豆-禾混播草地中紫花苜蓿比例对其固氮效率的影响及潜在生理机制

Influence of Medicago sativa Proportion on Its Individual Nitrogen Fixation Efficiency and Underlying Physiological Mechanism in Legume-Grass Mixture Grassland

【目的】通过研究豆草比例对紫花苜蓿-羊草混播草地豆科植物生物固氮的影响及其生理生态机制,深化对混播群落结构和生物固氮功能关系的理解,辅助豆-禾混播草地的科学建植和管理,提高混播草地生物固氮和土壤肥力,提升草地资源生产和生态保障能力。【方法】2017年5月,利用紫花苜蓿和羊草为试验材料,采取随机区组设计,于中国科学院长岭草地农牧生态研究站内建植不同豆草比例(紫花苜蓿占比为25%、50%、75%、100%)的紫花苜蓿-羊草混播草地,4次重复。建植一年后,通过样方取样法调查混播草地群落结构变化,测定紫花苜蓿叶片、枝条和根系生长发育、光合和水分等生理代谢特征,在测定根系结瘤特征基础上,采取15N同位素自然丰度法评估紫花苜蓿固氮效率,结合土壤水分动态监测,分析豆草比例对紫花苜蓿生物固氮的影响及其生理生态机制。【结果】(1)混播草地建植一年后,对应25%、50%、75%和100%设计的豆草混播比例,混播草地实际紫花苜蓿占比分别为11%、27%、53%和100%。(2)对比25%、75%和100%的初始豆草种植比例,50%的初始豆草比例下,生长季平均土壤水分含量分别增加了21.4%、36.4%和51.7%。(3)50%豆草种植比例下,紫花苜蓿植株有更大的枝条和根系生物量、叶片数量、叶片面积、叶片厚度和叶片生物量,上述指标最小值出现在100%的豆草播种比例。(4)50%豆草种植比例下,紫花苜蓿具有最大的光合速率,枝条和根系中淀粉含量最高,但枝条和根系中可溶性糖含量最低。(5)50%的初始豆草混播比例下,紫花苜蓿根瘤发育更完善,其生物固氮率较25%、75%和100%的豆草混播比例分别提高了13.5%、44.6%和79.2%。回归分析表明,随豆草比例变化,紫花苜蓿生物固氮效率与土壤含水量呈正相关。【结论】紫花苜蓿-羊草混播草地中,紫花苜蓿生物固氮能力与豆草比例间存在非直线型变化关系。当初始豆草种植比例为50%时,紫花苜蓿生物固氮率最大。豆草比例驱动土壤水分变化,进而通过调控叶片发育和光合等途径改变了紫花苜蓿植株和根瘤发育,是其影响生物固氮的潜在机制。

【Objective】The introduction of Nitrogen-fixing legumes into grasslands is the economical and ecological measurement to improve soil fertility and to increase forage yield and quality in grasslands.In legume-grass mixture grasslands,the proportion of legume plants is the key factor to determine their N fixation function.This research studied the influence of legume proportion on nitrogen fixation efficiency of Medicago sativa-Leymus chinensis mixture grassland and underlying physiological and ecological mechanism,aiming to improve our understanding to the relationship between community structure and biological N fixation in mixed grassland,and to assist the establishment and management of legume-grass mixture grassland for increasing the biological nitrogen fixation and soil fertility,and thus to improve the resources production and ecological stability of mixed grassland.【Method】In May 2017,using M.sativa and L.chinensis as experimental materials,a completely randomized block design with four repeats was applied to establish Medicago sativa-leymus chinensis mixture grassland with different legume proportion(25%,50%,75%,and 100%)in the field in Changling grassland farming research station.One year after establishment,based on the quadrat survey procedure,the changes in community structure,and growth status of leaf,shoot and root,physiological and metabolic characteristics were measured.After measuring nodulation of root,nitrogen fixation efficiency of M.sativa was measured using 15N natural isotope abundance method in mixed grasslands.Finally,the effect of legume proportion on biological nitrogen fixation of M.sativa and its mechanism were analyzed in combination with monitoring soil moisture【.Result】(1)One year after sowing,the observed proportions of legume plants were 11%,27%,53%and 100%,corresponding to initially sowed legume proportion of 25%,50%,75%and 100%in mixed grasslands,respectively.(2)Compared with the initial legume sowing proportions of 25%,75%and 100%,the initial legume sowing proportion of 50%increased the mean soil moisture content within growing season by 21.4%,36.4%and 51.7%,respectively.(3)M.sativa had greater shoot and root biomass,leaf number,leaf area,leaf thickness and leaf biomass under the legume sowing proportion of 50%.The minimum value of the above variables was found in the legume sowing proportion of 100%.(4)M.sativa had the greatest photosynthetic rate and the greatest starch concentrations in shoot and root,but the lowest soluble sugar concentration in shoot and root when legume sowing proportion was 50%.(5)Under initial legume sowing proportion of 50%,root nodule development of M.sativa was better and its biological nitrogen fixation capacity was increased by 13.5%,44.6%and 79.2%,respectively,compared with initial legume sowing proportions of 25%,75%and 100%.Regression analysis showed that the biological nitrogen fixation of M.sativa was positively correlated with soil water content following change in legume proportion.【Conclusion】In Medicago sativa-leymus chinensis mixture grassland,the relationship between biological nitrogen fixation and legume proportion was non-linear.M.sativa had the highest nitrogen fixation efficiency when the initial legume sowing proportion was 50%in mixed grassland.Legume proportion drived change in soil water availability,thus to regulate the growth and development of M.sativa and its root nodules via modification in leaf development and photosynthesis,which was the underlying mechanism for legume proportion to influence the biological nitrogen fixation.This study could help in determining the legume-grass ratio during establishment of mixed grasslands,and guiding the water management in mixed grasslands.