| Natural and anthropogenic factors such as global warming,fire and overgrazing have lead to the expansion of shrubs into grassland,and the coverage of shrubs has been increased.This process is known as "shrub encroachment",which has a significant influence on carbon and nitrogen pool,net primary productivity,plant community structure and ecological functions of ecosystem.The Qinghai-Tibet Plateau,which occupies an important ecological position,is currently experiencing rapid shrub encroachment.When the plant-soil-enzyme system is taken as a whole,its response to shrub encroachment is still unclear.Therefore,our study selected the grassland encroached by four typical kinds of shrub as research object: Spiraea alpina,Potentilla fruticosa,Caragana erinacea and Sibiraea angustata,which on the eastern edge of the Qinghai-Tibet Plateau.According to the coverage of shrub,the shrub-encroached grassland is divided into four levels: grassland(native grassland with the shrub coverage of 0),mild shrub-encroached grassland(shrub coverage of5%-30%),moderate shrub-encroached grassland(shrub coverage of 30%-55%),and severe shrub-encroached grassland(shrub coverage≥55%).The response of plants,soil nutrient,and soil enzymes to shrub encroachment were investigated,and the relationship between the three components and soil physicochemical properties were analyzed.The purpose was to reveal the response of the plant-soil-enzyme system to shrub encroachment,providing theoretical reference for predicting the ecological effects and management measures of shrub encroachment in alpine meadows on the Qinghai-Tibet Plateau.The main research results of this article are as follows:(1)The effects of shrub encroachment on the nutrient content and their stoichiometric ratio of herbaceous plants were generally not significant.However,the encroachment of Spiraea alpina and Sibiraea angustata led to a significant decrease in TC content of the aboveground part of herbaceous plants,and the TC content of the aboveground part of herbaceous plants decreased by 13.68% and 10.77%,respectively,in the severe shrub-encroached grassland compared with the native grassland.The encroachment of Caragana erinacea caused the C:N ratio of the aboveground part of the herbaceous plant continuously decreased,and the ratio of C:P and N:P increased but not significantly.Plant N:P also had no significant response to shrub encroachment.It could be seen from the herbaceous plant stoichiometric ratio that the growth and development of herbaceous plants in all native grasslands in the study sample plots were generally restricted by N,and the N restriction had been alleviated after encroachment of Caragana erinacea,but the N restriction had not been removed.The expansion of the other three non-legume nitrogen-fixing shrubs had no significant impact on the N restriction of herbaceous plants,and even the N restriction of herbaceous plants had been slightly enhanced in the plots of Potentilla fruticosa and Sibiraea angustata.In general,the C,N,P and stoichiometric ratio of aboveground and underground parts of plants were not significantly affected by soil BD,SWC and p H,but were generally significantly affected by soil stoichiometric ratio,especially the aboveground parts of herbaceous plants C:P and N: P were significantly positively correlated with soil C:N,C:P and N:P.(2)The influence of shrub expansion on the content of soil C,N and P and their stoichiometric ratio varied from different shrub species.Overall,the expansion of nitrogen-fixing shrubs increased the soil nutrient content and its stoichiometric ratio in grasslands,while the expansion of non-legume nitrogen-fixing shrubs reduced the soil nutrient content and its stoichiometric ratio.The expansion of Caragana erinacea increased the content of SOC and STN with the increasing of shrub.Compared with the native grassland,the content of SOC and STN in the 0-10 cm soil layer increased by 106.53% and 67.40% respectively in the stage of severe shrub expansion.The content of soil SOC and TN decreased after the expansion of the three non-legume nitrogen-fixing shrubs.The content of STP had no significant change after the expansion of Caragana erinacea,but decreased overall after the expansion of the other three non-legume nitrogen-fixing shrubs.The content of SOC,STN,and STP were the highest in soil layer of 0-10 cm,and decreased from top soil layer to deep soil layer.The change trend of soil nutrient content in different soil layers was almost identical with shrub encroachment.In this study,the ratio of soil C: N,C: P,and N: P showed a decrease after the expansion of three non-legume nitrogen-fixing shrubs.The expansion of Sibiraea angustata resulted in the soil C: N,C: P and N:P increased with the increasing of shrub.The soil stoichiometric ratio showed that the expansion of Caragana erinacea reduced the availability of soil P thus increased the P limit,and the expansion of three non-legume nitrogen-fixing shrubs increased the soil P availability so reduced the P limit.Soil C:N,C:P and N:P were significantly positively correlated with the contents of SOC,STN and STP,respectively.The contents of soil C,N,P and their stoichiometric ratios were significantly negatively correlated with soil BD and p H(only there were no significant correlation in soil P content with soil p H,soil C: N with soil p H),and significantly positively correlated with SWC.(3)The activity of β-1,4-glucosidase(BG)of 0-10 cm soil layer increased significantly after the expansion of Caragana erinacea,decreased significantly after the expansion of the Sibiraea angustata and decreased after the expansion of Spiraea alpina and Potentilla fruticosa,but the change was not significant.The activity ofβ-1,4-N-acetylglucosaminidase(NAG)in soil increased significantly in grassland encroached by Caragana erinacea and decreased significantly in the other three shrub-encroached grassland.The activity of L-leucine aminopeptidase(LAP)in soil increased significantly after the expansion of Caragana erinacea,and decreased significantly after the expansion of Sibiraea angustata.The expansion of Spiraea alpina and Potentilla fruticosa had no significant effect on LAP activity.The expansion of Caragana erinacea and Sibiraea angustata lead to the maximum of acid phosphatase(AP)activity in soil at mild shrub-encroached grassland.Based on the soil enzyme stoichiometric ratios and their comparison with the global average,it was found that the study area was generally lacking of P element.The changes in AP enzyme activity and vector angle indicate that the relative P restriction of microorganisms reached the maximum during the initial encroachment stage of these two species,and Caragana erinacea was more severely restricted by P than Sibiraea angustata in the mild stage of shrub encroachment.The expansion of Spiraea alpina and Potentilla fruticosa had overall alleviated the P restriction on microorganisms,but the restriction had not yet been completely removed.The soil microorganisms in native grassland before Caragana erinacea encroachment into were also subject to N restriction,after the encroachment by Caragana erinacea,the soil microorganisms were weakened by N restriction,but they were still restricted by N.In addition to N:P of enzyme,the other two enzyme stoichiometric ratios and four enzyme activities were significantly positively correlated with soil BD,SWC and p H.BG,NAG and LAP were significantly positively correlated with soil C,N,P and their stoichiometric ratios.In a conclusion,shrub encroachment had changed the nutrient constraints of the alpine meadow ecosystem in the Qinghai-Tibet Plateau.The plant-soil-enzyme system of alpine meadow in the Qinghai-Tibet Plateau was mainly limited by N and P.The expansion of nitrogen-fixing shrubs alleviated the N limitation of the plant-soil-enzyme system,but increased the P limitation.The expansion of non-legume nitrogen-fixing shrubs relatively alleviates the P limitation,but increases the N limitation. |
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