在全球變化的背景下，大氣氮濕沉降可能會(huì)改變兩種植物的種群結(jié)構(gòu)和動(dòng)態(tài)，但可能對(duì)這兩種植物的種間關(guān)系影響較小。

氮沉降；克隆植物；種間競(jìng)爭(zhēng)；入侵植物；本地植物；脈沖式施氮；相對(duì)產(chǎn)量

氮素是影響植物生長(zhǎng)的重要營(yíng)養(yǎng)元素之一[1-2]。自然界中氮素的供給并不是持續(xù)不斷，而總是以不同的頻率釋放，呈現(xiàn)出時(shí)空上的異質(zhì)性[3-7]。時(shí)間上的異質(zhì)性常常指短時(shí)間的大量氮素供給，如降雨和融雪會(huì)在短時(shí)間內(nèi)提高土壤中的氮素含量[8-9]。相應(yīng)的，某些植物對(duì)氮素的利用也可以在很短的時(shí)間內(nèi)完成[10]。有研究發(fā)現(xiàn)，與持續(xù)施氮相比，集中的大量氮沉降會(huì)顯著促進(jìn)植物的生長(zhǎng)[3,11]。由于不同植物對(duì)氮沉降的響應(yīng)能力存在差異[12-13]，因此，氮沉降可能會(huì)引起植物種間關(guān)系的變化。

入侵植物對(duì)資源的有效利用是其成功入侵的重要原因之一[14-16]。相比較于本地植物，成功入侵的植物往往可以從環(huán)境中獲得更多的營(yíng)養(yǎng)，因而具有較強(qiáng)的競(jìng)爭(zhēng)能力[17-20]。此外，許多外來(lái)物種具備在短時(shí)間內(nèi)高效利用養(yǎng)分的能力也是其成功入侵本地植物群落的原因之一[21]。當(dāng)養(yǎng)分水平在一定范圍內(nèi)增加，會(huì)有利于外來(lái)物種對(duì)本地植物群落的入侵[22-23]。當(dāng)養(yǎng)分水平持續(xù)增加，養(yǎng)分不再是植物生長(zhǎng)的限制因素時(shí)，種間競(jìng)爭(zhēng)關(guān)系是否會(huì)發(fā)生變化還不清楚。當(dāng)養(yǎng)分總量一定時(shí)，施氮頻率能顯著促進(jìn)一些沙生植物的生長(zhǎng)和種間競(jìng)爭(zhēng)[24-26]，然而對(duì)一些物種的生長(zhǎng)和種間競(jìng)爭(zhēng)無(wú)顯著影響[27-29]。同時(shí)，有研究發(fā)現(xiàn)如果競(jìng)爭(zhēng)物種對(duì)資源的利用效率較低，施加頻率可能也不會(huì)顯著影響物種間的競(jìng)爭(zhēng)關(guān)系[26]。因此，假設(shè)施氮總量和頻率的變化會(huì)改變?nèi)肭种参锖捅镜刂参锏姆N間競(jìng)爭(zhēng)關(guān)系。

將入侵植物空心蓮子草(Alternantheraphiloxeroides(Mart.)Griseb.)和本地植物蓮子草(Alternantherasessilis(Linn.)DC.)分別采用單獨(dú)種植和混合種植的方式栽培。模擬大氣氮濕沉降分別對(duì)其進(jìn)行由兩種不同施氮總量和兩種不同施氮頻率交叉組成的4種施氮處理，并以不施氮處理作為對(duì)照，擬回答以下科學(xué)問(wèn)題：(1)施氮總量是否會(huì)影響這兩種植物的生長(zhǎng)和種間競(jìng)爭(zhēng)關(guān)系？(2)施氮頻率是否會(huì)影響這兩種植物的生長(zhǎng)和種間競(jìng)爭(zhēng)關(guān)系？(3)如果施氮總量和頻率對(duì)這兩種植物的生長(zhǎng)和種間競(jìng)爭(zhēng)關(guān)系有顯著效應(yīng)，它們之間是否存在交互作用？

1 材料和方法

1.1 研究物種和實(shí)驗(yàn)材料培養(yǎng)

實(shí)驗(yàn)選用了在中國(guó)東南沿海普遍存在的惡性入侵植物空心蓮子草和本地植物蓮子草。這兩種植物均屬于莧科蓮子草屬，都具有匍匐生長(zhǎng)，水陸兩棲，無(wú)性繁殖能力強(qiáng)等特性。在野外這兩種植物一般通過(guò)無(wú)性繁殖的方式進(jìn)行擴(kuò)張，其匍匐莖的節(jié)點(diǎn)可以產(chǎn)生根而成為獨(dú)立存活的植株[30-31]。

空心蓮子草，原產(chǎn)于南美洲，后來(lái)被引入澳大利亞、美國(guó)、中國(guó)等許多國(guó)家，成為當(dāng)?shù)貝盒匀肭值耐鈦?lái)物種[32-34]。由于它對(duì)新環(huán)境具有較強(qiáng)的適應(yīng)性和繁殖能力，在我國(guó)東南沿海地區(qū)已經(jīng)對(duì)當(dāng)?shù)厣鷳B(tài)系統(tǒng)、經(jīng)濟(jì)發(fā)展以及人類健康造成巨大的危害[15,35-36]。蓮子草原產(chǎn)于中國(guó)，在野外常與空心蓮子草共生。

本實(shí)驗(yàn)所用的空心蓮子草和蓮子草于2011年5月初采集于浙江省西溪濕地公園，屬于陸生型。然后，在北京林業(yè)大學(xué)科技股份有限公司的溫室中進(jìn)行培養(yǎng)。

1.2 實(shí)驗(yàn)設(shè)計(jì)

在實(shí)驗(yàn)開(kāi)始之前，選取空心蓮子草和蓮子草植株各540棵，剪取每棵植株頂部20cm的莖節(jié)用于實(shí)驗(yàn)，將植株垂直的插入土壤中，土壤埋住植株末端的兩個(gè)莖節(jié)。在之后的一周內(nèi)，將死掉的植株替換以保證植物在實(shí)驗(yàn)開(kāi)始時(shí)全部存活。實(shí)驗(yàn)容器為直徑25cm，高30cm黑色塑料桶，桶內(nèi)裝有12cm高的基質(zhì)，基質(zhì)成分為底泥(2012年初取自北京翠湖濕地公園)、沙子和草炭，按照1∶1∶1的體積比均勻混合而成。

本實(shí)驗(yàn)設(shè)計(jì)采取替代系列實(shí)驗(yàn)[37-40]，對(duì)兩個(gè)物種均進(jìn)行競(jìng)爭(zhēng)和施氮處理，其中，競(jìng)爭(zhēng)包含兩個(gè)處理分別是：(1)單獨(dú)種植12株空心蓮子草或蓮子草于一個(gè)培養(yǎng)容器中，分為3排4列，不存在種間競(jìng)爭(zhēng)；(2)混合種植6株空心蓮子草和6株蓮子草于一個(gè)培養(yǎng)容器中，三排四列交叉種植，存在種間競(jìng)爭(zhēng)。施氮包括5個(gè)處理，分別是：Control(對(duì)照組，不施氮)、LALF(低總量和低頻率，總共施氮15g N m-2a-1，6次)、LAHF(低總量和高頻率，總共施氮15g N m-2a-1，18次)、HALF(高總量和低頻率，總共施氮30g N m-2a-1，6次)和HAHF(高總量和高頻率，總共施氮30g N m-2a-1，18次)。因此，對(duì)每個(gè)物種，實(shí)驗(yàn)一共10個(gè)處理(5種施氮處理 × 2種競(jìng)爭(zhēng)處理)，每個(gè)處理設(shè)置6組重復(fù)。實(shí)驗(yàn)中的氮素總量和頻率依據(jù)這兩種植物在我國(guó)自然分布區(qū)域的大氣氮濕沉降量和降水情況進(jìn)行設(shè)置[41-42]。氮素是以硝酸銨溶液的形式溶解在去離子水中，使用噴霧器對(duì)植株及土壤進(jìn)行噴施。5個(gè)施氮處理按照不同的頻率施加硝酸銨溶液，實(shí)驗(yàn)每次施加硝酸銨溶液200mL，每次施加的溶液中硝酸銨含量分別為：0g(Control)、0.03g(LAHF)、0.06g(HAHF)、0.09g(LALF)、0.18g(HALF)。在實(shí)驗(yàn)過(guò)程中，高頻處理共施氮18次，低頻處理共施氮6次；高施氮總量處理共施氮30g N m-2a-1，低施氮總量處理共施氮15g N m-2a-1。在每次施氮處理時(shí)，對(duì)照處理每次噴施200mL去離子水。

實(shí)驗(yàn)于2012年7月3日開(kāi)始，結(jié)束于2012年10月1日。在實(shí)驗(yàn)過(guò)程中，(1)除了施加硝酸銨溶液外，每周澆水2—3次以保證植物正常生長(zhǎng)；(2)所有實(shí)驗(yàn)容器每月移動(dòng)1次，以減少溫室中環(huán)境異質(zhì)性造成的實(shí)驗(yàn)誤差。本實(shí)驗(yàn)是在北京林業(yè)大學(xué)科技股份有限公司溫室中進(jìn)行的，實(shí)驗(yàn)過(guò)程中的日均溫度為25.0℃，平均濕度為74.2%。

1.3 實(shí)驗(yàn)數(shù)據(jù)采集

實(shí)驗(yàn)結(jié)束時(shí)，清數(shù)出每株植物的節(jié)數(shù)，并測(cè)量出每個(gè)植株的莖節(jié)長(zhǎng)度和總?cè)~面積。總?cè)~面積使用WinFOLIA多功能葉面積儀測(cè)量(WinFOLIA Pro 2004a, Regent Instruments, Quebec, Canada)。隨后，將植株的根、莖、葉分開(kāi)，并在70℃的烘箱內(nèi)烘干48h，分別測(cè)定各部分生物量。

1.4 數(shù)據(jù)分析

空心蓮子草和蓮子草的數(shù)據(jù)分別進(jìn)行了分析。首先，運(yùn)用雙因素方差分析(Two-way ANOVA)的方法分析了競(jìng)爭(zhēng)和施氮處理對(duì)兩種植物的各生長(zhǎng)和形態(tài)指標(biāo)(包括總生物量、根生物量、莖生物量、葉生物量、總莖長(zhǎng)、總節(jié)點(diǎn)數(shù)、葉面積和根冠比)的影響(表1)。

表1 種間競(jìng)爭(zhēng)和施氮處理對(duì)空心蓮子草和蓮子草各生長(zhǎng)指標(biāo)的影響Table 1 Effects of interspecific competition (C) and nitrogen (N) addition on growth traits of Alternanthera philoxeroides and A.sessilis.

種間競(jìng)爭(zhēng)和施氮處理以及它們的交互作用對(duì)兩種植物的生長(zhǎng)指標(biāo)的影響采用雙因素方差分析方法(Two-way ANOVA)進(jìn)行分析;a: 數(shù)據(jù)經(jīng)過(guò)開(kāi)方處理；b: 數(shù)據(jù)經(jīng)過(guò)自然對(duì)數(shù)轉(zhuǎn)換

運(yùn)用三因素方差分析(Three-way ANOVA)檢驗(yàn)競(jìng)爭(zhēng)、施氮總量和施氮頻率3種因素對(duì)植物生長(zhǎng)和形態(tài)指標(biāo)的影響以及它們之間的交互作用(該分析不涉及對(duì)照組數(shù)據(jù)，表2)。方差分析之前，對(duì)不符合方差齊性檢驗(yàn)的數(shù)據(jù)進(jìn)行開(kāi)二次方或者取自然對(duì)數(shù)的數(shù)據(jù)轉(zhuǎn)換，已分別在方差分析表中標(biāo)注。

為探討兩種蓮子草屬植物的種間關(guān)系對(duì)5種施氮處理的響應(yīng)，基于兩種植物的生物量指標(biāo)，計(jì)算相對(duì)產(chǎn)量Relative Yield(RY)。已有研究表明，RY能有效地衡量植物的種間競(jìng)爭(zhēng)強(qiáng)度[43-45]，其計(jì)算公式如下：

RY=Yab/Ya

式中，Ya表示植物a在單獨(dú)種植處理中每株植物生物量的平均值，Yab表示植物a在混合種植處理中每株植物生物量的平均值。當(dāng)RY=1表明種內(nèi)競(jìng)爭(zhēng)強(qiáng)度與種間競(jìng)爭(zhēng)強(qiáng)度相似；當(dāng)RY>1表明種內(nèi)競(jìng)爭(zhēng)強(qiáng)度大于種間競(jìng)爭(zhēng)強(qiáng)度；當(dāng)RY<1表明種間競(jìng)爭(zhēng)強(qiáng)度大于種內(nèi)競(jìng)爭(zhēng)強(qiáng)度。計(jì)算了兩種植物總生物量的RY值，并運(yùn)用了單因素方差分析(One-way ANOVA)檢驗(yàn)不同施氮處理對(duì)植物總生物量RY值的影響。所有實(shí)驗(yàn)數(shù)據(jù)統(tǒng)計(jì)分析在SPSS(18.0)統(tǒng)計(jì)軟件中進(jìn)行。

2 結(jié)果

2.1 不同施氮處理對(duì)空心蓮子草和蓮子草生長(zhǎng)的影響

施氮顯著促進(jìn)了空心蓮子草和蓮子草植株的生長(zhǎng)和生物量的積累(表1，圖1，圖2)，空心蓮子草的根生物量例外(F4, 50=0.86,P=0.498, 表1)。排除對(duì)照組的三因素方差分析得出施氮總量顯著影響了兩種植物的葉生物量和所有形態(tài)指標(biāo)，而施氮頻率對(duì)兩種植物生長(zhǎng)的影響較小(表2)。施氮總量和頻率的交互作用對(duì)兩種植物的生長(zhǎng)和生物量積累的無(wú)顯著影響。

2.2 不同施氮處理對(duì)空心蓮子草和蓮子草種間競(jìng)爭(zhēng)關(guān)系的影響

不同施氮處理對(duì)空心蓮子草和蓮子草植株的相對(duì)產(chǎn)量(RY)的影響差異都不顯著(圖3)。空心蓮子草總生物量大于1和小于1的RY值各占一半，說(shuō)明其種內(nèi)競(jìng)爭(zhēng)和種間競(jìng)爭(zhēng)強(qiáng)度基本相似；而蓮子草5種處理中的RY值均大于1，說(shuō)明其種間競(jìng)爭(zhēng)強(qiáng)度小于種內(nèi)競(jìng)爭(zhēng)(圖3)。

表2 種間競(jìng)爭(zhēng)(C)、施氮總量(A)和施氮頻率(P)對(duì)空心蓮子草和蓮子草各生長(zhǎng)指標(biāo)的影響

Table 2 Effects of interspecific competition(C), nitrogen(N)amount(A)and nitrogen pulse(P)on growth traits of Alternanthera philoxeroides and A.sessilis

性狀Trait種間競(jìng)爭(zhēng)Competition施氮頻率Pulse施氮總量AmountC×PC×AA×PC×A×P(1)空心蓮子草A．philoxeroides總生物量Totalbiomass0．57ns1．88ns1．97ns6．53?0．49ns0．38ns0．43ns根生物量Rootbiomass247．30??0．61ns0．44ns3．30ns0．15ns0．16ns2．44ns莖生物量Stembiomassb16．15??2．48ns2．12ns5．23?1．10ns1．38ns0．04ns葉生物量Leafbiomass0．69ns0．66ns23．76??0．19ns0．50ns0．31ns0．51ns總莖長(zhǎng)Stemlength0．05ns0．02ns8．13??1．20ns0．07ns0．04ns0．19ns總節(jié)數(shù)No．ofnodes0．48ns0．01ns8．86??2．99ns0．07ns0．13ns0．52ns葉面積Leafarea0．11ns0．23ns11．39??0．37ns0．02ns0．22ns0．08ns根冠比Root：shootratiob426．70??2．85ns7．48??0．76ns0．77ns0．30ns2．63ns(2)蓮子草A．sessilis總生物量Totalbiomass4．85?0．09ns2．21ns0．31ns0．72ns0．24ns1．68ns根生物量Rootbiomassa119．73??0．41ns0．06ns0．06ns0．02ns0．51ns1．71ns莖生物量Stembiomass9．62??0．23ns1．75ns0．01ns1．06ns1．31ns1．21ns葉生物量Leafbiomass1．44ns2．67ns9．34??2．93ns0．00ns2．26ns0．10ns總莖長(zhǎng)Stemlength0．30ns6．86?14．30??5．20?0．68ns1．08ns0．22ns總節(jié)數(shù)No．ofnodesa0．06ns0．95ns19．50??1．32ns0．05ns1．41ns0．14ns葉面積Leafarea0．92ns3．64ns9．90??2．28ns0．06ns1．35ns0．47ns根冠比Root：shootratiob301．45??0．46ns2．50ns0．20ns1．04ns3．14ns0．57ns

種間競(jìng)爭(zhēng)、施氮總量和施氮頻率以及它們的交互作用對(duì)兩種植物的生長(zhǎng)指標(biāo)的影響采用三因素方差分析方法(Three-way ANOVA)進(jìn)行分析;顯著性水平：**P<0.01, * 0.010.05；表中所有數(shù)據(jù)的自由度均為(1,40)；a: 數(shù)據(jù)經(jīng)過(guò)開(kāi)二次方處理；b: 數(shù)據(jù)經(jīng)過(guò)自然對(duì)數(shù)轉(zhuǎn)換

圖1 種間競(jìng)爭(zhēng)和施氮處理對(duì)空心蓮子草植株總生物量、根生物量、莖生物量、葉生物量、總莖長(zhǎng)、總節(jié)數(shù)、葉面積和根冠比的影響(均值+標(biāo)準(zhǔn)誤差)Fig.1 Effects of interspecific competition and nitrogen (N) addition on total biomass, root biomass, stem biomass, leaf biomass, stem length, number of nodes, leaf area and root to shoot ratio (mean + SE) of Alternanthera philoxeroidesControl: 對(duì)照組，不施氮no N;LALF: 低總量和低頻率low N amount and low N frequency;LAHF: 低總量和高頻率low N amount and high N frequency;HALF: 高總量和低頻率high N amount and low N frequency;HAHF: 高總量和高頻率high N amount and high N frequency

圖2 種間競(jìng)爭(zhēng)和施氮處理對(duì)蓮子草植株總生物量、根生物量、莖生物量、葉生物量、總莖長(zhǎng)、總節(jié)數(shù)、葉面積和根冠比的影響(均值+標(biāo)準(zhǔn)誤差)Fig.2 Effects of interspecific competition and nitrogen (N) addition on total biomass, root biomass, stem biomass, leaf biomass, stem length, number of nodes, leaf area and root to shoot ratio (mean + SE) of Alternanthera sessilis Control(對(duì)照組，不施氮no N);LALF: 低總量和低頻率low N amount and low N frequency;LAHF: 低總量和高頻率low N amount and high N frequency;HALF: 高總量和低頻率high N amount and low N frequency;HAHF: 高總量和高頻率high N amount and high N frequency

2.3 種間競(jìng)爭(zhēng)對(duì)空心蓮子草和蓮子草生長(zhǎng)的影響

種間競(jìng)爭(zhēng)和施氮處理的交互作用對(duì)空心蓮子草和蓮子草植株的生長(zhǎng)和生物量積累無(wú)顯著影響(表1)。存在種間競(jìng)爭(zhēng)時(shí)，種間競(jìng)爭(zhēng)顯著提高了空心蓮子草植株莖生物量的積累(與無(wú)種間競(jìng)爭(zhēng)處理的均值相比大約增加60%)，降低了根生物量的積累(表1，圖1)。因而，顯著降低了植株的根冠比(F1, 50=338.04;P<0.001)。與空心蓮子草不同，蓮子草的總生物量積累顯著增加；其適應(yīng)策略是增加對(duì)根生物量的分配(與無(wú)種間競(jìng)爭(zhēng)處理的均值相比大約增加250%)，而降低對(duì)莖生物量的分配(表1，圖2)，相應(yīng)地，顯著提高了蓮子草的根冠比。

圖3 不同施氮處理對(duì)空心蓮子草和蓮子草總生物量相對(duì)產(chǎn)量(RY)的影響(均值+標(biāo)準(zhǔn)誤差)Fig.3 Effects of nitrogen (N) addition on relative yield based on total biomass of Alternanthera philoxeroides and A.sessilis (mean + SE)單因素方差分析(One-way ANOVA)得出兩種植物的相對(duì)產(chǎn)量在5個(gè)不同施氮處理之間均無(wú)顯著差異;Control: 對(duì)照組，不施氮no N;LALF: 低總量和低頻率low N amount and low N frequency;LAHF: 低總量和高頻率low N amount and high N frequency;HALF: 高總量和低頻率high N amount and low N frequency;HAHF: 高總量和高頻率high N amount and high N frequency

3 討論

氮素是限制植物生長(zhǎng)的重要營(yíng)養(yǎng)元素之一[1-2]。施氮會(huì)直接影響到植物的生長(zhǎng)[46-47]，因而可能影響植物種間的競(jìng)爭(zhēng)關(guān)系。本研究結(jié)果表明，施氮總量的增加顯著促進(jìn)了兩種植物的生長(zhǎng)，但并沒(méi)有顯著影響兩種植物的種間競(jìng)爭(zhēng)關(guān)系(表1, 圖3)。存在種間競(jìng)爭(zhēng)時(shí)，兩種蓮子草屬植物表現(xiàn)出幾乎相同的競(jìng)爭(zhēng)力，甚至更有利于本地植物蓮子草的生長(zhǎng)。對(duì)多樣性-可入侵性假說(shuō)的研究發(fā)現(xiàn)物種功能群多樣性(C3禾本科植物、C4植物、非禾本科草本植物和豆科植物)與群落的可入侵性具有顯著的負(fù)相關(guān)關(guān)系，即相同功能群的物種在群落中對(duì)資源和空間的競(jìng)爭(zhēng)要比不同功能群之間的物種激烈[48]。蓮子草和喜旱蓮子草屬于相同形態(tài)學(xué)功能群且同屬，研究發(fā)現(xiàn)蓮子草對(duì)空心蓮子草的入侵有強(qiáng)烈的抵制作用。還有一些研究也發(fā)現(xiàn)，入侵植物在面對(duì)本地優(yōu)勢(shì)物種時(shí)也會(huì)表現(xiàn)出較低的競(jìng)爭(zhēng)力[49-52]。陳躍等[53]研究也發(fā)現(xiàn)蓮子草和空心蓮子草在脅迫條件下表現(xiàn)出非常相似的形態(tài)適應(yīng)特征。因此，施氮總量雖然促進(jìn)了兩種植物的生長(zhǎng)，卻并不顯著影響兩種植物的種間競(jìng)爭(zhēng)關(guān)系可能是由于兩個(gè)物種利用和競(jìng)爭(zhēng)資源的能力相當(dāng)。

在自然條件下，空心蓮子草和蓮子草具有極為相似的形態(tài)特征[54]。當(dāng)存在種間競(jìng)爭(zhēng)時(shí)，兩種植物表現(xiàn)出不同的生物量分配策略(表1，圖1和圖2)。入侵物種空心蓮子草將更多的生物量分配到莖的生長(zhǎng)，加速擴(kuò)大其占領(lǐng)空間以獲取更多的資源如光照。在光照、養(yǎng)分和水分等資源充足且土壤營(yíng)養(yǎng)元素(如氮素)的分布主要限于表層時(shí)，植物降低對(duì)根生物量的投資，而增加對(duì)地上部分(如匍匐莖)的投資將有助于提高其入侵能力，從而提高整個(gè)植株的適應(yīng)性[31-32]。因此，空心蓮子草在野外可以成功的入侵到各種生態(tài)系統(tǒng)，但是蓮子草卻不能成為入侵物種[30]。與之相對(duì)應(yīng)地，本地種蓮子草表現(xiàn)出更強(qiáng)的扎根能力并將更多的生物量用于根的生長(zhǎng)，即在種間競(jìng)爭(zhēng)處理中的根生物量的均值是單獨(dú)種植處理中的3.5倍(圖2)。在一定的空間和資源水平條件下，較多的根生物量分配增強(qiáng)了蓮子草獲取更多的土壤養(yǎng)分和水分能力。因此，蓮子草表現(xiàn)出較強(qiáng)的種間競(jìng)爭(zhēng)能力。

施氮頻率對(duì)兩種植物的生長(zhǎng)和種間關(guān)系都沒(méi)有顯著影響(表2，圖3)。國(guó)內(nèi)外許多研究表明施氮頻率對(duì)植物生長(zhǎng)是否具有促進(jìn)作用決定于物種對(duì)氮素的利用能力、植株發(fā)育階段、施氮季節(jié)、施氮多少以及土壤含水量等因素[6,55-56]。在本實(shí)驗(yàn)中，為植物提供的水分和除氮素而外的養(yǎng)分很充足。實(shí)驗(yàn)中的低氮處理(15g N m-2a-1)已經(jīng)能滿足植株的生長(zhǎng)需求[3,28]。在資源相對(duì)充足的條件下，不同的施氮頻率可能就不會(huì)顯著影響植物的生長(zhǎng)，進(jìn)而不會(huì)影響植物的種間競(jìng)爭(zhēng)；其次，Grime[57]指出對(duì)環(huán)境脅迫具有較強(qiáng)耐受能力的植物對(duì)施氮頻率不敏感。由于這兩種植物對(duì)環(huán)境脅迫如土壤養(yǎng)分和水分含量都具有較強(qiáng)的耐受能力[30,54]。因此，它們可能對(duì)施氮頻率的響應(yīng)較弱。再次，施氮周期較短也可能會(huì)影響這兩種植物對(duì)施氮頻率的響應(yīng)。

總之，施氮顯著提高了兩種植物的生長(zhǎng)，卻不影響其種間競(jìng)爭(zhēng)強(qiáng)度。全球變化背景下的氮沉降可能改變兩種植物的種群結(jié)構(gòu)和動(dòng)態(tài)。下一步的研究將在考慮土壤養(yǎng)分和水分條件、物種對(duì)環(huán)境脅迫的耐性以及施氮強(qiáng)度和季節(jié)的基礎(chǔ)上，進(jìn)一步研究施氮頻率對(duì)兩種植物生長(zhǎng)和種間競(jìng)爭(zhēng)的影響。

致謝：感謝王奧、姜星星、張秋秋、呂仁猛、鄭立地和姚新穎在實(shí)驗(yàn)過(guò)程和收獲中給予的幫助。

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Effects of nitrogen addition on interspecific competition betweenAlternantheraphiloxeroidesandAlternantherasessilis

ZHOU Jian1, LI Hongli1, LUO Fangli1,*, HUANG Wenjun2, ZHANG Mingxiang1, YU Feihai1

1SchoolofNatureConservation,BeijingForestryUniversity,Beijing100083,China2SichuanAcademyofForestry,Chengdu610081,China

Nitrogen (N) is one of the most important soil nutrients for plants, and the amount and frequency of N release in soils is heterogeneous.Many studies have focused on the effects of heterogeneous N addition on a single plant species, but little is known about the effects of N amount and short-term N pulse on interspecific competition between invasive and native plant species.A replacement series experiment was conducted under greenhouse conditions to examine the effects of N amount and frequency release on the growth and interspecific competition between an invasive wetland clonal plant,Alternantheraphiloxeroides(alligator weed), which is native in South America but highly invasive in China, and its native congenerAlternantherasessilis(sessile joyweed).Plant materials ofA.philoxeroidesandA.sessiliswere collected from five locations in Xixi Wetland Park in Hangzhou, Zhejiang Province, China.Stem fragments of similar length (20cm), with a stem tip for each species, were grown in monoculture (12ramets in one container, no interspecific competition) and in mixture (six invasive plants with six native plants, with interspecific competition) in five different N treatments: control (no N added), low amount and high frequency (a total of 15g N m-2a-1added every 5days), low amount and low frequency (a total of 15g N m-2a-1added every 15days), high amount and high frequency (a total of 30g N m-2a-1added every 5days), and high amount and low frequency (a total of 30g N m-2a-1added every 15days).The results showed that N addition significantly increased the growth of bothA.philoxeroidesandA.sessilis.No significant difference was observed between the control and treatments for the relative yield (yield of each species in mixture divided by yield in its monoculture) for both species, indicating that N addition did not significantly affect the interspecific competition between the two species.The probable reason is that both species are from the same functional group and may have comparable capacities to compete and utilize the soil nutrients.The N pulse did not significantly modify growth or interspecific competition for either species.The sufficient soil water content and availability of nutrients other than N may explain why there were no effects of a N pulse on competition between the two species.Alternatively, high tolerance of both species to low soil nutrients and water content may lead to weaker responsiveness to a N pulse.With interspecific competition, the invasive species,A.philoxeroides, invested more biomass to stems, i.e., the stem biomass increased by 60%, which significantly decreased the root to shoot ratio.The native species,A.sessilis, allocated more biomass to roots, i.e., the root biomass increased by 250%, which significantly increased the root to shoot ratio.The results indicate that increasing atmospheric N deposition in the context of climate change may change population structure and dynamics of both species, but may not affect the interspecific competition of these plants.

atmospheric N deposition;clonal plant;interspecific competition;invasive plant;pulse;native plant;relative yield

中央高?；究蒲袠I(yè)務(wù)費(fèi)專項(xiàng)資金資助(TD-JC-2013-1);高等學(xué)校博士學(xué)科點(diǎn)專項(xiàng)科研基金新教師類資助課題(20120014120001);國(guó)家自然科學(xué)基金項(xiàng)目(31200313, 31200314)

2014-05-28; < class="emphasis_bold">網(wǎng)絡(luò)出版日期:

日期:2015-05-21

10.5846/stxb201405281098

*通訊作者Corresponding author.E-mail: ecoluofangli@163.com

周建，李紅麗，羅芳麗，黃文軍，張明祥，于飛海.施氮對(duì)空心蓮子草(Alternantheraphiloxeroides)和蓮子草(Alternantherasessilis)種間關(guān)系的影響.生態(tài)學(xué)報(bào),2015,35(24):8258-8267.

Zhou J, Li H L, Luo F L, Huang W J, Zhang M X, Yu F H.Effects of nitrogen addition on interspecific competition betweenAlternantheraphiloxeroidesandAlternantherasessilis.ActaEcologicaSinica,2015,35(24):8258-8267.