| The tableland of Loess Plateau is located in the northwest of China.Soil erosion is increasingly serious because of the special geographical environment and climatic characteristics,hydraulic erosion,wind erosion and freezing and thawing erosion are interlaced,and frequent human activities damage the grassland and natural vegetation,making it inherently fragile.The ecological environment is getting worse.Consequently,China has adopted bio-engineering measures such as the construction of the Three-North Shelterbelt,returning farmland to forests and grasslands,and has achieved certain ecological benefits.As an important gauge for measuring the ecological effects of vegetation restoration,vegetation carbon sequestration can provide reference for vegetation restoration in ecological restoration areas.In this paper,different vegetation types in the study area were used as research objects.The CASA model and soil respiration estimation model were used to simulate the vegetation NPP and Rh in the past 30 years.The spatio-temporal characters of vegetation carbon sequestration?NEP?were analysed.The influencing factors were analyzed,and combined with the MRT?mean residence time?of soil organic carbon,the temporal and spatial distribution characteristics of terrestrial carbon cycle under the ecological restoration background of the study area were discussed and predicted.The research results are as follows:?1?The inter-annual variation of NPP in the tableland of Loess Plateau were estimated by CASA model,which shows that the NPP with a significant growth trend from 1985 to1999;the annual NPP decreased first and then increased from 2000 to 2015.The seasonal variations of NPP are obvious,high in late spring and early autumn,and low in winter.The spatial distribution of annual NPP varied significantly over the region,with NPP hign in the southwest,but low in the northeast.The NPP in the area shows an increasing trend.Annual NPP values significantly increased are mainly concentrated in grassland cover areas;the areas where NPP declines are mainly scattered in the construction.The inter-annual NPP of different vegetations changed smoothly.From 1985 to 1999,the annual NPP of each vegetation was forest land>cultivated land>grassland>bare land and sparse vegetation.Among them,bare land and sparse vegetation showed significant downward trend;The annual NPP of each vegetation was deciduous broad-leaved forest>mixed forest>cultivated land>grassland>shrub>bare land and sparse vegetation during 2000-2015.Mixed forest,grassland and bare land and sparse vegetation showed a significant upward trend.?2?The soil respiration model was used to estimate the Rh of the tableland of Loess Plateau.The annual soil Rh in study area showed a significant growth trend from 1985 to2015.The soil Rh performance during the year was low in summer and high in winter.The spatially distribution of Rh is hign in the southwest,but low in the northeast.From 1985 to1999,the annual soil Rh value of different vegetations was forest land>cultivated land>grassland>bare land and sparse vegetation.The inter-annual soil Rh value of vegetation changed smoothly,without significant increase or decrease trend.The importance of annual Rh in vegetation was deciduous broad-leaved forest>mixed forest>cultivated land>shrub>grassland>bare land and sparse vegetation.Bare land and sparse vegetation showed a significant upward trend,but others with unsignificant change.?3?As a whole,the study area is a carbon sink.From 1985 to 1999,the NEP in the tableland of Loess Plateau was in a fluctuatly increasing trend.During the year,the seasonal variations of NEP were significantly,the lowest value appeared in July,and the highest value appeared in May.In the period of time,NEP showed a trend of lower first and then rising,and the whole was carbon sink year,but the carbon source year appeared in individual years.The lowest value appears in June.From the perspective of seasonal changes,the first half of the year in the study area is characterized by carbon accumulation and the second half is carbon consumption.Among all vegetation types,forest land belongs to high carbon sequestration vegetation(140 g C·m-2·yr-1 or higher),followed by mixed forest(130 g C·m-2·yr-1),NEP value for cultivated land and shrub is close,and the carbon sink capacity of grassland and bare land and sparse vegetation is weak.Among them,from 1985 to 1999,the carbon sink area accounted for more than 74.91%of the total area,mainly distributed in the area of forest,and construction land were concentrated as carbon sources;from 2000 to 2015,the carbon sink area was about 60.08%of the total area.?4?The spatial variation characteristics of NPP,Rh and NEP are the result of the combination of geography,meteorology and vegetation.Vegetation elements are the decisive factors to NPP,meteorological elements are the determinants to Rh,and NEP was affected by various factors such as temperature,precipitation,solar radiation,latitude and longitude,elevation,slope,and aspect,the influence degree of each influencing factor is latitude>solar radiation>temperature>longitude>precipitation>elevation>slope>slope direction.?5?There are interaction between SOC,NPP and Rh in the tableland of Loess Plateau.The soil carbon density increased by the carbon balance model for the past 30 years,decreased first and then increased.The mean retention time of SOC in the study area was between 4.9 and 7.1 yr,and it was extended year by year,indicating that the local carbon cycle has an accelerated trend. |
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