放牧生態系統枯落物及其作用

馬周文，王迎新，王宏，阿不滿，張貞明，侯扶江*

(1.草地農業生態系統國家重點實驗室，蘭州大學草地農業科技學院，甘肅 蘭州 730020；2.瑪曲縣阿孜畜牧科技示范園區，甘肅 瑪曲 747300；3.甘肅省草原技術推廣總站，甘肅 蘭州 730000)

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放牧生態系統枯落物及其作用

馬周文1，王迎新1，王宏2，阿不滿3，張貞明3，侯扶江1*

(1.草地農業生態系統國家重點實驗室，蘭州大學草地農業科技學院，甘肅 蘭州 730020；2.瑪曲縣阿孜畜牧科技示范園區，甘肅 瑪曲 747300；3.甘肅省草原技術推廣總站，甘肅 蘭州 730000)

草地枯落物是連接放牧生態系統中“土—草”界面的主要媒介，是調控地上-地下生態過程的關鍵因子，對草地物種多樣性、生產力、退化草地恢復等具有重要意義。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，且與放牧強度、制度、季節、家畜種類聯合響應；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物生命活動提供有利場所及能量，進而影響草地生態系統結構及功能。枯落物也能夠改變土壤理化性質和物質循環，產生化感物質，影響種子發芽、幼苗生長，導致草地群落構建和演替。本研究綜述了國內外相關文獻，探討放牧生態系統中的枯落物及其作用，旨在明確草地生態系統中“放牧-枯落物-土壤-綠色植物”之間的相互作用機制，為后續草地可持續性管理研究提供理論依據。

放牧；生態系統；草地枯落物；功能；群落演替

放牧是草地利用最普遍、最經濟的利用方式[1]。在放牧生態系統中，枯落物是土—草界面過程的主要介質，是調控地上—地下生態過程的關鍵因子[2-3]，其積累與分解是生態系統物質循環的重要過程[4-5]。家畜的采食、踐踏、排泄等行為調控枯落物的積累和分解，且與放牧強度、制度、季節、家畜種類密切相關[6-8]；放牧減少枯落物積累，降低火災風險[9-10]。枯落物的分解是放牧生態系統重要的過程，其能量流動和養分循環影響草地生產力、生物多樣性、退化草地的恢復等[11-12]。枯落物分解改變土壤理化性質和土壤養分循環，產生化感物質，影響種間競爭和群落結構[13-15]。同時，枯落物在草地表面形成“緩沖層”，減緩土-草-畜之間的物理作用強度，防止降雨對表層土壤的沖擊，調節土壤表層微氣候(溫度、含水量、光照等)，直接或間接影響植被生長和地下生態過程[16-17]。

國內外對草地枯落物的研究主要集中在以下3個方面：1)放牧等管理因素、氣候和地形等環境因素[18]、土壤微生物和動物、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)枯落物分解對生態系統營養元素循環的貢獻(C、N、P等)，枯落物分解過程中木質素、C、N、P等化學成分及其相互比例的生物地球化學循環規律[8,20]；3)枯落物對植被動態、放牧家畜行為和生產力、土壤微生物和動物組成與活動等的作用[21-22]。目前，草地枯落物對放牧系統土-草-畜相互作用研究比較少，其中機制尚不清楚。為此，分析國內外關于放牧對枯落物以及枯落物對草地結構與功能作用機制，以期為草地可持續放牧管理提供科學依據。

圖1 草地生態系統中“放牧-枯落物-綠色植物”之間的相互影響機制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，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，補償性生長，品質，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，提高枯落物營養品質，也促進家畜采食[25]。2)適度放牧刺激植物補償性生長，提高枯落物生物量[26]。3)放牧影響群落結構，改變枯落物組成，進而導致家畜對枯落物的采食或者枯落物的分解發生變化[27]。4)家畜踐踏造成牧草機械損傷，加速枯落物的形成[1]，同時破碎的枯落物易隨風飄失。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、溫度等微環境，影響枯落物的分解[30]。可見，放牧可以多途徑地調控枯落物產生和分解，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境條件改變，枯落物積累與分解對其有極大的響應，同時枯落物中營養元素以不同方式的信息傳遞，維持生態系統中“土-草-畜”之間的平衡，及草地的可持續利用[31-32]。與輪牧和不放牧相比，內蒙古荒漠草原連續放牧土壤呼吸減少顯著降低枯落物積累量[33]。澳大利亞時間控制性放牧系統，枯落物積累隨著時間的推移因土壤環境變化而增加[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量控制，干旱年份則主要受水分供應和土地利用方式影響[35]。科爾沁沙地連續放牧導致枯落物量減少，加速了土壤風蝕，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碳儲量的年際變化圍欄禁牧>劃區輪牧>自由放牧，其可能是自由放牧減少了植物碳庫向生態系統碳庫的碳輸入[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，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速率、改變土壤理化性質所致。總之，不同放牧制度改變植被、土壤微環境，調控枯落物積累與分解。

1.2 放牧率的影響

適宜的放牧率調控地上枯落物的物種豐富度和積累量、分解與養分歸還，提高牧草品質、維持草地健康[44-45]。過度放牧導致生物多樣性喪失，降低枯落物的產生[46]。隨放牧強度的增加，家畜采食與土壤微氣候的協同改變，使短花針茅荒漠草原枯落物產生量降低，分解速率提高[47]。放牧促進阿根廷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生負面的影響；輕度放牧下，枯落物質量高(C/N小、多酚含量低)、土壤水分好，促進硝化作用[8]。內蒙古典型草原枯落物積累受放牧強度的顯著影響，其從不放牧到高強度放牧減少了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枯落物與土壤接觸、排泄物增強微生物活性，加速其分解速率。可見，放牧率直接或間接影響植物的補償生長、營養物質周轉，土壤生物活動，對放牧生態系統枯落物有著多方面的響應。

1.3 放牧季節的影響

放牧季節不同，枯落物積累、分解、養分循環及產生的生態效應不同，以不同方式調控草地生態系統物質與能量的平衡，以及結構功能的穩定[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；減少枯落物氮損失，調節生態系統碳氮循環[54]。黃土高原典型草原，家畜在不同季節放牧行為差異較大，致使同一放牧率下暖季放牧地的枯落物及其養分分解速率高于冷季放牧地[55]。可見，放牧季節改變植被所需的物質能源，刺激植物生長及養分含量平衡，多方面影響草地生態系統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，進而導致枯落物的物種組成、品質和數量的改變。同時，放牧家畜喜食營養價值高的植物，導致植物群落產生大量難分解(營養含量較低)的枯落物，降低了養分(C、N 等)循環速率[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，導致相同放牧強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程，影響營養物質循環，同時分解微生物有其適應的枯落物類型：輕度放牧區灌木枯落物分解較快，而禾本科是在重度放牧區[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，山羊對灌木植被的喜食性指數(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]。總之，不同家畜選擇性采食，枯落物的物理破碎、化學物質循環等調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建和演替、物種相互作用、物質和能量流動等基本過程，維持草地生態系統結構和功能穩定[61-62]。枯落物對草地生態系統的作用主要有以下方面：1)枯落物分解釋放營養元素，促進土壤養分循環[63]；2)覆蓋地表，不影響下滲，減少蒸發，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，促進種子萌發，幼苗生長，有利于越冬[66]；5)產生化感物質，影響植物生長，有利于植被間的競爭[67]；6)保護植物、土壤動物和土壤結構，減輕家畜踐踏的損害[68]；7)為土壤微生物活動提供場所和能量[69]。總之，枯落物可以多方面影響草地生態系統的結構和功能。

2.1 對種子萌發的作用

枯落物調控土壤溫度、水分、通透性、透光率、表土鹽分以及物理屏障等(圖2)，不僅影響其分解進程，也改變植物種子萌發和幼苗生長，進而影響群落演替[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，且物理性質與枯落物量有顯著的互作效應[72]。松嫩平原低于600 g/m2的枯落物覆蓋能夠促進植物幼苗的建植，過多則延遲出苗時間和降低出苗速率，其可能原因是枯落物積累會導致土壤溫度和表土鹽分降低，土壤水分增加[73]。而綜合陸地生態系統，地表枯落物積累量少于500 g/m2時能夠促進植物建植，超過就會產生抑制作用，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，適當的放牧擾動有助于將種子撒播于土層內，同時枯落物提供物理屏障，促進種子萌發和幼苗生長[75]。因而，“種子-枯落物”的位置關系影響著種子的出苗，其中環境變化(水分)起著關鍵性作用，因為枯落物能夠緩沖環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 對物質循環的作用

草地枯落物的養分含量對其分解起著主要的驅動作用[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%)顯著增加土壤無機氮和植物速效磷含量，調控土壤-植物間的養分平衡[79]。枯落物分解是土壤有機物形成、有機養分礦化和生態系統元素平衡較為關鍵的環節，枯落物分解速率與其N、P、K等含量正相關，與C/N、C/P、木質素/纖維素負相關[80]，其中木質素取決于主要的生物或非生物因素控制碳周轉，影響枯落物分解，維持生態系統的碳平衡[81]。阿根廷灌叢放牧系統，枯落物所含次生化合物(木質素和可溶性酚類物質)會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)半干旱生態系統植被蓋度降低，減少枯落物中氮、可溶性酚類物質和木質素向土壤輸入，引起植被結構變化[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，引起冠層結構變化，降低枯落物分解及土壤氮素水平穩定，影響草地抵抗力和恢復力[6]。內蒙古半干旱草原高強度放牧引起枯落物輸入減少，導致土壤有機碳(soil organic carbon, SOC)和顆粒有機物(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)通過釋放化學物質，直接或間接地對其他植物(含微生物)產生作用[86-87]，這些化學物質主要來源于植物的次生代謝。植物-枯落物-土壤之間通過化學物質相互作用(圖2)[88-89]。枯落物分解提供的營養物質，是放牧生態系統化學物質的重要來源[90-91]，其組成和多樣性影響有機質分解，維持草地生態系統生產力[20]。枯落物的化感作用濃度效應明顯，其作用效果隨化感物質濃度而增強，且具有低濃度促進、高濃度抑制的現象[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的敏感性不同，不同器官化感物質含量不同。一般表現為，對根長的化感效應強于苗高，這可能是胚根最先接觸到化感物質抑制了細胞分裂和伸長，根長先受到抑制，從而影響了苗的生長的結果[93]。植物幼苗存活率、莖葉比隨枯落物數量的增加而增加，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(Saposhnikoviadivaricata)和冰草(Agropyroncristatum)種苗株高、葉片數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、種群擴大與入侵中起著重要作用，導致植物群落組成和結構變化，進而造成單優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作用之間的轉變，但根系枯落物競爭產生的此作用比幼苗響應更顯著[95]。雜草花序比莖或根的提取物具有更大的抑制作用，同時馬先蒿(Pediculariskansuensis)的化感潛力將導致在牧草種群內生真菌感染的植物的頻率增加[96]。

2.4 對微生物的作用

土壤微生物的活動對改變枯落物多樣性的分解起著基礎性作用[97]。生態系統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物質，將其轉化為更簡單的形式，獲得能量和物質來構建和維護自身的生命活動及其生態系統的穩定[98-99]。另外，細菌和真菌驅動枯落物分解促進陸地生態系統碳循環[100]。地下枯落物數量和質量增加豐富了土壤微生物碳、氮含量，改變了真菌和細菌的數量，導致土壤微生物群落結構的變化，影響著半干旱草原的生產力[101]。在高寒環境下，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，進一步促進了微生物數量和活性的增加，促進了枯落物的分解，加速了生態系統的養分循環[102]。

3 枯落物對家畜和小草食動物的作用

凋落物的質量(ADF、N)潛在影響牧草生產和家畜選擇性采食，同時為冬季提供高品質飼草創造了有利條件。降低了冬季飼養成本，保持草地利用的可持續性[103]。小草食動物也可能會通過改變枯落物的化學性質(增加氮、磷濃度，降低木質素/N和C/N的比率)增加養分循環速率，從而影響生態系統的功能[104]。

3.1 對家畜的作用

內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，枯落物自身養分失衡,使得綿羊體內氮含量下降。在其間補飼,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)枯落物會對綿羊瘤胃微生物的活性和數量產生不利影響[107]。羊草草地枯落物的保護性屏障效應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度，枯落物量不同，導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，家畜行走步數和時間增加，增強踐踏;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作用，降低踐踏強度[23]。

3.2 對小草食動物的作用

法國南部斯太普草原：輪牧能夠維持植被結構時間和空間異質性，有利于枯落物的積累，相比不放牧對直翅目昆蟲產生的負面影響，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相關[111]。滇西北藏區草地，凋落物的遮陰幫助蚯蚓免受紫外線的傷害。同時，凋落物量的積累對蚯蚓食物的豐富度具有重要的影響[112]。地表甲蟲個體數量、物種豐富度、Shannon-Wiener多樣性指數與枯落物厚度、蓋度呈顯著正相關；均勻度指數則與枯落物厚度呈負相關。枯落物是地表甲蟲群落組成的關鍵影響因素，同時為地表甲蟲棲息、產卵與繁殖、藏身等生命活動提供了有利場所[113-114]，幫助食葉甲蟲越冬，避免暴雨侵襲，降低被捕食風險[115]。另外，嚙齒動物群落指數與凋落物積累密切相關[116]。

圖3 枯落物對放牧生態系統的響應機制Fig.3 Response mechanism of litter to grazing ecosystem

4 展望

在放牧生態系統中，草地枯落物顯著影響植物群落的構建和功能，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功能和機制將是放牧生態學領域經久不衰的話題，未來可在以下幾個方面著重開展工作：1)草地枯落物的積累量決定著草地的發展，而放牧是調控草地枯落物的重要手段，通過控制性放牧試驗，探討和明確草地枯落物的生態閾值及其相應的最優放牧模式。2)枯落物是生態系統碳、氮儲量的重要載體。草地枯落物多樣性中礦質元素的有效性對草地的響應有著不可忽視的作用，它與土壤間的養分循環利用，對于建立植被-土壤間的穩定動態不可或缺。3)在農田和森林生態系統中，人們對枯落物的化感作用研究方興未艾，但放牧生態系統中，缺乏對草地枯落物化感作用的系統性研究，其中機制尚不清楚。

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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

國家自然科學基金(31672472),甘肅省退牧還草科技支撐(2015620111000485)和國家科技支撐計劃(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. 放牧生態系統枯落物及其作用. 草業學報, 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.