| The restoration of Land Used for Qinghai-Tibet Highway Construction (LUQHC) which is usually considered the key to ensuring the safety of roadbed and erosion control, is an important part to restore degraded alpine grassland in the Qinghai-Tibet Plateau, China. In the future, due to the national macroeconomic policies of building railway and implementation of "One Belt and One Road", the potential of transportation construction in western China will be further released, thus the lands used for transportation will be created numerously. However, the alpine grassland in the Qinghai-Tibet Plateau which is fragile and sensitive to human disturbance, is difficult to recovery after damage caused by human disturbance, so the research concerned the restoration of plant on land used for highway construction is important and necessary. C, N, P stoichiometry is considered an effective tool to disclose the synergistic adaptation of plant growth and element availability in the process of plant restoration, and the spatial pattern of plant population is considered an important means to analyze the response of plant restorable communities to differentiated environment on LUQHC. A field survey along Qinghai-Tibet Highway was carried out in 2012 and 2013, to clarify the response of the spatial pattern of plant population in restorable community to the differentiated environment caused by Qinghai-Tibet Highway Construction, to investigate whether the environment caused by LUQHC contributes to shifts in the C, N, and P stoichiometry of the leaves and soil of the plant community, to explore the correlation between the spatial pattern of plant population and C, N, and P stoichiometry of the leaves and soil of the plant community, and to identify the N-or P-limitation status of plants in the natural community and restorable communities growing for nearly 18-19 years as part of the natural regeneration of LUQHC. The main conclusions are as follows:1. The restorable degrees of restorable community varied by locations and the succession direction of restorable communities in some sites remained unclear.The restorable plant of LUQHC experienced nearly 20 years of natural restoration had gradually achieved the goal of plant establishment. The coverage, biomass, and composition of restorable community showed natural resilience to some extent, but the plant composition still showed a difference between restorable communities and natural communities. The succession direction of restorable communities in some sites still remained unclear. The new damage caused by road rebuilt and human disturbance should be avoided and some necessary assisted recovery measures may been taken, to insure an appropriate condition for plant establishment on LUQHC.2. The spatial pattern of restorable plant population was spatially aggregated, and the intensity of clumped populations in restorable community was higher than that of the paired populations in natural community.The spatial pattern of population in restorable and natural community were both spatially aggregated, and the intensity of clumped distribution of population in restorable was mainly higher than that of clumped population in natural community. This spatial pattern of population was controlled by the biological characteristics and the differentiated environment on LUQHC. The plant populations in restorable community preferred to clumped populations were in favor of increasing the plant adaptation to habitat and population effect which was beneficial to the development of plant population. In addition, this pattern of clumped population could reduce the interspecific competition in restorable community, which was of benefit to promoting richness of species, plant coverage and the restoration of plant on LUQHC.3. The differentiated environment caused by LUQHC contributed to shifts in the C, N, and P stoichiometry of the leaves.The C concentration in restorable community was consistent with that in natural community. The leaf N and P concentration of restorable communities were both higher than those of natural communities. A high percentage legume plant and relatively adequate available N supplement may be the major reasons for higher leaf N concentration in restorable communities. Restorable community was easy to accumulate the P from adjacent grassland by surface and subsurface runoff because of the lower terrain in LUQHC. The lower coverage in restorable community could enhance the soil temperature, furthermore, the warmer soil temperature is benefit for promoting microbiological activity which is correlated with P decomposition from plant litter. There were no significant differences between restorable community and natural community in terms of leaf C:N and C:P ratios, indicating that the efficiency of N, P nutrition utilization was not apparently influenced by the LUQHC.4. The differentiated environment caused by LUQHC contributed to shifts in the stoichiometry of main nutrient elements in soils.This study showed that the plant and soil system had gradually restored after nearly two decades natural restoration, and the soil fertility had been undergoing a positive evolution. The soil organic carbon (SOC) of the restorable plots at both 0-10 cm and 10-20 cm soil depths were significantly lower than those of natural plots, and the soil total nitrogen (STN) shared the similar pattern with SOC. The soil total P (STP) of the restorable plots was same with that in natural plots. The SOC:STN in restorable community had no significant difference with that in natural community. The SOC:STP in restorable was lower than that of natural community, indicating that the restorable community had relative higher efficiency of P element utilization. STN: STP had significant differences between restorable community and natural community.5. The correlation existed in the three kinds of variables:spatial pattern of plant population, leave C, N, and P stoichiometry and soil main nutrient stoichiometrySpatial pattern of plant population in this study was significantly correlated with leaf P concentration and C:P ratio, indicating that the higher P concentration in restorable community was influenced by spatial pattern of clumped populations to some extent. The K had significantly positive correlation with STN, and this result indicated that the restorable plant on LUQHC preferred to more clumped distribution when soil N element of restorable plots was relatively lower. Leaf P was significantly positively correlated with STP, and leaf C:N and C:P ratios were significantly negatively correlated with STP.6. Plants in study area were mainly P-limited, and the patterns of N, P-limitation in restorable community and natural community varied by yearsIn northern Qinghai-Tibet Plateau, the alpine grassland plant leaf N:P of 2012 had no significant difference with that of 2013 at species level. Using the thresholds of N, P-limitation, and the results of the correlation between leaf elements and soil nutrients, this study shows that plants were probably P-limited in the study region, with plants in the restorable communities suffering from more intense P-limited conditions than those in the natural communities in 2012. However, in 2013, plants in the restorable communities tended to N and P co-limitation. |
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