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半干旱黄土区草地植物多样性和生产力对封育和管理措施的响应

Response of Plant Diversity and Productivity to Grazing Exclusion and Grassland Management on the Semiarid Loess Plateau

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【Author in Chinese】 刘建

【Supervisor】 程积民邱莉萍

【Author's Information】 西北农林科技大学, 草学, 2020, 博士

【Abstract in Chinese】 草地是陆地上最大的生态系统之一,在生物多样性保护、水土保持、碳蓄积和畜牧业生产中发挥着重要的作用。然而,在气候变化和人类活动的影响下,草地大多面临着物种多样性降低和生产力下降等退化问题,尤其在生态环境脆弱的半干旱黄土区。封育作为半干旱地区退化草地恢复的有效措施,可以增加植物多样性和生产力,改善土壤理化性质,提高生态系统稳定性,但长期封育会降低植物生产力和多样性。目前已有大量研究报道了封育对退化草地生态系统的影响,但长期封育草地植物多样性与生产力下降的原因尚需深入探究。因此,本研究以半干旱黄土区封育草地为研究对象,分析了不同封育年限(10、27、35年,自由放牧草地作为对照)与不同坡向(阴、阳坡)对植物多样性、地上生物量和土壤水氮动态的影响,以期揭示封育后期植物多样性和生产力降低的机制。同时,针对封育后期出现的植物多样性降低和生产力下降问题,本研究分析了不同的草地管理措施(刈割、火烧、增水和氮、磷添加)对封育27年草地植物多样性、地上生产力和土壤水氮动态的影响。主要研究结果如下:(1)植物地上生物量随封育时间的延长先增加后稳定(封育27—35年)。植物多样性不受封育年限的影响。土壤水分和矿质氮含量随封育年限的延长而增加,但氮矿化速率随封育年限的延长而降低。封育对土壤氮动态的影响随土层变化,表现出在0—10 cm比10—20 cm封育降低了更多的氮矿化速率但增加了更多的矿质氮含量。枯落物的积累是影响封育草地恢复的关键因子,其在封育前期和中期促进了草地的恢复,但在封育后期限制了草地的恢复。因此封育并非退化草地恢复的长久之计。鉴于枯落物在封育过程中发挥的关键作用,枯落物的管理应作为长期封育草地管理的重点。(2)坡向显著影响了地表植被和土壤水氮动态,调节了退化草地对封育的响应。阴坡比阳坡有更高的植被盖度、群落地上生物量、禾草地上生物量和土壤水分和矿质氮含量,同时也有更低的杂类草地上生物量和氮矿化速率。坡向通过影响水分和养分等因素调控了封育草地的恢复过程,使植被盖度、植物地上生物量与土壤水分、矿质氮含量在阴坡中随封育时间的延长而增加的更多,但也使氮矿化速率随封育时间的延长而降低的更多。物种丰富度在阴坡中对封育的响应更显著,表现出在封育10年中显著增加而在封育27年中显著降低,但在阳坡不受封育的影响。因此在封育后期的草地管理中应注重水分和养分的管理。(3)氮添加增加了植物地上生物量、土壤水分与矿质氮含量,但不影响植物多样性和氮矿化速率。磷添加增加了植物地上生物量,降低了物种丰富度与Shannon-Wiener多样性指数。增水促进了磷添加对植物地上生物量的增加,并且减轻了磷添加对植物多样性的抑制作用。整体而言,水肥管理对地上生物量的影响有限,并且降低了物种多样性,因此不适合作为封育后期的管理措施。(4)刈割显著增加了物种丰富度和Shannon-Wiener多样性指数,降低了植物地上生物量、土壤水分与矿质氮含量,但不影响土壤氮矿化速率。水肥管理会促进刈割后禾草地上生物量和矿质氮含量的恢复,但不影响植物多样性和杂类草地上生物量对刈割的响应。因此,在封育草地后期的管理中,可使用刈割来促进植物多样性的恢复,同时辅以水肥以控制地上生物量和土壤氮有效性的降低。(5)火烧不影响植物多样性和土壤氮矿化速率,并降低了植物地上生物量、土壤水分和矿质氮含量。增水和氮添加有利于火烧后地上生物量的恢复,但未改变植物多样性、土壤水分、矿质氮和矿化速率对火烧的响应。因此,火烧不适合作为封育后期草地的管理措施。本研究分析了封育年限与坡向对草地的影响,得出以下结论:枯落物的积累是影响封育草地恢复的关键因素;坡向可以通过其对水分和养分的影响去调控封育草地的恢复。因此,封育后期草地的管理应重视枯落物和水肥的作用。通过分析刈割、火烧与水肥管理对封育27年草地的影响,本研究发现刈割+水肥在增加植物多样性的同时降低了植物地上生物量的损失,因而可作为封育后期的重要管理措施。

【Abstract】 Grassland,one of the largest terrestrial ecosystems in the world,is important for the protection of biodiversity,the conservation of soil and water,carbon sequestration and livestock production.As climate change and human activities,grassland suffers great degradation,especially on the semiarid Loess Plateau.Grazing exclusion is an effective management for restoring semiarid degraded grassland,which could increase plant productivity and diversity.However,long-term fencing decreases plant productivity and diversity,which limits the sustainable development of grassland.Therefore,we examined the effects of grazing exclusion chronosequence(10,27 and 35,and grazing grassland as control)and slope aspect(north and south slope)on plant diversity,aboveground biomass and soil water and nitrogen(N)dynamics in a grazed grassland on the Loess Plateau,and tried to reveal the underlying mechanism for grassland degradation resulted from long-tern fencing.Meanwhile,focused on the grassland degradation(as characterized by the decrease of plant productivity and diversity)in long-term fencing,we examined the effects of grassland managements(clipping,fire and additions of water,N and phosphorus(P))on plant diversity,aboveground biomass and soil water and N dynamics,which could help to the sustainable management of the long-term grazing excluded grassland.Main results are as follows:(1)Plant aboveground biomass firstly increased with the increasing of grazing exclusion chronosequence,and stabilized during the late stage of fencing(27 a-35 a).Plant diversity was not affected by grazing exclusion.Soil moisture and mineral N content increased with the increasing of fenced time,while soil net N mineralization rate decreased with that.The effects of grazing exclusion on soil net N mineralization and mineral N were larger at 0-10 cm than those at 10-20 cm,with more decrease of soil net N mineralization rate and more increase of mineral N content.Litter accumulation was important for restoring fenced grassland,which promoted the restoration during early and mid-stage of fencing but hindered the restoration during late stage.Therefore,grazing exclusion is not effective in long-term run.The control of litter accumulation is important for the management of grasslands that suffered degradation in long-term grazing exclusion.(2)Slope aspect significantly affected the vegetation and the dynamics of soil water and nitrogen,and regulated the effects of grazing exclusion.North slope had the higher plant canopy cover,grass biomass,soil moisture,mineral N contents but the lower forb biomass and soil net N mineralization rates than south slope.Slope aspect regulated the effects of grazing exclusion through its controlling on soil moisture and nutrients availability,with more increase of plant canopy cover,aboveground biomass,soil moisture and mineral N contents but more decrease of net N mineralization rates on the north slope than those on the south slope.Species richness was increased after 10 years of fencing and decreased after 27 years of fencing on the north slope,but was not affected by fencing on the south slope.Therefore,the management of water and nutrients is important for long-term fenced grassland.(3)N addition enhanced plant aboveground biomass,soil moisture,mineral N and nitrate contents,but did not change plant diversity and soil net N mineralization rates.P addition increased plant aboveground biomass but decreased plant diversity.Water addition amplified the effects of P addition on plant diversity and aboveground biomass.Additions of water,N and P exerted limited effects on plant aboveground biomass,and decreased plant diversity,which was not a appreciate management for long-term fenced grassland.(4)Clipping significantly increased species richness and Shannon-Wiener index,decreased plant aboveground biomass,soil moisture and mineral N contents,but did not change soil net N mineralization rates.Additions of water,N and P could facilitate the compensatory growth of grass and the increase of mineral N after clipping,but did not alter the effects of clipping on plant diversity and aboveground biomass of forb.Therefore,clipping+ additions of water,N and P could be helpful to restore the grassland of the late stage of exclusion.(5)Fire did not affect plant diversity,but decreased plant aboveground biomass,soil moisture and mineral N contents.Additions of water and/or N could facilitate the compensatory growth of aboveground plant after fire,but did not change the effects of fire on plant diversity,soil moisture,mineral N and net N mineralization.Therefore,fire is not a appreciate management for long-term fenced grassland.Through examining the effects of grazing exclusion and slope aspect on grassland,we found litter accumulation was the key factor during the restoration of fenced grassland;slope aspect could regulate the restoration of excluded grassland through its controlling on soil moisture and nutrients.Thus,litter accumulation,water and fertilization were important for the restoration of grasslands in long-term fencing.Through analyzing the effects of grassland managements on grassland,our study found clipping+water+fertilization could increase plant diversity while least decreased plant productivity.Therefore,clipping+water+fertilization is an important tool for the management of long-term fenced grasslands.

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