引用本文
马周文, 王迎新, 王宏, 阿不满, 张贞明, 侯扶江. 放牧生态系统枯落物及其作用.草业学报, 2015,26(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 PRATACULTUAE SINICA, 2015,26(7): 201-212  
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放牧生态系统枯落物及其作用
马周文1, 王迎新1, 王宏2, 阿不满3, 张贞明3, 侯扶江1,*
1.草地农业生态系统国家重点实验室,兰州大学草地农业科技学院,甘肃 兰州 730020
2.玛曲县阿孜畜牧科技示范园区,甘肃 玛曲 747300
3.甘肃省草原技术推广总站,甘肃 兰州 730000
*通信作者Corresponding author. E-mail:cyhoufj@lzu.edu.cn

作者简介:马周文(1991-),男,甘肃陇西人,在读硕士。E-mail:mazhw2015@lzu.edu.cn

收稿日期: 2016-09-07
基金: 国家自然科学基金(31672472),甘肃省退牧还草科技支撑(2015620111000485)和国家科技支撑计划(2012BAD13B05)资助
摘要

草地枯落物是连接放牧生态系统中“土—草”界面的主要媒介,是调控地上-地下生态过程的关键因子,对草地物种多样性、生产力、退化草地恢复等具有重要意义。一方面,家畜的采食、践踏、排泄物等减少枯落物积累和加速其分解,且与放牧强度、制度、季节、家畜种类联合响应;另一方面,枯落物影响家畜的选择性采食、蹄压等行为,为微生物和小草食动物生命活动提供有利场所及能量,进而影响草地生态系统结构及功能。枯落物也能够改变土壤理化性质和物质循环,产生化感物质,影响种子发芽、幼苗生长,导致草地群落构建和演替。本研究综述了国内外相关文献,探讨放牧生态系统中的枯落物及其作用,旨在明确草地生态系统中“放牧-枯落物-土壤-绿色植物”之间的相互作用机制,为后续草地可持续性管理研究提供理论依据。

关键词: 放牧; 生态系统; 草地枯落物; 功能; 群落演替
doi: 10.11686/cyxb2016342
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.State Key Laboratory of Grassland Agro-ecosystems, College of Pastoral Agriculture Science and Technology, Lanzhou University, Lanzhou 730020, China
2.Animal Husbandry Science and Technology Demonstration Park of Maqu County, Maqu 747300, China
3.Chief Station of Gansu Grassland Technology Extension, Lanzhou 730000, China
Abstract

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.

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

放牧是草地利用最普遍、最经济的利用方式[1]。在放牧生态系统中, 枯落物是土— 草界面过程的主要介质, 是调控地上— 地下生态过程的关键因子[2, 3], 其积累与分解是生态系统物质循环的重要过程[4, 5]。家畜的采食、践踏、排泄等行为调控枯落物的积累和分解, 且与放牧强度、制度、季节、家畜种类密切相关[6, 7, 8]; 放牧减少枯落物积累, 降低火灾风险[9, 10]。枯落物的分解是放牧生态系统重要的过程, 其能量流动和养分循环影响草地生产力、生物多样性、退化草地的恢复等[11, 12]。枯落物分解改变土壤理化性质和土壤养分循环, 产生化感物质, 影响种间竞争和群落结构[13, 14, 15]。同时, 枯落物在草地表面形成“ 缓冲层” , 减缓土-草-畜之间的物理作用强度, 防止降雨对表层土壤的冲击, 调节土壤表层微气候(温度、含水量、光照等), 直接或间接影响植被生长和地下生态过程[16, 17]。

国内外对草地枯落物的研究主要集中在以下3个方面:1)放牧等管理因素、气候和地形等环境因素[18]、土壤微生物和动物、植物生物多样性等生物因素[19]对枯落物形成与分解的影响; 2)枯落物分解对生态系统营养元素循环的贡献(C、N、P等), 枯落物分解过程中木质素、C、N、P等化学成分及其相互比例的生物地球化学循环规律[8, 20]; 3)枯落物对植被动态、放牧家畜行为和生产力、土壤微生物和动物组成与活动等的作用[21, 22]。目前, 草地枯落物对放牧系统土-草-畜相互作用研究比较少, 其中机制尚不清楚。为此, 分析国内外关于放牧对枯落物以及枯落物对草地结构与功能作用机制, 以期为草地可持续放牧管理提供科学依据。

1 放牧对枯落物的影响

枯落物是对放牧最敏感的指标之一[23, 24]。家畜对枯落物的作用主要有以下方面:1)家畜直接采食枯落物, 在冬春季牧草供给不足时较为常见, 牧草生长旺期较少采食[23]; 由于放牧增加枯落物氮含量、降低C/N, 提高枯落物营养品质, 也促进家畜采食[25]。2)适度放牧刺激植物补偿性生长, 提高枯落物生物量[26]。3)放牧影响群落结构, 改变枯落物组成, 进而导致家畜对枯落物的采食或者枯落物的分解发生变化[27]。4)家畜践踏造成牧草机械损伤, 加速枯落物的形成[1], 同时破碎的枯落物易随风飘失。5)家畜践踏破碎和浅埋枯落物, 加速枯落物的分解[28]。6)家畜排泄物含有大量氮素, 增强土壤微生物的活性, 加速其对枯落物的分解[29]。7)放牧改变了草地的辐射、温度等微环境, 影响枯落物的分解[30]。可见, 放牧可以多途径地调控枯落物产生和分解, 从而影响枯落物的累积量(图1)。

图1 草地生态系统中“ 放牧-枯落物-绿色植物” 之间的相互影响机制
①放牧率, 放牧制度, 放牧季节, 家畜种类等Stocking rate, grazing system, grazing season, livestock species, etc.; ②枯落物积累量, 密度, 高度等Litter accumulation, density, height, etc.; ③物理作用, 化学作用(分解、化感), 生物作用等Physical function, chemical function (decomposition, chemical), biological function, etc.; ④采食, 践踏, 排泄物等Feeding, trampling, excrement, etc.; ⑤生长速率, 补偿性生长, 品质, 物种多样性等Growth rate, compensatory growth, quality, species diversity, etc.; ⑥竞争, 地上(地下)生物量等Competition, above-ground (under-ground) biomass, etc.Fig.1 The interaction mechanism among the “ grazing-litter-green plants” in the grassland ecosystem

1.1 放牧制度的影响

不同放牧制度致使环境条件改变, 枯落物积累与分解对其有极大的响应, 同时枯落物中营养元素以不同方式的信息传递, 维持生态系统中“ 土-草-畜” 之间的平衡, 及草地的可持续利用[31, 32]。与轮牧和不放牧相比, 内蒙古荒漠草原连续放牧土壤呼吸减少显著降低枯落物积累量[33]。澳大利亚时间控制性放牧系统, 枯落物积累随着时间的推移因土壤环境变化而增加[34]。大针茅(Stipa grandis)草原枯落物积累量依次为放牧< 割草< 围封; 湿润年份, 枯落物分解主要受其质量控制, 干旱年份则主要受水分供应和土地利用方式影响[35]。科尔沁沙地连续放牧导致枯落物量减少, 加速了土壤风蚀, 枯落物的高度、碳含量随休牧而增加[36]。短花针茅(Stipa breviflora)荒漠草原枯落物有机碳储量的年际变化围栏禁牧> 划区轮牧> 自由放牧, 其可能是自由放牧减少了植物碳库向生态系统碳库的碳输入[37]。在河北温带草原牧草不同生育期研究得出:在高度、延迟和适度放牧制度下, 土壤氮重新分配成了植物氮或枯落物氮, 导致植物地上部分的氮含量显著提高, 土壤氮损失增加[38]。退化的山地草原枯落物分解速率随封育时间而降低, 但显著高于自由放牧地, 是由于枯落物最大持水率的变化所致[39, 40]。放牧制度改变羊草(Leymus chinensis)草原植被盖度、土壤含水量和土壤微生物量, 显著影响枯落物的分解速率[41]。南非Namakaroo(那抹卡鲁)萨王纳, 高强度、低频率的非选择性轮牧枯落物量显著高于围栏禁牧[42]; 加拿大Nova Scotia(新斯科舍)轮牧系统, 枯落物量依次为集约化放牧< 半集约化放牧< 粗放轮牧[43], 都是由于家畜践踏和排泄物提高生物量周转速率、改变土壤理化性质所致。总之, 不同放牧制度改变植被、土壤微环境, 调控枯落物积累与分解。

1.2 放牧率的影响

适宜的放牧率调控地上枯落物的物种丰富度和积累量、分解与养分归还, 提高牧草品质、维持草地健康[44, 45]。过度放牧导致生物多样性丧失, 降低枯落物的产生[46]。随放牧强度的增加, 家畜采食与土壤微气候的协同改变, 使短花针茅荒漠草原枯落物产生量降低, 分解速率提高[47]。放牧促进阿根廷Patagonia(巴塔哥尼亚)草灌丛草原枯落物碳、氮的释放, 因而轻度放牧和围栏封育草地枯落物碳含量高于过度放牧草地[48]。放牧改变枯落物组分, 高浓度抑制因子[氨氧化细菌(ammonia-oxidizing bacteria, AOB)、氨氧化古生菌(ammonia-oxidizing archaea, AOA)]对干旱土壤硝化作用产生负面的影响; 轻度放牧下, 枯落物质量高(C/N小、多酚含量低)、土壤水分好, 促进硝化作用[8]。内蒙古典型草原枯落物积累受放牧强度的显著影响, 其从不放牧到高强度放牧减少了83%[49]; 西北高山草甸相比不放牧, 重度放牧条件下垂穗鹅观草(Roegneria nutans)凋落物相对重量、碳、氮损失分别增加了6.8%、6.8%和5.5%, 四川嵩草(Kobresia setchwanensis)凋落物分别增加了12.2%、12.5%和13.4%[50], 原因是家畜践踏促进枯落物与土壤接触、排泄物增强微生物活性, 加速其分解速率。可见, 放牧率直接或间接影响植物的补偿生长、营养物质周转, 土壤生物活动, 对放牧生态系统枯落物有着多方面的响应。

1.3 放牧季节的影响

放牧季节不同, 枯落物积累、分解、养分循环及产生的生态效应不同, 以不同方式调控草地生态系统物质与能量的平衡, 以及结构功能的稳定[51, 52]。冬季放牧, 家畜采食大量地上枯落物, 为后来植物生长提供了良好的环境, 有利于植物再生和幼苗的建植, 增加植物的相对增长速率, 使冷季放牧草地枯落物积累高于暖季放牧草地[53]。美国Oklahoma(俄克拉荷马州)高草草原气候变暖增加C4和减少C3植物枯落物生产; 减少枯落物氮损失, 调节生态系统碳氮循环[54]。黄土高原典型草原, 家畜在不同季节放牧行为差异较大, 致使同一放牧率下暖季放牧地的枯落物及其养分分解速率高于冷季放牧地[55]。可见, 放牧季节改变植被所需的物质能源, 刺激植物生长及养分含量平衡, 多方面影响草地生态系统枯落物的积累与分解。

1.4 放牧家畜种类的影响

不同放牧家畜食性不同, 引起群落植被组成的改变, 进而导致枯落物的物种组成、品质和数量的改变。同时, 放牧家畜喜食营养价值高的植物, 导致植物群落产生大量难分解(营养含量较低)的枯落物, 降低了养分(C、N 等)循环速率[56, 57]。青藏高原高寒草甸由于藏羊和牦牛蹄压的不同, 造成枯落物破碎的差异, 导致相同放牧强度下牦牛放牧地枯落物损失率高于藏羊放牧地[58]。欧洲Norway(挪威)苔原驯鹿践踏改变枯落物分解进程, 影响营养物质循环, 同时分解微生物有其适应的枯落物类型:轻度放牧区灌木枯落物分解较快, 而禾本科是在重度放牧区[25]。欧洲Spitsbergen(斯匹次卑尔根岛)草地鹅践踏没有显著改变枯落物氮释放, 但通过抑制其积累间接影响氮释放率[59]。灌丛草地, 山羊对灌木植被的喜食性指数(33%)极显著高于草本植物, 而对杂类草和禾草仅为4%和9%, 增加草本植物枯落物积累[60]。总之, 不同家畜选择性采食, 枯落物的物理破碎、化学物质循环等调控着草地枯落物的返还。

2 枯落物对草地的作用

枯落物参与草地群落构建和演替、物种相互作用、物质和能量流动等基本过程, 维持草地生态系统结构和功能稳定[61, 62]。枯落物对草地生态系统的作用主要有以下方面:1)枯落物分解释放营养元素, 促进土壤养分循环[63]; 2)覆盖地表, 不影响下渗, 减少蒸发, 保持土壤水分[64]; 3)隔离种子与土壤, 不利于植被的建植[65]; 4)春季保温, 夏季遮阴, 促进种子萌发, 幼苗生长, 有利于越冬[66]; 5)产生化感物质, 影响植物生长, 有利于植被间的竞争[67]; 6)保护植物、土壤动物和土壤结构, 减轻家畜践踏的损害[68]; 7)为土壤微生物活动提供场所和能量[69]。总之, 枯落物可以多方面影响草地生态系统的结构和功能。

2.1 对种子萌发的作用

枯落物调控土壤温度、水分、通透性、透光率、表土盐分以及物理屏障等(图2), 不仅影响其分解进程, 也改变植物种子萌发和幼苗生长, 进而影响群落演替[70, 71]。贝加尔针茅(Stipa baicalensis)草原表层土壤容重、表面硬度随枯落物的增加而减小, 且物理性质与枯落物量有显著的互作效应[72]。松嫩平原低于600 g/m2的枯落物覆盖能够促进植物幼苗的建植, 过多则延迟出苗时间和降低出苗速率, 其可能原因是枯落物积累会导致土壤温度和表土盐分降低, 土壤水分增加[73]。而综合陆地生态系统, 地表枯落物积累量少于500 g/m2时能够促进植物建植, 超过就会产生抑制作用, 可能是由于透光率减弱所致[74]。围封天然羊草草地:种子在土壤表面较难萌发, 适当的放牧扰动有助于将种子撒播于土层内, 同时枯落物提供物理屏障, 促进种子萌发和幼苗生长[75]。因而, “ 种子-枯落物” 的位置关系影响着种子的出苗, 其中环境变化(水分)起着关键性作用, 因为枯落物能够缓冲环境突变带来的不利影响[76]。

图2 草地枯落物对植物的影响Fig.2 Effects of litter on plant in grassland

2.2 对物质循环的作用

草地枯落物的养分含量对其分解起着主要的驱动作用[77], 它的分解过程对枯落物物种丰富度和生物多样性比较敏感[78]。内蒙古典型草原枯落物输入增多(年均的120%)显著增加土壤无机氮和植物速效磷含量, 调控土壤-植物间的养分平衡[79]。枯落物分解是土壤有机物形成、有机养分矿化和生态系统元素平衡较为关键的环节, 枯落物分解速率与其N、P、K等含量正相关, 与C/N、C/P、木质素/纤维素负相关[80], 其中木质素取决于主要的生物或非生物因素控制碳周转, 影响枯落物分解, 维持生态系统的碳平衡[81]。阿根廷灌丛放牧系统, 枯落物所含次生化合物(木质素和可溶性酚类物质)会降低其分解微生物酶活性, 从而减缓其氮释放到土壤中的速率[27]。阿根廷Patagonian Monte(巴塔哥尼亚)半干旱生态系统植被盖度降低, 减少枯落物中氮、可溶性酚类物质和木质素向土壤输入, 引起植被结构变化[82]。青藏高原草甸草地, 枯落物增加土壤碳矿化和微生物碳, 且放牧对枯落物 C释放影响有抑制作用, 其中 N、P 元素为释放或富集-释放模式, 但在沼泽草地, N、P 元素则为释放-富集-释放模式[83, 84]。阿根廷Patagonian Monte干旱草原放牧增加枯落物的顽拗性, 引起冠层结构变化, 降低枯落物分解及土壤氮素水平稳定, 影响草地抵抗力和恢复力[6]。内蒙古半干旱草原高强度放牧引起枯落物输入减少, 导致土壤有机碳(soil organic carbon, SOC)和颗粒有机物(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)通过释放化学物质, 直接或间接地对其他植物(含微生物)产生作用[86, 87], 这些化学物质主要来源于植物的次生代谢。植物-枯落物-土壤之间通过化学物质相互作用(图2)[88, 89]。枯落物分解提供的营养物质, 是放牧生态系统化学物质的重要来源[90, 91], 其组成和多样性影响有机质分解, 维持草地生态系统生产力[20]。枯落物的化感作用浓度效应明显, 其作用效果随化感物质浓度而增强, 且具有低浓度促进、高浓度抑制的现象[92]。对同一种植物而言, 化感作用存在器官差异性, 可能是由于受体植物的不同器官对同一种化感物质的敏感性不同, 不同器官化感物质含量不同。一般表现为, 对根长的化感效应强于苗高, 这可能是胚根最先接触到化感物质抑制了细胞分裂和伸长, 根长先受到抑制, 从而影响了苗的生长的结果[93]。植物幼苗存活率、茎叶比随枯落物数量的增加而增加, 且受枯落物和植物物种差异的显著影响[73]。羊草草地枯落物对柴胡(Bupleurum chinense)、披碱草(Elymus dahuricus)、防风(Saposhnikovia divaricata)和冰草(Agropyron cristatum)种苗株高、叶片数及地上生物量均随着枯落物的增加而增大, 且在200 g/m2处理株高达到最大值[75]。青藏高原高寒草地, 黄帚橐吾(Ligularia virgaurea)化感作用在其生存竞争、种群扩大与入侵中起着重要作用, 导致植物群落组成和结构变化, 进而造成单优势种群落的形成和草地退化[94]。加拿大稀树草原植物幼苗枯落物能够显著地影响植物从抑制作用到促进作用之间的转变, 但根系枯落物竞争产生的此作用比幼苗响应更显著[95]。杂草花序比茎或根的提取物具有更大的抑制作用, 同时马先蒿(Pedicularis kansuensis)的化感潜力将导致在牧草种群内生真菌感染的植物的频率增加[96]。

2.4 对微生物的作用

土壤微生物的活动对改变枯落物多样性的分解起着基础性作用[97]。生态系统中枯落物分解很大程度上取决于微生物, 通过分解非生命的有机物质, 将其转化为更简单的形式, 获得能量和物质来构建和维护自身的生命活动及其生态系统的稳定[98, 99]。另外, 细菌和真菌驱动枯落物分解促进陆地生态系统碳循环[100]。地下枯落物数量和质量增加丰富了土壤微生物碳、氮含量, 改变了真菌和细菌的数量, 导致土壤微生物群落结构的变化, 影响着半干旱草原的生产力[101]。在高寒环境下, 随着温度的上升, 微生物呼吸作用提高, 再加上牲畜践踏和粪便的作用, 进一步促进了微生物数量和活性的增加, 促进了枯落物的分解, 加速了生态系统的养分循环[102]。

3 枯落物对家畜和小草食动物的作用

凋落物的质量(ADF、N)潜在影响牧草生产和家畜选择性采食, 同时为冬季提供高品质饲草创造了有利条件。降低了冬季饲养成本, 保持草地利用的可持续性[103]。小草食动物也可能会通过改变枯落物的化学性质(增加氮、磷浓度, 降低木质素/N和C/N的比率)增加养分循环速率, 从而影响生态系统的功能[104]。

3.1 对家畜的作用

内蒙古典型草原枯落物为绵羊提供的硫元素极少(小于4%)[105]。短花针茅荒漠草原进入枯草期, 可供绵羊采食利用的枯落物氮不足, 枯落物自身养分失衡, 使得绵羊体内氮含量下降。在其间补饲, 将减少绵羊枯草期的氮损失[106]。美国Montana(蒙大拿州)草地, 乳浆大戟(Euphorbia esula)枯落物会对绵羊瘤胃微生物的活性和数量产生不利影响[107]。羊草草地枯落物的保护性屏障效应影响牛的选择性采食, 在轻度放牧的条件下, 其作用是有限的; 但高放牧强度下, 大量枯落物影响牛对草地的利用率[108]。羊草草甸草原不同放牧强度, 枯落物量不同, 导致家畜的采食速率间的差异[109]。草地退化, 牧草不足, 凋落物积累量减少, 降低觅食效率, 家畜行走步数和时间增加, 增强践踏; 草地健康, 枯落物缓冲了放牧家畜的冲击, 对于家畜畜蹄起到保护作用, 降低践踏强度[23]。

3.2 对小草食动物的作用

法国南部斯太普草原:轮牧能够维持植被结构时间和空间异质性, 有利于枯落物的积累, 相比不放牧对直翅目昆虫产生的负面影响, 其对直翅目昆虫是有益的[110]。青藏高原高寒草地门源草原毛虫体型大小与植物枯落物量呈负相关[111]。滇西北藏区草地, 凋落物的遮阴帮助蚯蚓免受紫外线的伤害。同时, 凋落物量的积累对蚯蚓食物的丰富度具有重要的影响[112]。地表甲虫个体数量、物种丰富度、Shannon-Wiener多样性指数与枯落物厚度、盖度呈显著正相关; 均匀度指数则与枯落物厚度呈负相关。枯落物是地表甲虫群落组成的关键影响因素, 同时为地表甲虫栖息、产卵与繁殖、藏身等生命活动提供了有利场所[113, 114], 帮助食叶甲虫越冬, 避免暴雨侵袭, 降低被捕食风险[115]。另外, 啮齿动物群落指数与凋落物积累密切相关[116]。

图3 枯落物对放牧生态系统的响应机制Fig.3 Response mechanism of litter to grazing ecosystem

4 展望

在放牧生态系统中, 草地枯落物显著影响植物群落的构建和功能, 是“ 放牧-枯落物-土壤-绿色植物-健康家畜” 综合体不可或缺的组成部分(图3)。研究草地枯落物的生态功能和机制将是放牧生态学领域经久不衰的话题, 未来可在以下几个方面着重开展工作:1)草地枯落物的积累量决定着草地的发展, 而放牧是调控草地枯落物的重要手段, 通过控制性放牧试验, 探讨和明确草地枯落物的生态阈值及其相应的最优放牧模式。2)枯落物是生态系统碳、氮储量的重要载体。草地枯落物多样性中矿质元素的有效性对草地的响应有着不可忽视的作用, 它与土壤间的养分循环利用, 对于建立植被-土壤间的稳定动态不可或缺。3)在农田和森林生态系统中, 人们对枯落物的化感作用研究方兴未艾, 但放牧生态系统中, 缺乏对草地枯落物化感作用的系统性研究, 其中机制尚不清楚。

The authors have declared that no competing interests exist.

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