Abstract:

【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.

Key words: Medicago sativa, Leymus chinensis, mixed grassland, biological nitrogen fixation, soil water content, photosynthesis