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放牧对内蒙古典型草原生态系统磷平衡的影响

Effects of Grazing on Phosphorus Balance in A Typical Steppe of Inner Mongolia

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【Author in Chinese】 乌力吉

【Supervisor】 白永飞赵萌莉

【Author's Information】 内蒙古农业大学, 生态学, 2020, 博士

【Abstract in Chinese】 放牧是草地生态系统最主要和直接的利用方式,其中放牧强度是决定草地生态系统养分输入和输出的关键因子。磷(P)是植物生长和繁殖的必需元素,并以各种形式参与到有机体的生理生化过程。目前,国内外有关生物地球化学循环的研究主要集中在碳(C)和氮(N),有关磷平衡的过程和机制仍不十分清楚,特别是放牧对草地生态系统磷平衡的研究亟待加强。为此,本研究以内蒙古典型草原为研究对象,依托中国科学院内蒙古草原生态系统定位研究站建立的长期放牧实验平台,采用随机设计,分为2个地形和7个放牧强度,共14个放牧处理。对草地生态系统不同组分的磷含量、磷库以及磷的周转等进行了两年(2016、2017年)的研究,取得了以下几个方面的研究结果和结论:(1)放牧对土壤表层0~20cm全磷含量和磷库均具有显著的影响,同时地形也是影响土壤全磷含量和磷库的重要因子。在平地放牧系统中,放牧显著降低了土壤全磷含量和磷库(P<0.05);在坡地放牧系统中,随着放牧强度增加土壤全磷含量和磷库表现出略微增加的趋势。不同地形条件下,土壤全磷含量和磷库均表现为平地高于坡地(P<0.001)。放牧强度虽然没有改变土壤全磷含量的空间异质性,但是有时空间异质性的效应大于放牧处理的效应,平地的空间异质性大于坡地。放牧强度和地形均显著影响土壤有效磷含量(P<0.05),放牧显著增加了两年的土壤有效磷含量,同时表现出平地系统高于坡地系统。(2)放牧对植物群落地上生物量磷含量没有显著的影响(P>0.05),但对地上生物量磷库具有显著的影响(P<0.05),其差异主要是由于生物量不同导致的。植物群落地上生物量磷含量和磷库在不同地形条件下和不同年份间均表现出显著的差异(P<0.001)。不论平地系统还是坡地系统,两年数据均表明放牧显著影响地上生物量磷库,具体你响应表现为,随放牧强度增加,2016年呈“单峰型”变化,而2017年呈现降低的趋势。平地和坡地系统中,地上生物量磷含量和磷库均表现为2016高于2017(P<0.001),这种差异性主要是由有两年含量变化引起的。(3)放牧对凋落物磷含量的影响不显著(P>0.05),但是显著降低了其磷库的大小(P<0.01),主要是生物量的相对贡献率较大。地形条件对凋落物磷含量具有显著的影响(P<0.01),但对凋落物磷库没有显著的影响(P>0.05)。(4)放牧对活根、死根、地下净初级生产力磷含量均没有显著的影响(P>0.05),但放牧显著降低了三者的磷库(P<0.001),三者生物量的相对贡献大于含量的相对贡献。活根磷含量在平地中显著高于坡地(P<0.01),但地形对死根和地下净初级生产力磷含量没有影响(P>0.05)。不同地形和年份对活根和死根磷库均没有显著影响(P>0.05),但地下净初级生产力的磷库在不同年份间差异显著(P<0.05)。(5)地上凋落物残留率不受放牧强度的影响(P>0.05),但是放牧通过减少凋落物的数量,进而显著降低了磷的释放量(P<0.001)。不同地形下,三个分解阶段的地上凋落物残留率和磷释放量均没有显著差异。根系凋落物的分解速率和养分释放受放牧强度和地形的影响(P<0.05)。在平地系统中,残留率随放牧强度的增加呈“单峰型”变化,而坡地系统中残留率与放牧强度呈正相关。放牧对根系凋落物磷释放速率的影响在不同地形下基本一致,放牧通过减少了根系生物量进而降低了磷的释放量。放牧对绵羊粪便的分解速率没有显著影响(P>0.05),但放牧通过增加粪便排泄量改变了磷的释放量,且平地的释放量显著大于坡地(P<0.001)。(6)放牧对磷的重吸收效率具有显著影响(P<0.05)。平地放牧系统中,重吸收效率均与放牧强度呈显著负相关(P<0.05),而坡地系统中二者随放牧强度增加呈“单峰型”变化(P<0.01)。(7)家畜的采食量和排泄量均随放牧强度的增加而呈线性增加或“单峰型”变化,在重度放牧下家畜的采食量最大,但在GI=9.0sheepha-1时,家畜采食量下降。绵羊个体增重和每公顷羊增重的变化呈现相反的趋势,个体增重在轻度或中度放牧条件下最大,而每公顷羊增重在重度放牧下最大(P<0.05)。本研究表明,既保持家畜体重增加又不损坏草地健康的最大放牧强度为GI=6.0sheep ha-1。放牧强度对啮齿动物的分布有也一定的影响,当放牧较轻时主要是以体型较小的黑线仓鼠为主,而随着放牧强度的增加,达乌尔黄鼠的数量有所增加。放牧强度对昆虫生物量的影响年际间变化较大(P<0.001),在2016年放牧显著降低了昆虫生物量,而2017年放牧对昆虫生物量没有显著影响。综上所述,放牧对典型草原生态系统磷循环具有重要的影响。放牧不仅加速了磷循环,同时影响了生态系统的可持续性,轻度放牧不会破坏草地生态系统,但是畜产品输出较少、经济效益不高;重度放牧尽管增加了畜产品的输出但是也增加了水土流失和养分损失,减低了草地的生态功能和可持续性。因此,确定合理的放牧强度是维持草地生态功能和生产功能的关键。

【Abstract】 Livestock grazing is the most important and direct way to use grassland ecosystems.Grazing intensity(GI)is a key factor for nutrient input and output of grassland ecosystems.Phosphorus(P)is an essential element for plant growth and reproduction involving in the physiological and biochemical processes of organisms in many forms.Most studies focus on the carbon and nitrogen cycling,however,the process and mechanism of P balance remain understudied,especially for effects of grazing on P balance in grassland ecosystems.This study was carried out at long term grazing experiment in Inner Mongolia Grassland Ecosystem Research Station the Chinese Academy of Science.This study carried out in a randomized block design with 2 topography and 7 grazing intensities,including 14 grazing treatments in total.P concentration,P pool and P turnover of different grassland components were studied for 2 years.Main results as follows:(1)At the depth of 0-20cm,grazing had significant effects on soil total P concentration and P pool,and topography was also the main factor affecting the soil total P concentration and P pool.Soil total P concentration and P pool decreased(P<0.05)with an increase in GI in the flat grazing system,whereas they increased(P<0.05)slightly with an increase in GI in the slope grazing system.Soil total P concentration and P pool were higher(P<0.001)in the flat than in the slope.GI had no effects on the spatial heterogeneous of soil total P concentration,whereas the effects of spatial heterogeneous was greater than grazing treatment.The spatial heterogeneous of the flat grazing system was greater than the slope.GI and topography had significant effects on soil available-P(P<0.05),it increased with GI in two years.Soil available-P was higher(P<0.001)in the flat than in the slope.(2)Grazing had little effect on P concentration of the plant community(P>0.05),but the aboveground biomass P pool was influenced(P<0.001),and the differences was due to the different contribution of biomass.The aboveground biomass P concentration and P pool were also impacted by the year and topography(P<0.001).Grazing had significant effects on aboveground biomass P pool in 2 years in the flat and the slope system(P<0.05).Aboveground biomass P pool was hump-shaped in 2016 while decreased in 2017 with an increase in GI.In flat and slope system,aboveground biomass P concentration and P pool were higher(P<0.001)in 2016 than in 2017,the differences was caused by the P concentration of two years.(3)Grazing had little effect on litter P concentration(P>0.05),but litter P pool decreased(P<0.01)with an increase in GI,and the differences was due to the different contribution of biomass.Litter P concentration was affected by the topography(P<0.001)but litter P pool was not impacted(P>0.05).(4)Grazing had little effects on P concentration of live root,dead root and BNPP(P>0.05),but their P pool decreased(P<0.001)with GI increased,this trends was caused by the contribution of biomass is greater than the P concentration.The topography affected P concentration of live root(P<0.01)but did not impact P concentration of dead root and BNPP(P>0.05).P concentration of live root was higher(P<0.01)in the flat than in the slope.Neither P pool of live root or dead root was affected by the topography and the year(P>0.05),but P pool of BNPP differed(P<0.05)between years.(5)GI had little effect on litter residual rate(P>0.05),but P release decresed with an increase in GI due to litter mass decreased(P<0.001).Litter residual rate and P release was not affected(P>0.05)by the topography,whereas root litter residual rate and P release were affected by both GI and topography of three decomposition time(P<0.05).Root decomposition rate was affected by GI and topography(P<0.05).Root litter residual rate was hump-shaped with GI increased in the flat grazing system,while increased with an increase in GI in the slope grazing system.The effects of grazing on root litter P release was consistent in the flat or slope system,grazing decreased the root P release throuth decreased the root biomass.The decomposition rate of sheep feces was not influenced by GI(P>0.05),but P release of sheep feces altered due to feces was increased by grazing.P release of sheep feces was greater in the flat than in the slope(P<0.001).(6)P resorption proficiency and efficiency was affectedby GI(P<0.05).P resorption proficiency and efficiency decreased with an increase in GI in the flat topography(P<0.05),but was hump-shaped with an increase in GI in the slope(P<0.01).(7)Feed intake and excrement of sheep were hump-shaped or increased with an increase in GI.Feed intake was the highest in the heavy grazing but decreased in GI=9.0 sheep ha-1.Live mass gain per sheep and live mass gain per ha had the opposite trend.Live mass gain per sheep was the highest in light and moderate grazing,while live mass gain per ha was the highest in heavy grazing(P<0.05).In this study,the optimun GI to get live mass gain and keep grassland healthy was 6.0 sheep ha-1.GI also influenced the distribution of rodents,the dominate rodent species was Cricetulus barabensis with small body in low GI,and Spermophilus dauricus increased with an increase in GI.The effect of GI on insect biomass varied annually(P<0.001),which decreased with an icrease in GI in 2016 but was not affected in 2017.In summary,grazing had important influences on P cycling in the typical grassland ecosystem.Grazing accelerated P cycling and affected the ecosystem sustainability.Light grazing did not demage the grassland ecosystem,but animal products and the economic benefits were low.Heavy grazing increased animal products and herders’ incomes but increased soil erosions and nutrents losses,lowered grassland function and sustainability.Therefore,optimum GI is crucial for maintaining ecological function and pruductivity of grassland ecosystems.

【Keywords in Chinese】 典型草原放牧强度P循环P含量P库地形
【Key words】 typical grasslandgrazing intensityP cyclingP concent rationtP pooltopography
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