Effect Of Clipping On Compensatory Growth Of Leymus Chinensis Under Various Saline-alkali Conditions

Leymus chinensis?Trin.?Tzvel,a perennial rhizomatous grass,is dominant species in Northeast Songnen meadow steppe.Grazing and mowing have been the most common land use types.However,over the last 50 years,large area of L.chinensis grassland has been seriously degraded and saline-alkalized mainly due to unlimited reclamation and overgrazing of human beings.Plant compensatory growth is commonly defined as a positive response of plants to clipping or grazing.The amount?or degree?of compensation depends on the net effect of promotion and inhibition about defoliation,which is closely related to defoliation intensity and resource availability.At present,there are many studies on plant compensatory growth after clipping or grazing under different nitrogen or phosphorus conditions.Although saline-alkali meadow steppe is an important habitat of plant growth,the study on compensatory growth mechanism under various saline-alkali conditions is currently very deficient.Therefore a field experiment was conducted with saline-alkali and clipping levels in Songnen meadow steppe.Our objective was to improve and enrich the theory of plant compensation growth,which provides scientific basis for rational use and restoration of saline-alkali meadow steppe.L.chinensis was transplanted into the field in 2013 and was used in our study in 2014and 2015.The experiment was conducted in a completely randomized block design with four replications.Each block includes three clipping levels and three saline-alkali levels.Clipping levels included no clipping?C0?,moderate clipping?C1,30%of shoot removal?and severe clipping?C2,70%of shoot removal?in twice a year.Saline-alkali levels included no saline-alkali addition?S0?,moderate saline-alkali(S1,155.5g mixed saline-alkali m^-22 yr^-1addition,NaCl:Na₂CO₃:NaHCO₃=2:1:1),severe saline-alkali?S2,310.5g mixed saline-alkali m^-22 yr^-11 addition,NaCl:Na₂CO₃:NaHCO₃=2:1:1?,the saline-alkali was added three times a year in the early growing season).In 2014 and 2015,the vegetation density and height were investigated in May,June and July;photosynthesis was measured in July;aboveground biomass was harvested in early August,and belowground was destructively harvested by a soil core for rhizome and root biomass after removal aboveground part.Proline,organic acids,cations and small molecular sugars were measured in the lab.Based on a 2-year field experiment,the main results are present as follows:?1?There were no significant interactions of year,clipping and salt–alkali on total biomass,aboveground and rhizome biomass of L.chinensis.Total biomass,aboveground and rhizome biomass in 2015 were lower than those in 2014,which was mainly caused by slower growth rate and more biomass allocated to belowground under relative drought condition in2015.?2?No significant interactions existed between saline-alkali and clipping on total biomass and aboveground biomass of L.chinensis.Moderate saline-alkali significantly increased total biomass,while there was no significant change at severe saline-alkali level.Saline-alkali has no significant effect on aboveground biomass.R/S ratio,population density,as well as total soluble sugar contents?especially mannitol?of rhizome and root increased at moderate saline-alkali level,but there were no obvious changes at severe saline-alkali level.Additionally,proline,organic acids contents of rhizome and Na⁺contents of each organs increased,K⁺content maintained unchanged,and K⁺/Na⁺ratio reduced with increasing saline-alkali intensity.Moreover K⁺/Na⁺ratio of shoot was higher than that of rhizome and root.L.chinensis enhanced belowground biomass allocation and expanded root growth space under moderate saline-alkali condition,which help increase accumulations of proline,soluble sugar and organic acids in root.At the same time maintaining a relatively higher K⁺/Na⁺ratio in shoot enhances saline-alkaline tolerance.Both root and shoot physiological strategies help to promote tillering and biomass accumulation.?3?L.chinensis exhibited equal-compensatory growth at moderate clipping level with no obviously different accumulations of total biomass and aboveground biomass as compared with those unclipped plants under whatever saline-alkali conditions.Oppositely,total biomass and aboveground biomass greatly decreased at severe clipping level which exhibited under-compensatory growth.R/S ratio,as well as proline,total soluble sugar and organic acid contents of shoot did not show any significant changes,additionally population density improved at moderate clipping level.But they all declined except of R/S ratio increasing at severe clipping level.Therefore balanced allocation of resources and enough accumulations of osmotic substances including proline,total soluble sugar and organic acids in shoot can promote clipping tolerance and further improve aboveground compensatory growth.?4?Interestingly,unlike total biomass and aboveground biomass,moderate clipping significantly increased rhizome and root biomass of L.chinensis under moderate saline-alkali condition,showing over-compensatory growth with 1.21 and 1.25 compensatory index,while there were no over-compensatory growth under no saline-alkali and severe saline-alkali conditions.Severe clipping significantly reduced rhizome and root biomass under whatever saline-alkali levels,showing under over-compensatory growth with compensatory index about0.5-0.7.Contents of total soluble sugar,organic acid and K⁺,and K⁺/Na⁺ratio in root decreased after clipping treatments under no saline-alkali and severe saline-alkali conditions.However moderate saline-alkali promoted their accumulations at moderate clipping level.In the moderate clipping treatment,root total soluble sugar contents including sucrose,fructose and glucose were 1.19 times higher than those in no clipping treatment,root organic acid content was 1.08 times higher than that in no clipping treatment,root K⁺content was 1.68times higher than that in no clipping treatment under moderate saline-alkali condition.All these values were higher than those under no saline-alkali and severe saline-alkali conditions.The“antagonistic”interaction between clipping and saline-alkali treatment on belowground compensatory growth was attributed to the fact that moderate saline-alkali could promote total soluble sugar,organic acid and K⁺accumulations in root,and then improve capacity of belowground compensatory growth.However,when L.chinensis was severely clipped,belowground compensatory growth was not dependent on saline-alkali,but clipping intensity.Therefore L.chinensis exhibited under-compensatory growth at severe clipping level.Compensatory growth of L.chinensis was affected not only by clipping intensity,but also saline-alkali condition.Moderate clipping could promote compensatory growth of total biomass and aboveground biomass,whereas severe clipping had an opposite effect on them.The interaction between clipping and saline-alkali on belowground compensatory growth was antagonistic.Moderate saline-alkali can improve belowground compensatory growth.Our results suggest that moderate use?grazing or haymaking?is feasible for the light saline-alkalized meadow steppe of L.chinensis since moderate clipping have no negative even positive effect on productivity.