| The grassland ecosystem is an important part of the natural ecosystem.The level of plant primary productivity is an important indicator of various grassland functions,and determines the strength of the grassland ecosystem.Previous studies about the vegetation productivity have mostly focused on the water and nitrogen changed and the large geographic scales.However,the seasonal changes,population dynamics and productivity formation of ground vegetation are more dependent on bud banks in grassland ecosystems,which dominated by perennial clonal plants.Previous studies on the bud bank have focused on the effects of ecological characteristics,different habitats or restricted resources on bud density under natural conditions.However,little is known about the response of bud bank traits to the changes in the main limiting factors of water and nitrogen in arid and semi-arid areas from the perspectives of bud number,bud biomass,bud size and the spatial location of the bud bank.Research on the relationship between bud bank traits and vegetation ontology and productivity formation is relatively rare.In the present study,Leymus chinensis,a typical rhizome clonal plant in temperate semi-arid meadow grassland,was used to investigate the response and growth strategies of the characteristics and dynamics of bud bank on water and nitrogen changes in different seasons(spring and autumn)by water and nitrogen additional experiments.Meanwhile,we also clarified the internal mechanisms of seasonal changes in bud banks,and the relationship between L.chinensis bud bank and its productivity of aboveground part by analyzing seasonal changes in bud banks combined of the relationships among soil resources,individual growth and development,photosynthetic and physiological changes and bud banks and different types of buds.The main findings of this study are list as follows:(1)The bud traits and spatial location of bud bank of L.chinensis were significantly affected by water and nitrogen treatments in different seasons.Among them,the axillary shoot buds were sensitive to the additional of water and nitrogen in the two seasons,but the rhizome buds were only sensitive to the addition of water and nitrogen in spring.The additional of water and nitrogen in different seasons did not change the ecological rule of bud bank dynamic changes of L.chinensis.Spring and autumn were the main input seasons of bud bank,and the dynamic changes of axillary shoot bud were similar to the variation rule of the total buds,which were the main influencing factors of bud bank dynamic changed.The different adaptive strategies of the two types of buds in spring and autumn were important reason for their seasonal variation.In spring,the bud bank was mainly composed of rhizome buds,and the population mainly adopted the "guerrilla" growth strategy of expanding territory to expand the population and occupy more ecological niche.In autumn,the population adopted the "intensive" and "guerrilla" growth strategies to consolidate the territory,and the number and proportion of axillary shoot buds increased significantly after water and nitrogen treatments,and the population adopted the "intensive" growth mode under natural conditions.(2)The seasonal addition of water and nitrogen did not produce seasonal changes in the soil physicochemical properties of L.chinensis population,and the additional of water and nitrogen in two seasons significantly increased soil water content and the contents of available nitrogen and phosphorus in the soil nutrient matrix,so as to improve the bud traits of L.chinensis population,which showed a significantly correlation with the bud traits.The additional of water and nitrogen improved the stability of soil characters.There was no significant correlation between the variation coefficient of soil properties and axillary shoot bud traits in spring and autumn,but the rhizome bud traits were significantly positively correlated with it in spring.The rhizome bud traits were more sensitive to the changes in soil traits than axillary shoot bud traits,and were also more susceptible to the changes in soil resources and environment.The additional of water and nitrogen in different seasons did not change the main influencing factors of the number of buds,and the main influencing factor of the number of the buds were soil water content and available nitrogen content in both spring and autumn.(3)Water and nitrogen additional in spring and autumn significantly improved photosynthetic traits index of L.chinensis,but there were significant seasonal differences in the photosynthetic traits.The photosynthetic water use efficiency,total ecosystem production and net photosynthetic rate had significant responses to water and nitrogen supplementation in spring.The characteristics of axillary shoot buds are closely related to the photosynthetic capacity and more dependent on the nutrient supply of the mother plant.Due to different insertion position of rhizome buds,their growth and development mainly depend on the absorption of soil resources by underground root system of L.chinensis,which have no significant correlation with the photosynthetic characteristics.As the main influencing factor of axillary shoot buds,photosynthetic water use efficiency was not affected by seasonal changes.After additional of water and nitrogen,the auxin content decreased significantly,which decreased the inhibition of cytokinin and promoted the input of axillary shoot buds.The seasonal variation of auxin content in axillary shoot buds was the main substance that affected the seasonal variation of lower axillary shoot buds of L.chinensis.(4)The additional of water and nitrogen in spring could significantly improve the individual development of L.chinensis,so that it had a larger underground rhizome system,and the plant height and leaf traits were also significantly improved,which enhanced the ability of L.chinensis to acquire aboveground and underground resources,and then improved the size of its bud bank and bud traits.axillary shoot buds was positively correlated with ontogeny of L.chinensis.After the population growth rate slowed down in autumn,it was easier to obtain nutrient supply from the mother plant for growth and development,which made it had higher input and output in autumn.The hay productivity and greenfeed productivity of L.chinensis were significantly affected by the additional of water and nitrogen in spring and autumn,and the axillary shoot buds were the main contributors to the regulation of L.chinensis productivity in both spring and autumn.In general,the additional of water and nitrogen in spring and autumn reduced the limitation of water and nitrogen,the main influencing factors of Songnen grassland on the growth and development of L.chinensis,improved the comprehensive characteristics of the bud bank,and increased the spatial location of the bud bank,made the number of L.chinensis buds significantly increased.Among them,axillary shoot buds are mainly affected by the growth and development of L.chinensis,while rhizome buds are sensitive to the changes in soil resources and environment.The seasonal difference of auxin in axillary shoot buds is the main influencing and regulating factor of the seasonal changes in the composition of bud bank,which make the two types of buds have significant seasonal differences in spring and autumn.As a result,L.chinensis has different growth strategies in different seasons.The spring bud bank is mainly composed of rhizome buds,the population mainly adopts the "guerrilla" growth strategy of expanding territory.The population adopts the "phalanx" and "guerrilla" of consolidating territories under natural conditions in autumn.After water and nitrogen treatments,the population adopts the "phalanx" growth strategy.Although L.chinensis has different growth strategies in different seasons,axillary shoot buds are the main contributors to its productivity of aboveground part in both spring and autumn. |
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