Exploring The Responses Of Carbon And Nitrogen Dynamics Of Leymus Chinensis To Clipping Treatment By Using The ¹³C And ¹⁵N Isotope Labelling Technology

Grazing and clipping are main ways of grassland utilization.The regeneration of herbage after defoliation is a slow and complicated process.Study on the distribution and transfer of carbon and nitrogen resources after clipping is very important to explain the regeneration mechanisms of herbage.Using the pot experiment,we studied carbon and nitrogen dynamics and regrowth of typical species of Songnen meadow,Leymus chinensis.We set up moderate clipping treatment and control treatment and set up 10 sampling times which lasted 1 month and covered the early,middle and late stages after defoliation.Through characterization of the redistribution of storage compounds,the assimilation rate of the residues and root system’s absorbing of nitrogen;we try to understand carbon and nitrogen dynamics and the leading role in different stages of regeneration,then to explore the regeneration mechanism after defoliation.At the same time,¹³C and ¹⁵N stable isotope labelling technology was used to track the distribution of the newly absorbed carbon and nitrogen elements in the leaves,stems and roots and the changes over time.We comparied and analysed the shift of ¹³C and ¹⁵N recovery and allocation in leaves,stems and roots.We also measured the morphological parameters,such as plant height,biomass and the physiological indexes,including leaf gas exchange and contents of soluble sugar,ammonium nitrogen and nitrate nitrogen.The main results are as follows:i.In the second week after moderate clipping treatment,we detected the highest increasing rate in the accumulated aboveground biomass and total biomass.The quickest increment of belowground biomass was observed in the third week.L.chinensis can compensate and guarantee the leaf regeneration by sacrificing the growth of the belowground part.L.chinensis had achieved overcompensation since the second week after the clipping.ii.The stored carbohydrates were converted into non-structured soluble sugar and used for the growth of new leaves,which played a major role in the resistance to clipping damage of L.chinensis during the early stage after the clipping.Soluble sugar content increased in leaves and decreased in stems and roots,which suggests that L.chinensis remobilize the stored carbohydrates to the aboveground part.Leaf net photosynthetic rate gradually increased and the remaining soluble sugar of leaves was transferred to the stems and roots for storage.iii.The effects of moderate clipping treatment on photosynthesis were significant.The first two days after defoliation were physiological injury period,after that,the leaf net photosynthetic rate gradually recovered and played a major role in regrowth from the 4^th day to the 14^th day after the clipping.The higher net photosynthetic rate was also maintained till the end of the experiment.iv.The moderate clipping treatment increased the distribution of newly absorbed carbon and nitrogen to the aboveground part.The leaf ¹³C recovery in the clipping treatment was significantly higher than in the control treament from the 7^th day to the35^th day after the clipping,and the ¹³C recovery rates of stems and roots were decreased in the middle and late stages after the clipping.At the 21^st day,28^th day and35^th day after clipping treatment,the allocation of ¹⁵N to the aboveground part was higher than the roots.The distribution of new absorption of ¹⁵N is the main effect factor during the late period of the regrowth of L.chinensis.v.The moderate clipping treatment promoted available nitrogen content in the soil,which was helpful to the absorption of L.chinensis’s roots,and increased the photosynthesis and regeneration.The results of this study indicated that,in the early stage after moderate clipping treatment,the storage carbohydrate was decomposed into soluble sugar to supply leaf growth,which played a major role in the regeneration of L.chinensis.The recovery and enhancement of leaf gas exchange played a very important role in the regeneration of L.chinensis except physiological injury period right after the clipping.The distribution of newly absorbed ¹⁵N to the aboveground parts is the main effect factor of the regeneration of L.chinensis during the late period after the clipping.This study provides a theoretical basis for the compensation growth strategy and regeneration mechanism of defoliated L.chinensis.