放牧生態(tài)系統(tǒng)枯落物及其作用

馬周文，王迎新，王宏，阿不滿，張貞明，侯扶江*

(1.草地農(nóng)業(yè)生態(tài)系統(tǒng)國家重點(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；2.瑪曲縣阿孜畜牧科技示范園區(qū)，甘肅 瑪曲 747300；3.甘肅省草原技術(shù)推廣總站，甘肅 蘭州 730000)

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放牧生態(tài)系統(tǒng)枯落物及其作用

馬周文1，王迎新1，王宏2，阿不滿3，張貞明3，侯扶江1*

(1.草地農(nóng)業(yè)生態(tài)系統(tǒng)國家重點(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；2.瑪曲縣阿孜畜牧科技示范園區(qū)，甘肅 瑪曲 747300；3.甘肅省草原技術(shù)推廣總站，甘肅 蘭州 730000)

草地枯落物是連接放牧生態(tài)系統(tǒng)中“土—草”界面的主要媒介，是調(diào)控地上-地下生態(tài)過程的關(guān)鍵因子，對草地物種多樣性、生產(chǎn)力、退化草地恢復(fù)等具有重要意義。一方面，家畜的采食、踐踏、排泄物等減少枯落物積累和加速其分解，且與放牧強(qiáng)度、制度、季節(jié)、家畜種類聯(lián)合響應(yīng)；另一方面，枯落物影響家畜的選擇性采食、蹄壓等行為，為微生物和小草食動(dòng)物生命活動(dòng)提供有利場所及能量，進(jìn)而影響草地生態(tài)系統(tǒng)結(jié)構(gòu)及功能。枯落物也能夠改變土壤理化性質(zhì)和物質(zhì)循環(huán)，產(chǎn)生化感物質(zhì)，影響種子發(fā)芽、幼苗生長，導(dǎo)致草地群落構(gòu)建和演替。本研究綜述了國內(nèi)外相關(guān)文獻(xiàn)，探討放牧生態(tài)系統(tǒng)中的枯落物及其作用，旨在明確草地生態(tài)系統(tǒng)中“放牧-枯落物-土壤-綠色植物”之間的相互作用機(jī)制，為后續(xù)草地可持續(xù)性管理研究提供理論依據(jù)。

放牧；生態(tài)系統(tǒng)；草地枯落物；功能；群落演替

放牧是草地利用最普遍、最經(jīng)濟(jì)的利用方式[1]。在放牧生態(tài)系統(tǒng)中，枯落物是土—草界面過程的主要介質(zhì)，是調(diào)控地上—地下生態(tài)過程的關(guān)鍵因子[2-3]，其積累與分解是生態(tài)系統(tǒng)物質(zhì)循環(huán)的重要過程[4-5]。家畜的采食、踐踏、排泄等行為調(diào)控枯落物的積累和分解，且與放牧強(qiáng)度、制度、季節(jié)、家畜種類密切相關(guān)[6-8]；放牧減少枯落物積累，降低火災(zāi)風(fēng)險(xiǎn)[9-10]?？萋湮锏姆纸馐欠拍辽鷳B(tài)系統(tǒng)重要的過程，其能量流動(dòng)和養(yǎng)分循環(huán)影響草地生產(chǎn)力、生物多樣性、退化草地的恢復(fù)等[11-12]?？萋湮锓纸飧淖兺寥览砘再|(zhì)和土壤養(yǎng)分循環(huán)，產(chǎn)生化感物質(zhì)，影響種間競爭和群落結(jié)構(gòu)[13-15]。同時(shí)，枯落物在草地表面形成“緩沖層”，減緩?fù)?草-畜之間的物理作用強(qiáng)度，防止降雨對表層土壤的沖擊，調(diào)節(jié)土壤表層微氣候(溫度、含水量、光照等)，直接或間接影響植被生長和地下生態(tài)過程[16-17]。

國內(nèi)外對草地枯落物的研究主要集中在以下3個(gè)方面：1)放牧等管理因素、氣候和地形等環(huán)境因素[18]、土壤微生物和動(dòng)物、植物生物多樣性等生物因素[19]對枯落物形成與分解的影響；2)枯落物分解對生態(tài)系統(tǒng)營養(yǎng)元素循環(huán)的貢獻(xiàn)(C、N、P等)，枯落物分解過程中木質(zhì)素、C、N、P等化學(xué)成分及其相互比例的生物地球化學(xué)循環(huán)規(guī)律[8,20]；3)枯落物對植被動(dòng)態(tài)、放牧家畜行為和生產(chǎn)力、土壤微生物和動(dòng)物組成與活動(dòng)等的作用[21-22]。目前，草地枯落物對放牧系統(tǒng)土-草-畜相互作用研究比較少，其中機(jī)制尚不清楚。為此，分析國內(nèi)外關(guān)于放牧對枯落物以及枯落物對草地結(jié)構(gòu)與功能作用機(jī)制，以期為草地可持續(xù)放牧管理提供科學(xué)依據(jù)。

圖1 草地生態(tài)系統(tǒng)中“放牧-枯落物-綠色植物”之間的相互影響機(jī)制Fig.1 The interaction mechanism among the “grazing-litter-green plants” in the grassland ecosystem ①放牧率，放牧制度，放牧季節(jié)，家畜種類等Stocking rate, grazing system, grazing season, livestock species, etc.；②枯落物積累量，密度，高度等Litter accumulation, density, height, etc.；③物理作用，化學(xué)作用(分解、化感)，生物作用等Physical function, chemical function (decomposition, chemical), biological function, etc.； ④采食，踐踏，排泄物等Feeding, trampling, excrement, etc.；⑤生長速率，補(bǔ)償性生長，品質(zhì)，物種多樣性等Growth rate, compensatory growth, quality, species diversity, etc.；⑥競爭，地上(地下)生物量等Competition, above-ground (under-ground) biomass, etc.

1 放牧對枯落物的影響

枯落物是對放牧最敏感的指標(biāo)之一[23-24]。家畜對枯落物的作用主要有以下方面：1)家畜直接采食枯落物，在冬春季牧草供給不足時(shí)較為常見，牧草生長旺期較少采食[23]；由于放牧增加枯落物氮含量、降低C/N，提高枯落物營養(yǎng)品質(zhì)，也促進(jìn)家畜采食[25]。2)適度放牧刺激植物補(bǔ)償性生長，提高枯落物生物量[26]。3)放牧影響群落結(jié)構(gòu)，改變枯落物組成，進(jìn)而導(dǎo)致家畜對枯落物的采食或者枯落物的分解發(fā)生變化[27]。4)家畜踐踏造成牧草機(jī)械損傷，加速枯落物的形成[1]，同時(shí)破碎的枯落物易隨風(fēng)飄失。5)家畜踐踏破碎和淺埋枯落物，加速枯落物的分解[28]。6)家畜排泄物含有大量氮素，增強(qiáng)土壤微生物的活性，加速其對枯落物的分解[29]。7)放牧改變了草地的輻射、溫度等微環(huán)境，影響枯落物的分解[30]?？梢?，放牧可以多途徑地調(diào)控枯落物產(chǎn)生和分解，從而影響枯落物的累積量(圖1)。

1.1 放牧制度的影響

不同放牧制度致使環(huán)境條件改變，枯落物積累與分解對其有極大的響應(yīng)，同時(shí)枯落物中營養(yǎng)元素以不同方式的信息傳遞，維持生態(tài)系統(tǒng)中“土-草-畜”之間的平衡，及草地的可持續(xù)利用[31-32]。與輪牧和不放牧相比，內(nèi)蒙古荒漠草原連續(xù)放牧土壤呼吸減少顯著降低枯落物積累量[33]。澳大利亞時(shí)間控制性放牧系統(tǒng)，枯落物積累隨著時(shí)間的推移因土壤環(huán)境變化而增加[34]。大針茅(Stipagrandis)草原枯落物積累量依次為放牧<割草<圍封；濕潤年份，枯落物分解主要受其質(zhì)量控制，干旱年份則主要受水分供應(yīng)和土地利用方式影響[35]?？茽柷呱车剡B續(xù)放牧導(dǎo)致枯落物量減少，加速了土壤風(fēng)蝕，枯落物的高度、碳含量隨休牧而增加[36]。短花針茅(Stipabreviflora)荒漠草原枯落物有機(jī)碳儲(chǔ)量的年際變化圍欄禁牧>劃區(qū)輪牧>自由放牧，其可能是自由放牧減少了植物碳庫向生態(tài)系統(tǒng)碳庫的碳輸入[37]。在河北溫帶草原牧草不同生育期研究得出：在高度、延遲和適度放牧制度下，土壤氮重新分配成了植物氮或枯落物氮，導(dǎo)致植物地上部分的氮含量顯著提高，土壤氮損失增加[38]。退化的山地草原枯落物分解速率隨封育時(shí)間而降低，但顯著高于自由放牧地，是由于枯落物最大持水率的變化所致[39-40]。放牧制度改變羊草(Leymuschinensis)草原植被蓋度、土壤含水量和土壤微生物量，顯著影響枯落物的分解速率[41]。南非Namakaroo(那抹卡魯)薩王納，高強(qiáng)度、低頻率的非選擇性輪牧枯落物量顯著高于圍欄禁牧[42]；加拿大Nova Scotia(新斯科舍)輪牧系統(tǒng)，枯落物量依次為集約化放牧<半集約化放牧<粗放輪牧[43]，都是由于家畜踐踏和排泄物提高生物量周轉(zhuǎn)速率、改變土壤理化性質(zhì)所致?？傊?，不同放牧制度改變植被、土壤微環(huán)境，調(diào)控枯落物積累與分解。

1.2 放牧率的影響

適宜的放牧率調(diào)控地上枯落物的物種豐富度和積累量、分解與養(yǎng)分歸還，提高牧草品質(zhì)、維持草地健康[44-45]。過度放牧導(dǎo)致生物多樣性喪失，降低枯落物的產(chǎn)生[46]。隨放牧強(qiáng)度的增加，家畜采食與土壤微氣候的協(xié)同改變，使短花針茅荒漠草原枯落物產(chǎn)生量降低，分解速率提高[47]。放牧促進(jìn)阿根廷Patagonia(巴塔哥尼亞)草灌叢草原枯落物碳、氮的釋放，因而輕度放牧和圍欄封育草地枯落物碳含量高于過度放牧草地[48]。放牧改變枯落物組分，高濃度抑制因子[氨氧化細(xì)菌(ammonia-oxidizing bacteria, AOB)、氨氧化古生菌(ammonia-oxidizing archaea, AOA)]對干旱土壤硝化作用產(chǎn)生負(fù)面的影響；輕度放牧下，枯落物質(zhì)量高(C/N小、多酚含量低)、土壤水分好，促進(jìn)硝化作用[8]。內(nèi)蒙古典型草原枯落物積累受放牧強(qiáng)度的顯著影響，其從不放牧到高強(qiáng)度放牧減少了83%[49]；西北高山草甸相比不放牧，重度放牧條件下垂穗鵝觀草(Roegnerianutans)凋落物相對重量、碳、氮損失分別增加了6.8%、6.8%和5.5%，四川嵩草(Kobresiasetchwanensis)凋落物分別增加了12.2%、12.5%和13.4%[50]，原因是家畜踐踏促進(jìn)枯落物與土壤接觸、排泄物增強(qiáng)微生物活性，加速其分解速率?？梢?，放牧率直接或間接影響植物的補(bǔ)償生長、營養(yǎng)物質(zhì)周轉(zhuǎn)，土壤生物活動(dòng)，對放牧生態(tài)系統(tǒng)枯落物有著多方面的響應(yīng)。

1.3 放牧季節(jié)的影響

放牧季節(jié)不同，枯落物積累、分解、養(yǎng)分循環(huán)及產(chǎn)生的生態(tài)效應(yīng)不同，以不同方式調(diào)控草地生態(tài)系統(tǒng)物質(zhì)與能量的平衡，以及結(jié)構(gòu)功能的穩(wěn)定[51-52]。冬季放牧，家畜采食大量地上枯落物，為后來植物生長提供了良好的環(huán)境，有利于植物再生和幼苗的建植，增加植物的相對增長速率，使冷季放牧草地枯落物積累高于暖季放牧草地[53]。美國Oklahoma(俄克拉荷馬州)高草草原氣候變暖增加C4和減少C3植物枯落物生產(chǎn)；減少枯落物氮損失，調(diào)節(jié)生態(tài)系統(tǒng)碳氮循環(huán)[54]。黃土高原典型草原，家畜在不同季節(jié)放牧行為差異較大，致使同一放牧率下暖季放牧地的枯落物及其養(yǎng)分分解速率高于冷季放牧地[55]?？梢?，放牧季節(jié)改變植被所需的物質(zhì)能源，刺激植物生長及養(yǎng)分含量平衡，多方面影響草地生態(tài)系統(tǒng)枯落物的積累與分解。

1.4 放牧家畜種類的影響

不同放牧家畜食性不同，引起群落植被組成的改變，進(jìn)而導(dǎo)致枯落物的物種組成、品質(zhì)和數(shù)量的改變。同時(shí)，放牧家畜喜食營養(yǎng)價(jià)值高的植物，導(dǎo)致植物群落產(chǎn)生大量難分解(營養(yǎng)含量較低)的枯落物，降低了養(yǎng)分(C、N 等)循環(huán)速率[56-57]。青藏高原高寒草甸由于藏羊和牦牛蹄壓的不同，造成枯落物破碎的差異，導(dǎo)致相同放牧強(qiáng)度下牦牛放牧地枯落物損失率高于藏羊放牧地[58]。歐洲Norway(挪威)苔原馴鹿踐踏改變枯落物分解進(jìn)程，影響營養(yǎng)物質(zhì)循環(huán)，同時(shí)分解微生物有其適應(yīng)的枯落物類型：輕度放牧區(qū)灌木枯落物分解較快，而禾本科是在重度放牧區(qū)[25]。歐洲Spitsbergen(斯匹次卑爾根島)草地鵝踐踏沒有顯著改變枯落物氮釋放，但通過抑制其積累間接影響氮釋放率[59]。灌叢草地，山羊?qū)嗄局脖坏南彩承灾笖?shù)(33%)極顯著高于草本植物,而對雜類草和禾草僅為4%和9%，增加草本植物枯落物積累[60]?？傊煌倚筮x擇性采食，枯落物的物理破碎、化學(xué)物質(zhì)循環(huán)等調(diào)控著草地枯落物的返還。

2 枯落物對草地的作用

枯落物參與草地群落構(gòu)建和演替、物種相互作用、物質(zhì)和能量流動(dòng)等基本過程，維持草地生態(tài)系統(tǒng)結(jié)構(gòu)和功能穩(wěn)定[61-62]。枯落物對草地生態(tài)系統(tǒng)的作用主要有以下方面：1)枯落物分解釋放營養(yǎng)元素，促進(jìn)土壤養(yǎng)分循環(huán)[63]；2)覆蓋地表，不影響下滲，減少蒸發(fā)，保持土壤水分[64]；3)隔離種子與土壤，不利于植被的建植[65]；4)春季保溫，夏季遮陰，促進(jìn)種子萌發(fā)，幼苗生長，有利于越冬[66]；5)產(chǎn)生化感物質(zhì)，影響植物生長，有利于植被間的競爭[67]；6)保護(hù)植物、土壤動(dòng)物和土壤結(jié)構(gòu)，減輕家畜踐踏的損害[68]；7)為土壤微生物活動(dòng)提供場所和能量[69]。總之，枯落物可以多方面影響草地生態(tài)系統(tǒng)的結(jié)構(gòu)和功能。

2.1 對種子萌發(fā)的作用

枯落物調(diào)控土壤溫度、水分、通透性、透光率、表土鹽分以及物理屏障等(圖2)，不僅影響其分解進(jìn)程，也改變植物種子萌發(fā)和幼苗生長，進(jìn)而影響群落演替[70-71]。貝加爾針茅(Stipabaicalensis)草原表層土壤容重、表面硬度隨枯落物的增加而減小，且物理性質(zhì)與枯落物量有顯著的互作效應(yīng)[72]。松嫩平原低于600 g/m2的枯落物覆蓋能夠促進(jìn)植物幼苗的建植，過多則延遲出苗時(shí)間和降低出苗速率，其可能原因是枯落物積累會(huì)導(dǎo)致土壤溫度和表土鹽分降低，土壤水分增加[73]。而綜合陸地生態(tài)系統(tǒng)，地表枯落物積累量少于500 g/m2時(shí)能夠促進(jìn)植物建植，超過就會(huì)產(chǎn)生抑制作用，可能是由于透光率減弱所致[74]。圍封天然羊草草地：種子在土壤表面較難萌發(fā)，適當(dāng)?shù)姆拍翑_動(dòng)有助于將種子撒播于土層內(nèi)，同時(shí)枯落物提供物理屏障，促進(jìn)種子萌發(fā)和幼苗生長[75]。因而，“種子-枯落物”的位置關(guān)系影響著種子的出苗，其中環(huán)境變化(水分)起著關(guān)鍵性作用，因?yàn)榭萋湮锬軌蚓彌_環(huán)境突變帶來的不利影響[76]。

圖2 草地枯落物對植物的影響Fig.2 Effects of litter on plant in grassland

2.2 對物質(zhì)循環(huán)的作用

草地枯落物的養(yǎng)分含量對其分解起著主要的驅(qū)動(dòng)作用[77]，它的分解過程對枯落物物種豐富度和生物多樣性比較敏感[78]。內(nèi)蒙古典型草原枯落物輸入增多(年均的120%)顯著增加土壤無機(jī)氮和植物速效磷含量，調(diào)控土壤-植物間的養(yǎng)分平衡[79]?？萋湮锓纸馐峭寥烙袡C(jī)物形成、有機(jī)養(yǎng)分礦化和生態(tài)系統(tǒng)元素平衡較為關(guān)鍵的環(huán)節(jié)，枯落物分解速率與其N、P、K等含量正相關(guān)，與C/N、C/P、木質(zhì)素/纖維素負(fù)相關(guān)[80]，其中木質(zhì)素取決于主要的生物或非生物因素控制碳周轉(zhuǎn)，影響枯落物分解，維持生態(tài)系統(tǒng)的碳平衡[81]。阿根廷灌叢放牧系統(tǒng)，枯落物所含次生化合物(木質(zhì)素和可溶性酚類物質(zhì))會(huì)降低其分解微生物酶活性，從而減緩其氮釋放到土壤中的速率[27]。阿根廷Patagonian Monte(巴塔哥尼亞)半干旱生態(tài)系統(tǒng)植被蓋度降低，減少枯落物中氮、可溶性酚類物質(zhì)和木質(zhì)素向土壤輸入，引起植被結(jié)構(gòu)變化[82]。青藏高原草甸草地，枯落物增加土壤碳礦化和微生物碳，且放牧對枯落物 C釋放影響有抑制作用，其中 N、P 元素為釋放或富集-釋放模式，但在沼澤草地，N、P 元素則為釋放-富集-釋放模式[83-84]。阿根廷Patagonian Monte干旱草原放牧增加枯落物的頑拗性，引起冠層結(jié)構(gòu)變化，降低枯落物分解及土壤氮素水平穩(wěn)定，影響草地抵抗力和恢復(fù)力[6]。內(nèi)蒙古半干旱草原高強(qiáng)度放牧引起枯落物輸入減少，導(dǎo)致土壤有機(jī)碳(soil organic carbon, SOC)和顆粒有機(jī)物(particulate organic matter, POM)含量降低[85]。

2.3 化感作用

化感作用指植物(含微生物)通過釋放化學(xué)物質(zhì)，直接或間接地對其他植物(含微生物)產(chǎn)生作用[86-87]，這些化學(xué)物質(zhì)主要來源于植物的次生代謝。植物-枯落物-土壤之間通過化學(xué)物質(zhì)相互作用(圖2)[88-89]?？萋湮锓纸馓峁┑臓I養(yǎng)物質(zhì)，是放牧生態(tài)系統(tǒng)化學(xué)物質(zhì)的重要來源[90-91]，其組成和多樣性影響有機(jī)質(zhì)分解，維持草地生態(tài)系統(tǒng)生產(chǎn)力[20]?？萋湮锏幕凶饔脻舛刃?yīng)明顯，其作用效果隨化感物質(zhì)濃度而增強(qiáng)，且具有低濃度促進(jìn)、高濃度抑制的現(xiàn)象[92]。對同一種植物而言，化感作用存在器官差異性，可能是由于受體植物的不同器官對同一種化感物質(zhì)的敏感性不同，不同器官化感物質(zhì)含量不同。一般表現(xiàn)為，對根長的化感效應(yīng)強(qiáng)于苗高，這可能是胚根最先接觸到化感物質(zhì)抑制了細(xì)胞分裂和伸長，根長先受到抑制，從而影響了苗的生長的結(jié)果[93]。植物幼苗存活率、莖葉比隨枯落物數(shù)量的增加而增加，且受枯落物和植物物種差異的顯著影響[73]。羊草草地枯落物對柴胡(Bupleurumchinense)、披堿草(Elymusdahuricus)、防風(fēng)(Saposhnikoviadivaricata)和冰草(Agropyroncristatum)種苗株高、葉片數(shù)及地上生物量均隨著枯落物的增加而增大，且在200 g/m2處理株高達(dá)到最大值[75]。青藏高原高寒草地，黃帚橐吾(Ligulariavirgaurea)化感作用在其生存競爭、種群擴(kuò)大與入侵中起著重要作用，導(dǎo)致植物群落組成和結(jié)構(gòu)變化，進(jìn)而造成單優(yōu)勢種群落的形成和草地退化[94]。加拿大稀樹草原植物幼苗枯落物能夠顯著地影響植物從抑制作用到促進(jìn)作用之間的轉(zhuǎn)變，但根系枯落物競爭產(chǎn)生的此作用比幼苗響應(yīng)更顯著[95]。雜草花序比莖或根的提取物具有更大的抑制作用，同時(shí)馬先蒿(Pediculariskansuensis)的化感潛力將導(dǎo)致在牧草種群內(nèi)生真菌感染的植物的頻率增加[96]。

2.4 對微生物的作用

土壤微生物的活動(dòng)對改變枯落物多樣性的分解起著基礎(chǔ)性作用[97]。生態(tài)系統(tǒng)中枯落物分解很大程度上取決于微生物，通過分解非生命的有機(jī)物質(zhì)，將其轉(zhuǎn)化為更簡單的形式，獲得能量和物質(zhì)來構(gòu)建和維護(hù)自身的生命活動(dòng)及其生態(tài)系統(tǒng)的穩(wěn)定[98-99]。另外，細(xì)菌和真菌驅(qū)動(dòng)枯落物分解促進(jìn)陸地生態(tài)系統(tǒng)碳循環(huán)[100]。地下枯落物數(shù)量和質(zhì)量增加豐富了土壤微生物碳、氮含量，改變了真菌和細(xì)菌的數(shù)量，導(dǎo)致土壤微生物群落結(jié)構(gòu)的變化，影響著半干旱草原的生產(chǎn)力[101]。在高寒環(huán)境下，隨著溫度的上升，微生物呼吸作用提高，再加上牲畜踐踏和糞便的作用，進(jìn)一步促進(jìn)了微生物數(shù)量和活性的增加，促進(jìn)了枯落物的分解，加速了生態(tài)系統(tǒng)的養(yǎng)分循環(huán)[102]。

3 枯落物對家畜和小草食動(dòng)物的作用

凋落物的質(zhì)量(ADF、N)潛在影響牧草生產(chǎn)和家畜選擇性采食，同時(shí)為冬季提供高品質(zhì)飼草創(chuàng)造了有利條件。降低了冬季飼養(yǎng)成本，保持草地利用的可持續(xù)性[103]。小草食動(dòng)物也可能會(huì)通過改變枯落物的化學(xué)性質(zhì)(增加氮、磷濃度，降低木質(zhì)素/N和C/N的比率)增加養(yǎng)分循環(huán)速率，從而影響生態(tài)系統(tǒng)的功能[104]。

3.1 對家畜的作用

內(nèi)蒙古典型草原枯落物為綿羊提供的硫元素極少(小于4%)[105]。短花針茅荒漠草原進(jìn)入枯草期,可供綿羊采食利用的枯落物氮不足，枯落物自身養(yǎng)分失衡,使得綿羊體內(nèi)氮含量下降。在其間補(bǔ)飼,將減少綿羊枯草期的氮損失[106]。美國Montana(蒙大拿州)草地，乳漿大戟(Euphorbiaesula)枯落物會(huì)對綿羊瘤胃微生物的活性和數(shù)量產(chǎn)生不利影響[107]。羊草草地枯落物的保護(hù)性屏障效應(yīng)影響牛的選擇性采食，在輕度放牧的條件下，其作用是有限的；但高放牧強(qiáng)度下，大量枯落物影響牛對草地的利用率[108]。羊草草甸草原不同放牧強(qiáng)度，枯落物量不同，導(dǎo)致家畜的采食速率間的差異[109]。草地退化，牧草不足，凋落物積累量減少，降低覓食效率，家畜行走步數(shù)和時(shí)間增加，增強(qiáng)踐踏;草地健康，枯落物緩沖了放牧家畜的沖擊，對于家畜畜蹄起到保護(hù)作用，降低踐踏強(qiáng)度[23]。

3.2 對小草食動(dòng)物的作用

法國南部斯太普草原：輪牧能夠維持植被結(jié)構(gòu)時(shí)間和空間異質(zhì)性，有利于枯落物的積累，相比不放牧對直翅目昆蟲產(chǎn)生的負(fù)面影響，其對直翅目昆蟲是有益的[110]。青藏高原高寒草地門源草原毛蟲體型大小與植物枯落物量呈負(fù)相關(guān)[111]。滇西北藏區(qū)草地，凋落物的遮陰幫助蚯蚓免受紫外線的傷害。同時(shí)，凋落物量的積累對蚯蚓食物的豐富度具有重要的影響[112]。地表甲蟲個(gè)體數(shù)量、物種豐富度、Shannon-Wiener多樣性指數(shù)與枯落物厚度、蓋度呈顯著正相關(guān)；均勻度指數(shù)則與枯落物厚度呈負(fù)相關(guān)?？萋湮锸堑乇砑紫x群落組成的關(guān)鍵影響因素，同時(shí)為地表甲蟲棲息、產(chǎn)卵與繁殖、藏身等生命活動(dòng)提供了有利場所[113-114]，幫助食葉甲蟲越冬，避免暴雨侵襲，降低被捕食風(fēng)險(xiǎn)[115]。另外，嚙齒動(dòng)物群落指數(shù)與凋落物積累密切相關(guān)[116]。

圖3 枯落物對放牧生態(tài)系統(tǒng)的響應(yīng)機(jī)制Fig.3 Response mechanism of litter to grazing ecosystem

4 展望

在放牧生態(tài)系統(tǒng)中，草地枯落物顯著影響植物群落的構(gòu)建和功能，是“放牧-枯落物-土壤-綠色植物-健康家畜”綜合體不可或缺的組成部分(圖3)。研究草地枯落物的生態(tài)功能和機(jī)制將是放牧生態(tài)學(xué)領(lǐng)域經(jīng)久不衰的話題，未來可在以下幾個(gè)方面著重開展工作：1)草地枯落物的積累量決定著草地的發(fā)展，而放牧是調(diào)控草地枯落物的重要手段，通過控制性放牧試驗(yàn)，探討和明確草地枯落物的生態(tài)閾值及其相應(yīng)的最優(yōu)放牧模式。2)枯落物是生態(tài)系統(tǒng)碳、氮儲(chǔ)量的重要載體。草地枯落物多樣性中礦質(zhì)元素的有效性對草地的響應(yīng)有著不可忽視的作用，它與土壤間的養(yǎng)分循環(huán)利用，對于建立植被-土壤間的穩(wěn)定動(dòng)態(tài)不可或缺。3)在農(nóng)田和森林生態(tài)系統(tǒng)中，人們對枯落物的化感作用研究方興未艾，但放牧生態(tài)系統(tǒng)中，缺乏對草地枯落物化感作用的系統(tǒng)性研究，其中機(jī)制尚不清楚。

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Litter and its functions in grazing ecosystems

MA Zhou-Wen1, WANG Ying-Xin1, WANG Hong2, A Bu-Man3, ZHANG Zhen-Ming3, HOU Fu-Jiang1*

1.StateKeyLaboratoryofGrasslandAgro-ecosystems,CollegeofPastoralAgricultureScienceandTechnology,LanzhouUniversity,Lanzhou730020,China； 2.AnimalHusbandryScienceandTechnologyDemonstrationParkofMaquCounty,Maqu747300,China； 3.ChiefStationofGansuGrasslandTechnologyExtension,Lanzhou730000,China

Plant litter is the main medium at the soil-grass interface in grassland ecosystems, and is one of the key factors regulating above-ground ecological processes. Plant litter dynamics are important for grassland species diversity and productivity, as well as for the restoration of degraded grasslands. Livestock affect litter dynamics through feeding, trampling, and their excretions, which can reduce litter accumulation and accelerate its decomposition. These activities vary depending on the stocking rate, grazing season, grazing system, and animal species. Litter affects the selective feeding and hoof pressure of livestock. Plant litter can provide favorable sites for microorganisms and small herbivores, thus affecting the structure and function of the grassland ecosystem. Litter dynamics can also change the physical and chemical properties of soil and affect nutrient cycling. Soil nutrient status and allelochemicals in plant litter can affect seed germination and seedling growth, which contribute to grassland community structure and succession. In this paper, we review the literature related to litter and its role in grazed-grassland ecosystems to understand the interactions among grazing animals, litter, soil, and plants in these environments. Our long-term aim is to provide a theoretical basis for the sustainable management of grassland ecosystems.

grazing； ecological system； plant litter； function； community succession

10.11686/cyxb2016342

2016-09-07；改回日期：2016-11-28

國家自然科學(xué)基金(31672472),甘肅省退牧還草科技支撐(2015620111000485)和國家科技支撐計(jì)劃(2012BAD13B05)資助。

馬周文(1991-),男,甘肅隴西人,在讀碩士。E-mail:mazhw2015@lzu.edu.cn

*通信作者Corresponding author. E-mail: cyhoufj@lzu.edu.cn

http://cyxb.lzu.edu.cn

馬周文, 王迎新, 王宏, 阿不滿, 張貞明, 侯扶江. 放牧生態(tài)系統(tǒng)枯落物及其作用. 草業(yè)學(xué)報(bào), 2017, 26(7): 201-212.

MA Zhou-Wen, WANG Ying-Xin, WANG Hong, A Bu-Man, ZHANG Zhen-Ming, HOU Fu-Jiang. Litter and its functions in grazing ecosystems. Acta Prataculturae Sinica, 2017, 26(7): 201-212.