Study On The Effects Of Simulated Herbivory And Nitrogen Addition On Leymus Chinensis Spatial Expansion In Salinealkali Patchy Grassland

Grassland ecosystem is the largest terrestrial ecosystem in China,however,the climate change and unreasonable reclamation/grazing caused an increasing grassland degradation,which has seriously affected the functions and services of the grassland ecosystem.Therefore,it is necessary to explore effective managements or restoration measures to achieve the sustainable development of local animal husbandry and the security of ecological environment.Many researches have been carried out and a great progress has been made on the restoration of degraded grassland.In recent years,the spatial expansion characteristics of clonal plants in the ecological restoration of degraded grassland has been received extensive attention.Leymus chinensis,the dominant clonal plant species in Songnen grassland,can expand its space by asexual propagation,then colonize and grow in saline-alkali soil patches with the help of underground rhizomes,which has a potential restoration effect on saline-alkali soil patches.The grazing,fertilization and saline-alkali stress are the three key factors affecting the spatial expansion of L.chinensis.However,the effect and mechanisms of these three factors and their interactions on the expansion of L.chinensis from favorable habitats to saline-alkali soil patches are not clear.In order to solve the above problems,this study took L.chinensis as the research object,and the pot experiments were used to simulate the real situation of Songnen grassland.The simulated herbivory（cattle,sheep）,nitrogen addition(no nitrogen,nitrogen addition 10 g m^-2·yr^-1)and saline-alkali stress degrees（moderate stress,severe stress）were set as treatments,the researches were investigated including（1）the effects of simulated herbivory,nitrogen addition,saline-alkali stress and their interaction on morphological characteristics of L.chinensis;（2）the effects of simulated herbivory,nitrogen addition,saline-alkali stress and their interaction on biomass and biomass allocation of L.chinensis;（3）the effects of simulated herbivory,nitrogen addition,saline-alkali stress and their interaction on eco-physiological characteristics of L.chinensis.The main results were as follows:（1）The effects of simulated herbivory on the spatial expansion of L.chinensis to saline-alkali soil patches were related to the types of livestock and the degree of saline-alkali stress.Simulated sheep herbivory could significantly promote the spatial expansion and growth of L.chinensis under moderate saline-alkali stress conditions.Compared with the control,the total rhizome length,bud number,total biomass and rhizome biomass of the younger ramets of L.chinensis growing in saline-alkali soil patches increased by 67%,84%,42%and 78%,respectively（P<0.05）;while the simulated cattle herbivory inhibited the spatial expansion of L.chinensis to saline-alkali patches,and compared with the control,the total rhizome length,bud number and total biomass of the whole clones of L.chinensis in both favorable soil patches and saline-alkali patches decreased by 32%,36%and 42%,respectively（P<0.05）.The content of malondialdehyde（MDA）in the leaves of the mother ramets of L.chinensis growing in favorable soil patches was slightly affected by simulated sheep herbivory,indicating that the growth point of L.chinensis might not be damaged and the compensatory growth of L.chinensis might be stimulated.However,the simulated cattle herbivory significantly increased the MDA content in the leaves of L.chinensis mother ramets（P<0.05）,indicating that the leaf tissue of L.chinensis mother ramets was seriously damaged,and to some extent,the growth and expansion of L.chinensis in saline-alkali soil patches was inhibited.In addition,compared with the control,the stomatal conductance（g_s）and intercellular CO₂ concentration（Ci）of L.chinensis mother ramets leaves were significantly increased by 55%and 12%,which provided a material basis for the improvement of photosynthetic efficiency of the mother ramets of L.chinensis.（2）Nitrogen addition(10g m^-2·yr^-1)had no significant effect on the spatial expansion of L.chinensis to saline-alkali soil patches.Compared with the control,the total length of rhizome,bud number and total biomass of L.chinensis ramets growing in saline-alkali soil patches showed no significant changes under nitrogen addition treatments（P>0.05）.Additionally,the biomass of the whole clone of L.chinensis was slightly affected by nitrogen addition.Compared with the control,the proline content of the mother ramets of L.chinensis under severe saline-alkali stress was significantly increased by 85%（P<0.05）under nitrogen addition treatments.However,there was no significant change in root shoot ratio of whole plant under this treatment,which may be the main reason for the insignificant spatial expansion of L.chinensis.（3）The effects of nitrogen addition and simulated herbivory interaction on the spatial expansion of L.chinensis in saline-alkali soil patches also depended on the livestock types.Nitrogen addition could partly alleviate the negative effects of cattle herbivory on the spatial expansion of L.chinensis to saline-alkali patches,compared with the cattle herbivory.The total length of rhizome of L.chinensis under the moderate-saline stress increased by 66%under nitrogen addition and simulated herbivory interaction.However,the nitrogen addition inhibited the spatial expansion of L.chinensis to saline-alkali soil patches.Compared with the sheep herbivory,the nitrogen addition and simulated herbivory interaction caused the decrements of total rhizome length and bud number of L.chinensis under moderate saline-alkali stress conditions by 22%and 20%,respectively.On the one hand,compared with the simulated cattle herbivory,the nitrogen addition and simulated herbivory interaction increased the net photosynthetic rate（Pn）,transpiration rate（E）,stomatal conductance（g_s）,intercellular CO₂ concentration（Ci）and POD enzyme activity and reduced MDA content,which were the main reasons for the nitrogen addition to alleviate the negative effects of cattle herbivory on the spatial expansion of L.chinensis.On the other hand,compared with the simulated sheep herbivory,the nitrogen addition and simulated herbivory interaction increased the net photosynthetic rate,which might redistribute more resources to the aerial parts of the L.chinensis and the growth of the rhizomes was limited.In conclusion,the effects of different herbivory on the spatial expansion of L.chinensis in heterogeneous habitats are different.Coordination of nitrogen addition,types of herbivory and the degree of saline-alkali soil stress in grassland could improve the recovery efficiency of saline-alkali degraded grassland.It is of great importance to utilize the spatial expansion characteristics of clonal plants to restore the degraded grassland,and manage the grassland reasonably（eg.grazing or fertilization）in the future.