| Grassland ecosystem plays an important role in the global carbon cycle.Therefore,study the grassland ecosystem carbon reserves is important to accurate assessment of global ecosystem carbon balance.Mowing is one of the important utilization modes,on the one hand,it has influence on the input level of soil carbon by Net primary productivity(NPP),on the other hand,mowing has influence the soil respiration and temperature by change the coverage of vegetation,there all influence the process of carbon cycling in grassland ecosystems.As often,grassland utilization is concentrated on growing season,so the research in grassland carbon sequestration also focus on this period.But it remains unconfirmed whether grassland carbon sequestration will recover or continues to deteriorate.A large number of studies have shown that,modest clipping can be promote plant compensatory growth,and people study on it was generally tended how to improve the NPP,but lacked of relationship between compensatory growth and grassland carbon sequestration.So,it is important to combine plant compensatory growth and grassland sequestration for sustainable development in grassland and ecological protection.The aim of this study is to explore the relation between plant compensatory growth and grassland sequestration,and this article mainly from the flowing three aspects:(1)Under the aim of searching for the influence on grassland sequestration by clipping management,to determine the content of plant and soil carbon storage annual periodic(2012-2013).The results showed that,clipping has influenced the above-and belowground biomass carbon storage,but litter and soil carbon storage has not affected.During the growing season,the total carbon storage of the grassland has no change that compared with control.In addition,the grassland biomass had no significant difference with control in non-growing season.Under the aim of searching for the influence on the rate of ecosystem gas exchange,to determine the net ecosystem exchange(NEE),ecosystem repiration(ER),soil respiration rate(Rs)and gross ecosystem exchange(GEP)in the middle of each month in 2013.The result showed that,NEE,ER,Rs and GEP of unclipped and clipped grassland showed an obvious seasonal variation,and the whole variant trend is identical.Under growing season,unclipped and clipped grassland were all carbon sequestration period,the trend was "U" shape,and CO2 absorption has achieves maximum in July,but CO2 absorption of clipped grassland was lower than unclipping grassland.Under non-growing season,unclipped and clipped grassland were all carbon emission period,but CO2 emissions of clipped grassland was lower than unclipped grassland.Therefore,NEE of clipped grassland was not affected by clipped,and clipped grassland was a carbon sink grassland as unclipped.As compared with unclipped grassland,ER and RS of clipped grassland has decreased in few months,but the whole effect were not significant;Under growing season,GEP of clipped grassland was higher than unclipped grassland,but there were no significantly in non-growing season and full year.(2)Under the aim of searching for the influence on relationship between compensatory growth and grassland sequestration,three clipping intensities managements were proposed with one growing season.The result showed that,the community aboveground biomass was exhibited equal-and over-compensatory growth at 50%clipping intensities(MC)and 70%(HC)clipping treatment,and in the plant community,belowground biomass were all showed under-compensatory growth,but the total biomass was not affected by clipping;as compared with control,relative growth and carbon increments were significantly increased,and HC>MC treatment.The objectives of this study were to investigate the compensatory growth response patterns of community,Synusias,and plant individuals to defoliation removal at different clipping rate,to determine the dependence of community responses on species responses and interactions between the component species.The result showed that,clipping has a significant effect on top and middle species,but not sensitive to the lower species in community;the above-,belowground and total biomass were all decreased after clipping treatments,and showed under-compensatory growth;the middle species has adjusted the biomass allocation that after clipping treatment,and showed equal-compensatory growth;compared with control,the belowground of lower species has increased under MC treatment,the total biomass was not affected;the aboveground and total biomass of lower species has significantly increased in HC treatment,but not affected on belowground biomass;as a dominant species of upper and middle layer,the compensatory growth of M.sinensis and D.glomerata following clipping removal is an important mechanism underpinning community level compensatory responses,and the different methods of compensatory growth after clipping were further illustrates the difference bwtween biological characteristics;over-compensatory growth of T.repens might be related to its light environment.(3)This study was conducted to examine how light regimes influence the pattern of compensatory growth after defoliation;we found that red light altered above-to-belowground allometry,enhanced leaf C concentrations and induced N deficiency,which subsequently stimulated root growth.By contrast,leaf N concentrations of L.perenne were greater under shading treatment,which enhanced shoot growth along with decreased belowground growth concurrently.On the condition of defoliation,red light increased shoot growth,not at the expense of root growth,which was not the same case in shading treatment.Therefore,the specific allometric principle of various treatments is probably the main reason that caused the different patterns of compensatory growth after defoliation.Under the aim of searching for the influence on the relationship between compensatory growth and internal C/N ratio status.Our result showed that,Moreover,regardless of unclipped(no defoliation)and defoliation conditions,L.perenne biomass partitioning between roots and shoots was significantly correlated with leaf N concentrations and C/N ratio,which indicated that allometric biomass allocation can be largely modulated by signals related to plants C and N status.These results demonstrated that leaf C and N status would be an appropriate indicator to valuate compensatory growth after defoliation.In conclusion,this study has answered scientific problem that established at the initial period,and got the following conclusions:At moderate clipping intensity,the grassland have a self-adaptation function to external interference,and clipping has not affect on grassland carbon storage in short order.The rate of ecosystem gas exchange has changed in growing and non-growing season,but the effect of whole year were not significant,and grassland ecosystem of clipped treatment was showed carbon sequestration.Under growing season,clipping has stimulated the plant compensatory growth,and improved carbon sequestration.Therefore,compensatory growth was benefits for grassland carbon sequestration.The compensatory growths of top and middle specie were contributed to most of the increase in community biomass.The compensatory growth of M.sinensis and D.glomerata following clipping removal is an important mechanism underpinning community level compensatory responses,and the different methods of compensatory growth after clipping were further illustrates the difference bwtween biological characteristics;over-compensatory growth of T.repens might be related to its light environment.The compensatory growth was more likely to happen in non-limited light environment,and the patterns of compensatory growth might be related to allometric principle.Our study indicated that variation of light regime and defoliation can alter both C/N balance and ultimately biomass allocation in grass species.The allometric allocation is largely modulated by signals related to C and N status of plant.Hence,leaf C and N status can be an appropriate indicator of compensatory growth after defoliation. |
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