Changes Of Grassland Community Structure And CO₂ Exchange Characteristics Under Different Planting Years In Sanjiangyuan District

The natural grassland in Sanjiangyuan is seriously degraded.Most of the undegraded grasslands with the dominant species of Elymus nutans and Kobresia humilis were degraded to the severely degraded grasslands with the dominant species of Ligularia virgaurea.In order to make degraded grassland to maintain its ecological function,most of the area is restored by artificial cultivation.However,with the extension of the grassland planting years,the composition and structure of the community will change again,with a tendency of secondary degradation.Therefore,the relationship between grassland planting years and the community structure needs further discussion.In this paper,the artificial grassland of Elymus nutans with planting years of 11,12,13,14,17 and 18 years?Recorded as 11Y,12Y,13Y,14Y,17Y and 18Y,respectively?in Sanjiangyuan area was taken as the research object.And the natural grassland with different degrees of degradation?moderately degraded,severely degraded and not degraded?was used as a control to investigate the effects of planting years on plant community structure and CO₂ exchange characteristics of artificial grassland.?1?With the increase of planting years,the aboveground biomass,dominance index,plant height,relative abundance and net photosynthetic rate of Ligularia virgaurea increased gradually;the above index of Elymus nutans decreased first and then remained unchanged,and the minimum construction period is 17 years,and will remain unchanged afterwards.At 14 years,the aboveground biomass of this species was 127.24 g·m^-2,which was significantly larger than that of undegraded natural grassland.At 17 years,the biomass on the grass was 92.88 g·m^-2,which was associated with undegraded grassland.Compared with 101.44 g·m^-2,there was no significant difference between the two.?2?There are significant differences in the parameters of the light response curve of Ligularia virgaurea under different planting years.Among them,the apparent quantum efficiency,maximum net photosynthetic rate,and light saturation point was increasing with the planting years.L_spp and light energy utilization amplitude gradually increase.It showed that with the increase of the planting years,its ability to adapt to the light conditions was gradually strengthened,and the light energy utilization ability getting higher and higher.?3?The aboveground biomass and abundance of plant communities decreased first and then unchanged with the increase of planting years.The value reached the minimum at 17Y,which was 278.56 g·m^-2.The difference was not significant when compared with 261.80 g·m^-2of moderately degraded grassland.It was 315.44 g·m^-2at14Y,which was significantly lower than that of non-degraded grassland(336.88g·m^-2).It showed that in terms of above-ground biomass,artificial grassland has degraded when it was planted for 14 years,and reached a moderate degradation level at 17 years.The Shannon-Weiner Index and the richness first increased and then unchanged,and the peak appeared at 17Y,and its Shannon-Weiner Index was 1.51.The Shannon Weiner Index of non-degraded grassland,moderately degraded and severely degraded grassland were 1.74,1.41 and 0.98,respectively.It is indicated that with the increase of planting years,the diversity and stability of artificial grassland is were increasing.In addition,when the artificial grassland was planted for 17 years,its plant community stability reached the level of moderately degraded grassland.The natural grassland has a reduced community stability as the degree of degradation increased.?4?With the increase of planting years,the net photosynthetic rate,respiration rate and soil respiration rate of plant communities decreased gradually,and the net photosynthetic rate of grassland communities at planting years of 14 years was 2.35?mol CO₂·m^-2·s.^-1,moderately degraded grassland was 2.36?mol CO₂·m^-2·s^-1,and the difference between the two was not significant.It is indicated that with the succession of artificial grassland,the carbon fixation capacity of grassland is weakened,and by14 years,the fixed CO₂ capacity of the community is the same as that of moderately degraded grassland.In natural degraded grassland,with the increase of degradation degree,the respiration rate,net photosynthetic rate and soil respiration rate of the community decreased gradually,indicating that the carbon sequestration capacity of grassland gradually weakened with the increase of degradation degree.From the above results,it can be concluded that artificial grassland can guarantee a high level of productivity within 11-14 years of planting,and degradation occurs at14 years.By 17 years,there is no significant difference compared with moderately degraded grassland then stabilized.It showed that the method of planting artificial grassland can promote the ecological restoration of severe and extremely degraded grassland,and it will have a secondary degradation trend by 14 years.If the necessary manual management measures are not taken,the moderate degradation level will be reached in 17 years.Therefore,measures such as impurity removal,fertilization and reseeding should be carried out at 14 years to prevent secondary degradation of artificial grassland.