張靜，陳先江，侯扶江

(草地農(nóng)業(yè)生態(tài)系統(tǒng)國家重點(diǎn)實(shí)驗(yàn)室，農(nóng)業(yè)部草牧業(yè)創(chuàng)新重點(diǎn)實(shí)驗(yàn)室，蘭州大學(xué)草地農(nóng)業(yè)科技學(xué)院，甘肅 蘭州 730020)

家畜排泄物對牧草種子傳播和萌發(fā)的作用

張靜，陳先江，侯扶江

(草地農(nóng)業(yè)生態(tài)系統(tǒng)國家重點(diǎn)實(shí)驗(yàn)室，農(nóng)業(yè)部草牧業(yè)創(chuàng)新重點(diǎn)實(shí)驗(yàn)室，蘭州大學(xué)草地農(nóng)業(yè)科技學(xué)院，甘肅 蘭州 730020)

放牧調(diào)控牧草的生殖格局，包括牧草有性繁殖和無性繁殖分配、種子生產(chǎn)、傳播和萌發(fā)等，其中家畜的排泄物是重要的作用途徑之一。放牧家畜的排泄物促進(jìn)種子生產(chǎn)、遠(yuǎn)距離擴(kuò)散和種子萌發(fā)，同時也會損傷種子、抑制萌發(fā)等。本文從種子生產(chǎn)、傳播及萌發(fā)等方面闡述放牧家畜排泄物對牧草種子的作用，認(rèn)為其是牧草自然更新有效途徑之一。放牧家畜排泄物能夠增加種子產(chǎn)量、采食幾率、散布距離及改變萌發(fā)自身與生境條件,從而提高種子傳播數(shù)量和質(zhì)量，以促進(jìn)牧草的有效自然更新。通過了解家畜排泄物在牧草種子傳播和萌發(fā)中的作用并再此基礎(chǔ)上探討牧草自然更新的機(jī)制，以期為草地更新和退化草地恢復(fù)提供理論參考。

放牧；草地；有性繁殖；消化道；種子擴(kuò)散；土壤

牧草的更新方式通常有無性和有性繁殖兩種，Nathan和Muller將有性繁殖的時空模型歸納為植株、種子生產(chǎn)、短暫土壤種子庫(transient soil seed bank)、持久種子庫(persistent soil seed bank or permanent soil seed bank)和幼苗5個時期，種子生產(chǎn)、散布形式及種子域的形式(包含距離、方向的分布和貯藏種子的群集等)是核心[1]。家畜排泄物占據(jù)放牧地30%～40%的面積[2-3]，是家畜向草地返還物質(zhì)的主要途徑[4-5]，而且對牧草種子的擴(kuò)散和萌發(fā)、實(shí)生苗生長等有直接和間接作用[6]，排泄物在草地牧草自然更新方面的重要性就不言而喻了。

根據(jù)目前國內(nèi)外的研究，放牧家畜排泄物對牧草更新的作用主要包括排泄物分解利用、種子的消化道傳播和萌發(fā)等幾方面。在分解利用上，小尺度內(nèi)排泄物能改變土壤養(yǎng)分[7]，繼而可能影響種子形成[8]；而以糞排泄物為載體的傳播方式的研究[9]成為學(xué)者們關(guān)注熱點(diǎn)。在北美[10-11]、歐洲[12-13]、亞洲[14-15]等地區(qū)，探究了不同家畜對不同牧草種子傳播和萌發(fā)的作用，主要涉及了放牧家畜有牛[16-17]、羊[18-19]、鹿[20-21]等。為此，本文重點(diǎn)圍繞排泄物在牧草種子生產(chǎn)、傳播及萌發(fā)過程等方面的作用進(jìn)行論述，以闡明放牧家畜對天然草地牧草更新的機(jī)制，為合理利用草地及改善草地生態(tài)環(huán)境提供參考。

1 家畜排泄物對牧草種子生產(chǎn)的作用

放牧家畜排泄物中的營養(yǎng)成分轉(zhuǎn)化為溶解態(tài)后經(jīng)過淋溶滲漏[22]和微生物分解等方式進(jìn)入土壤，改善土壤pH，提高土壤C、N、P等元素的含量和土壤肥力(表1)，從而促進(jìn)植物的生長，同時改變草地養(yǎng)分再分配[29]，對草地生態(tài)系統(tǒng)的養(yǎng)分平衡起到一定的調(diào)節(jié)作用。N、P、K等元素都能促進(jìn)牧草生殖構(gòu)件的形成或增加種子的產(chǎn)量：N元素能增加無芒雀麥(Bromusinermis)[30]和羊草(Leymuschinensis)的生殖枝、小花數(shù)[31]、籽實(shí)和結(jié)實(shí)率[32]及燕麥(Avenasativa)的穗長及繁殖生物量[33]；P元素則具有促進(jìn)牧草生長、生殖器官形成和果實(shí)發(fā)育的作用[34]及增加禾本科種子產(chǎn)量[35]；而K元素同樣也能增加羊草籽實(shí)和結(jié)實(shí)率[32]。排泄物作為有機(jī)肥能夠提高禾草生殖分配[36]及增加豆科類植物種子的產(chǎn)量和質(zhì)量[37]。經(jīng)過施加腐熟羊糞，可以有效增加青海草地早熟禾(Poapratensiscv. Qinghai)的種子產(chǎn)量[38]。總之，排泄物的分解是一個漫長的過程，對牧草種子生產(chǎn)的影響也因此在短期內(nèi)不顯著，但是經(jīng)過處理腐熟的排泄物能有效地增加種子的產(chǎn)量。因此，在放牧草地上如何簡單、高效的利用放牧家畜排泄物這一方面需要更加深入的研究。

表1 糞斑下土壤養(yǎng)分差異Table 1 Effect of dung on soil nutrient content

2 家畜排泄物對牧草種子傳播的作用

2.1種子的傳播與保存

家畜消化道傳播(endozoochory)是家畜采食牧草種子后，種子在消化道內(nèi)隨家畜運(yùn)動、并通過糞便排泄，遠(yuǎn)離植物母體的過程[12]。家畜消化道處理促進(jìn)牧草種子萌發(fā)，糞便施肥可提高其出苗率和幼苗建植[39]。牧草種子通過家畜消化道的傳播，改變放牧草地植物種子的密度，加快退化草地的生態(tài)恢復(fù)[16]。家畜消化道傳播的有效性取決于種子攝入、消化道對種子的作用、種子傳播距離等。

種子攝入是消化道傳播的基礎(chǔ)。排泄物主要在牧草的氣味、營養(yǎng)成分方面影響家畜對種子的攝入而間接作用消化道傳播。其作用過程包括：1)糞排泄物因其氣味常導(dǎo)致家畜不喜歡采食糞斑處的牧草[4]而減少種子的攝入[40]；2)隨著排泄物養(yǎng)分進(jìn)入土壤，提高其附近牧草營養(yǎng)品質(zhì)、種子產(chǎn)量而促進(jìn)家畜采食[41-42]，增加種子攝入幾率。雖然糞排泄物能短暫減少家畜對牧草種子的攝入量，但從長遠(yuǎn)來看，家畜排泄物是有助于動物對牧草種子的攝入而間接貢獻(xiàn)于消化道傳播。

家畜消化道傳播的關(guān)鍵點(diǎn)是消化道對種子的作用，主要包含家畜咀嚼、胃及腸道的作用。種子的自身特性如：大小、形狀、壽命和種皮特性等都是消化道傳播的影響因子。在種子大小的研究上多數(shù)學(xué)者認(rèn)為小種子要比大種子更容易出現(xiàn)在糞排泄物里并更具有活力[43-44]。對這一結(jié)論，學(xué)者們給出了兩種解釋機(jī)制：較主流的解釋是小種子在動物消化道過程中受到的迫害小。小種子因其個體較小，容易逃避動物咀嚼迫害[45]且通過消化道的速度快[46]而減少損害。通過對200種不同植物種子的對比發(fā)現(xiàn)，植物種子的個體越小表示在消化過程中停留的時間就越短，從而增加了種子的存活率，使之能成功通過消化道傳播[47]。另一種解釋認(rèn)為每一個植物在其種子大小和種子數(shù)量上都是有著協(xié)調(diào)式的產(chǎn)出，因此種子較小的相應(yīng)產(chǎn)生的種子數(shù)量就較多了，這就增大了小種子出現(xiàn)在糞便中的幾率[48]。然而，也有學(xué)者提出了不同的觀點(diǎn)，認(rèn)為大種子對于小種子而言卻有著其自身的優(yōu)勢[49]。大種子由于其面積、質(zhì)量比較小，使得大種子具有不錯的抵抗消化道迫害的能力而增加消化道傳播的成功率。而種子的其它特性如：種子形狀的作用是影響種子通過消化道的速率，而種皮特性和種子壽命則是影響種子通過消化道后的活力(表2)。可見，相對而言種子若小、圓、且擁有較堅硬種皮則更能成功的經(jīng)家畜消化道來傳播。

放牧家畜消化道傳播為種子的遠(yuǎn)距離傳播提供了一個有效的途徑[55]，它滿足了遠(yuǎn)距離傳播的兩個前提條件：1)糞中包含有足夠數(shù)量的種子。卻因種子、家畜種類和放牧方式的差異而各異。在美國大草原，放牧野牛糞中的平均種子密度為每克干糞2～9粒[56]，結(jié)縷草(Zoysiajaponica)草地放牧的黃牛糞中含有的種子數(shù)量平均為每克干糞101.7粒[15]；荷蘭鈣質(zhì)草原，每克干綿羊糞中只含有(0.8±0.14)粒種子[57]，而結(jié)縷草草地放牧中山羊糞中含有每克干糞29.13粒[15]。2)家畜能攜帶種子產(chǎn)生較大的位移。有研究表明，放牧綿羊或山羊可以攜帶著采食進(jìn)入體內(nèi)的種子行走24～36 h[58]，在時間段之內(nèi)它們的行走最大距離3～17 km[59]。

家畜排泄物通過改變種子生存環(huán)境而使種子得到良好的保存。一方面糞便的覆蓋可以使在下層的種子逃避鼠、鳥等捕食者的捕食，從而增加種子的存活率；另一方面糞便的包裹為種子提供一種良好的庇護(hù)場所，改善種子的保存環(huán)境而使得種子得以保存。同時家畜的糞便是一些糞食型、捕食型無脊椎動物的取食和繁殖場所，在這些無脊椎動物的捕食過程中，糞便被移動到土壤表層下，其中的種子也隨之被帶入土壤下層并得到很好的保存。蜣螂作為一種典型的食糞無脊椎動物，在此過程中扮演著重要的角色[60-61]。家畜排泄物作為家畜消化道傳播種子的載體雖因動物和植物之間的物種差異而各異，但對于牧草來說卻是一種有效的擴(kuò)散形式。它能在一定程度上有效地擴(kuò)大種子傳播范圍，增加種子傳播的有效性。

表2 家畜影響種子的因素Table 2 Seed factors and influence on livestock biology

2.2土壤種子庫

土壤種子庫(soil seed bank)是存在于土壤上層凋落物和土壤中全部存活種子的總和。放牧能有效地改變草地土壤種子庫的密度和物種組成(表3)。土壤種子庫通常可以簡單的分為短暫土壤種子庫和持久土壤種子庫。持久土壤種子庫指種子在土壤中休眠期至少為一年，雖然糞在草地上保留的時間不是很長，但是其中包含的種子與持久土壤種子庫中的種子有相同的活力[67]。因此有學(xué)者認(rèn)為，糞中的種子對持久土壤種子庫影響更明顯，是植物通過種子入侵草地的重要途徑[68]；同時，也促進(jìn)退化草地的恢復(fù)[69-70]。北歐石灰?guī)r地區(qū)的草地，隨著牛糞的分解，糞斑下土壤的短暫種子庫所占比例減少，而其持久土壤種子庫占比增加[71]。不僅如此，糞排泄物還能增加了糞斑下土壤種子庫的物種豐富度和某些種子的密度。在Biaowiea森林中野牛糞斑下的表層土壤種子庫中的蕁麻(Urticadioica)、燈芯草(Juncuseffuses)以及苔草(Carexremota)等種子密度顯著增加，粟草(Miliumeffusum)、水蓼(Polygonumhydropiper)、玄參(Scrophularianodosa)等喜濕植物的種子可能從野牛糞中轉(zhuǎn)移到土壤里，增加了土壤種子庫的物種豐富度[72]。然目前國內(nèi)外在排泄物對土壤種子庫的影響方面研究較少，只是在一些動物和部分地區(qū)有研究，難以成為普適結(jié)論。

表3 放牧對土壤種子庫的影響Table 3 Effect of grazing on soil seed bank

3 家畜排泄物對草地種子萌發(fā)的作用

種子的萌發(fā)受其自身及環(huán)境條件的綜合影響(圖1)，而家畜排泄物對種子萌發(fā)的作用因素也較多，從作用效果上可歸為兩類：促進(jìn)或抑制萌發(fā)。排泄物能促進(jìn)種子萌發(fā)的作用途徑包括：1)家畜采食、消化后促進(jìn)萌發(fā)。一般，種子發(fā)芽率與家畜個體大小之間呈正相關(guān)關(guān)系[73-74]。日本結(jié)縷草種子經(jīng)黃牛采食后發(fā)芽率顯著提高，山羊采食后發(fā)芽率的提高不顯著[15]。然而，南非Thornveld草地，山羊和邦斯瑪拉牛采食代兒茶(Dichrostachyscinerea)和阿拉伯金合歡(Acacianilotica)后顯著促進(jìn)兩者種子萌發(fā)，且羊糞的萌發(fā)率高于牛糞[39]。2)直接改變種子萌發(fā)環(huán)境而促進(jìn)萌發(fā)。家畜糞可以改善種子萌發(fā)所需的水分條件，破碎和水分滲透后的羊糞能促進(jìn)種子發(fā)芽[57]并影響實(shí)生苗生長[75]，且在西歐，踐踏后的羊糞萌發(fā)物種數(shù)是未經(jīng)過踐踏的2～3.6倍[76]。3)間接改變種子萌發(fā)環(huán)境而促進(jìn)萌發(fā)。排泄物主要是改變了土壤中能影響種子萌發(fā)的pH和N、P等元素的含量。pH能通過改變種子內(nèi)在的代謝而顯著影響種子的萌發(fā)，對于堿蓬(Suaedasalsa)種子而言，較高的pH能提高其種子的萌發(fā)率[77]；N、P元素的增加可以促進(jìn)堿蓬[78]、老芒麥(Elymussibiricus)和諾丹冰草(Agropyrondesertorumcv. Norda)種子的萌發(fā)[79]。

圖1 影響種子萌發(fā)的因素Fig. 1 Factors affecting seed germination

排泄物主要是通過改變萌發(fā)環(huán)境條件而抑制種子萌發(fā)。如，新鮮牛糞覆蓋厚度的增加會阻礙干旱土壤中細(xì)葉百脈根(Loutstenuis)種子的萌發(fā)[80]；糞便的形狀也抑制種子萌發(fā)：羊糞為球形且有硬殼，阻礙糞中種子萌發(fā)，羊糞粒干燥迅速也不利于種子萌發(fā)和實(shí)生苗生長。而且糞中的有害物質(zhì)[66]和N濃度過高、pH不適宜等化學(xué)指標(biāo)也可能抑制種子的萌發(fā)[81]。對于羊草種子來說，其種子的發(fā)芽率隨著pH的增加而降低[82]。

4 討論與展望

4.1作用機(jī)制

以種子為繁殖體的更新方式是牧草自然更新的重要方式。種子傳播影響著植物種群續(xù)存及物種多樣性維系[83]；種子萌發(fā)特性影響著牧草繁殖的成功率及群落的建成與演替。放牧家畜排泄物能夠通過改良土壤而促進(jìn)植物有性生長、提高種子存活率等；能直接或間接地影響種子的傳播及萌發(fā)，為牧草以種子為繁殖體的更新方式提供有利的條件(圖2)。對放牧家畜排泄物在牧草種子傳播和萌發(fā)作用的研究可以從種子質(zhì)量和數(shù)量兩方面加深其對牧草更新的了解[84]。種子傳播數(shù)量是家畜采食牧草的頻次和其每次所傳播種子數(shù)量的乘積[85]。受排泄物影響，牧草營養(yǎng)增加、種子產(chǎn)量提高，家畜采食頻次和采食數(shù)量增加，從而提高傳播數(shù)量。種子傳播質(zhì)量取決于家畜消化、傳播距離和種子適宜生境萌發(fā)[86]，排泄物對其影響來自以下兩種機(jī)制：種子經(jīng)過家畜消化道作用并隨家畜運(yùn)動而產(chǎn)生位移，導(dǎo)致種子存活概率和傳播距離改變；經(jīng)過消化道作用改變種子萌發(fā)自身?xiàng)l件和受排泄物影響而改變種子萌發(fā)生境條件。這幾種機(jī)制直觀地體現(xiàn)了排泄物與牧草更新關(guān)系，通過了解排泄物對種子傳播和萌發(fā)作用的機(jī)制有助于理解草地植物群落更新及其演替規(guī)律。

圖2 排泄物對種子傳播與萌發(fā)的作用Fig. 2 Effect of feces in seed dispersal and germination

注：①增加排泄物分布；②經(jīng)過在家畜消化道內(nèi)隨家畜運(yùn)動并通過糞排泄物，達(dá)到傳播的目的；③排泄物的包裹和覆蓋能改變萌發(fā)環(huán)境，促進(jìn)或抑制萌發(fā)；④排泄物含有豐富的養(yǎng)分，能促進(jìn)牧草的生長與發(fā)育；⑤隨著排泄物的分解，養(yǎng)分進(jìn)入土壤，改變土壤理化性質(zhì)；⑥養(yǎng)分充足的土壤條件，能促進(jìn)牧草的生長；⑦牧草的生殖發(fā)育，影響種子的形成及質(zhì)量；⑧土壤pH、營養(yǎng)元素能作用于種子，影響萌發(fā)率；⑨通過消化道及其它方式傳播種子的過程中，種子受到外部條件作用而影響種子的萌發(fā)率。

Note: ①Increase excreta distribution; ②Through excreta excretion by livestock in the digestive tract with the livestock movement, to achieve the purpose of dispersal; ③With the excreta wrapped or coverage, it can change seed germination environment to promote or inhibit germination; ④Excreta is rich in nutrients, which can promote the growth and development of forage; ⑤With the decomposition of excreta, the nutrients into the soil and changing the physical and chemical properties of soil; ⑥The nutrient-rich soil conditions can promote the growth of forage;⑦The reproductive development of forage can affecting seed formation and quality; ⑧The soil pH and nutrient elements can act on the seeds to affecting the germination rate; ⑨In the process of dispersal seeds in the digestive tract and other ways, the seeds are affected by external conditions and affected the seed germination rate.

4.2研究展望

雖然目前現(xiàn)有的研究成果在排泄物對牧草種子傳播和萌發(fā)方面奠定了基礎(chǔ)，今后可以在以下幾個方面加強(qiáng)研究：1)加強(qiáng)排泄物對土壤種子庫的研究，關(guān)于這方面的研究較少，不能形成一種普適的結(jié)論；這也許能夠在保存和恢復(fù)植物物種與群落多樣性提供幫助。2)家畜排泄物對種子的傳播仍是國內(nèi)外研究的熱點(diǎn)，但大多還停留在對種子傳播作用的認(rèn)知上，在研究尺度、作用機(jī)理和有效利用方面需加強(qiáng)。3)加大排泄物在種子傳播中的有效性研究，有助于了解種子更新在區(qū)域性植被恢復(fù)中所產(chǎn)生的作用。4)以往研究都關(guān)注于改良栽培草地的牧草或?qū)δ敛萆L的影響，加強(qiáng)尿排泄物在種子萌發(fā)方面的研究，有助于為以種子為繁殖體的更新過程提供一種新途徑。

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Effectsoflivestockexcretaonherbageseeddispersalandgermination

Zhang Jing, Chen Xian-jiang, Hou Fu-jiang

(State Key Laboratory of Grassland Agro-Ecosystems, Key Laboratory of Grassland Livestock Industry Innovation, Ministry of Agriculture, China, College of Pastoral Agriculture Science and Technology, Lanzhou University, Lanzhou 730020, Gansu, China)

Grazing regulation of forage impacts forage consumption, growth, reproduction, seed production, dispersal, and germination. Dispersal of seeds through livestock excrement was one of the important means of seeds transport. While excreta from livestock can promote long-distance dispersal and seed germination, it can also damage the seeds, and affect germination. Therefore, we studied the effects of excreta on plant reproduction, propagation and germination of herbage seed. Our results show that livestock excreta served as a natural and effective means of herbage seed dispersal and regeneration. Livestock excreta increased seed production in herbage, improved likelihood of feeding, increased dispersal distance and also expanded the forage habitat. Through improved understanding of the role of excreta in herbage seed dispersal and germination it is possible to explore new approaches that may provide a theoretical framework to minimize grassland degradation and propose solutions for restoration of grasslands.

grazing; grassland; sexual reproduction; alimentary canal; seed dispersal; soil

Hou Fu-jiang E-mail:cyhoufj@lzu.edu.cn

10.11829/j.issn.1001-0629.2016-0475

張靜，陳先江，侯扶江.家畜排泄物對牧草種子傳播和萌發(fā)的作用.草業(yè)科學(xué),2017,34(10)：2070-2079.

Zhang J,Chen X J,Hou F J.Effects of livestock excreta on herbage seed dispersal and germination.Pratacultural Science，2017,34(10)：2070-2079.

S812.8;Q945.34

A

1001-0629(2017)10-2070-10

2016-09-13

2017-02-20

國家自然科學(xué)基金(31672472);教育部“創(chuàng)新團(tuán)隊(duì)發(fā)展計劃”(IRT-17R50)

張靜(1989-)，男，四川成都人，在讀碩士生，主要從事牧場管理研究。E-mail:zhangjing2014@lzu.edu.cn

侯扶江(1971-)，男，河南扶溝人，教授，博導(dǎo)，博士，主要從事草地-家畜互作研究。E-mail:cyhoufj@lzu.edu.cn

(責(zé)任編輯 張瑾)