| Vegetation restoration is the main measures to restoration and reconstruction of regionaldegraded ecosystem and its carbon sequestration benefits is one of the key factors to estimatethe effects of vegetation restoration. This study on the basis of predecessors’ work and ours, inview of the Loess Plateau ecological construction demand and the leading issues of ecologyand environmental science research, and taking restoration ecological system in the LoessPlateau as the research object, through the methods of combining the historical data collectionand field investigation, field experiment and indoor analysis, the typical sample sites and scaleexpansion, and in the background of the project of returning farmland to forest（Grain forGreen Program (GGP)）, to study carbon sequestration benefits evolution process and itsmechanism in the process of ecological restoration and reconstruction. This study evaluatedcarbon sequestration potential for “Grain for Green” Program; clarified the dynamiccharacteristics of the carbon sequestration and restrictive factors in the process of ecologicalrestoration and reconstruction; explored the relationship between the evolution of carbonsequestration benefits and vegetation restoration and reconstruction. These have importantreference value for scientific evaluation to carbon sequestration effect on vegetationrestoration environment, thereby to provide important scientific basis to guide the vegetationrestoration and reconstruction of ecological engineering. The main results are as follows:（1）‘‘Grain for Green’’ Program on the Loess Plateau plays a significant role infixing soil carbon storages. Restoration age is the main factor affecting the present soilcarbon sequestration.Land-use change is widely considered as a major factor that affects soil organic carbon（SOC）sequestration. The synthesis indicates that soil carbon sequestration potential of the“Grain for Green” Program in the Loess Plateau could reach0.59Tg yr-1, and the estimatedaverage soil carbon sequestration rate was0.29Mg ha-1yr-1; restoration age is the main factoraffecting the present soil carbon sequestration; annual average temperature and initial soilcarbon storages had a significant effect on the soil carbon sequestration, while the land-usechange types, climate zones and annual average precipitation showed no significant effect onthe soil carbon sequestration. When croplands converted to grasslands, the rate of soil carbon sequestration is the highest of the three land-use change types. The “Grain for Green”Program on the Loess Plateau plays a significant role in fixing soil carbon storages.（2） In the semi-arid environment, vegetation recovery following abandonment isslow and the improvement of soil properties is likely to require a considerably longperiod of time（>30yr）.The revegetation of abandoned farmland significantly influences soil organic carbon andtotal N（TN）. Top soil OC and TN were higher in older grassland, especially in the0-5cmsoil depths; deeper soil OC and TN was lower in younger grasslands（<20yr）, and higher inolder grasslands（30yr）. Soil OC and N storage（0-100cm）was significantly lower in theyounger grasslands（<20yr）, had increased in the older grasslands（30yr）, and at30yearsSOC had increased to pre-abandonment levels. Our results indicate that soil OC and TN weresignificantly and positively correlated, indicating that studies on the storage of soil OC andTN needs to focus on deeper soil and not be restricted to the uppermost（0-30cm）soil levels.Thus, in the semi-arid environment of the Loess Plateau, vegetation recovery followingabandonment is slow and the improvement of soil properties is likely to require aconsiderably long period of time（30yr）.（3） Vegetation restoration results in significant changes to ecosystem carbon poolproperties. Carbon storage in the soil was higher in the upper rather than the lower soillayers. However, the increments of carbon storage mainly changed in the lower soillayers.The results of investigate the temporal carbon sink and sequestration dynamics ofgrassland ecosystems in a long-term restoration period（~78yr）show that following theconversion of cropland to grassland, carbon sequestration values all increased at deeper soildepths of40-100cm; carbon storages within the0-40cm profile were largely unchanged.Five time intervals,0（cropland）,23,35,58and78years yielded carbon storages of7.69,14.58,16.25,19.22, and19.95kg m-2at the total carbon（plant and soil）storage scale. Themain finding indicates that the conversion of cropland to grassland results in significantchanges to ecosystem carbon pool properties. This finding has broad implications for theanthropogenic management of terrestrial carbon sequestration at the regional scale.（4） Changes in the carbon storage were the result of the accumulation of SOCand TN during forest succession and this capacity has shown to be positively related toforest succession.Land-use change resulting from natural restoration probably enhances the carbonsequestration capacity of terrestrial ecosystems. Long-term（~150yr）secondary forestsuccession the soil organic carbon storage, soil organic carbon, total nitrogen, and C/N ratio all increased rapidly and tended to be at their highest at roughly the50-years restoration mark.From this point onward the values gradually stabilized indicating that SOC and TNaccumulated mainly in the early restoration stages. Carbon storage was significantly andpositively correlated with SOC, TN, and C/N ratio（P<0.01）, and water storage in the soil wasnot the key factor influencing the soil carbon storage in the Ziwuling forest region where theannual rainfall is close to600mm. Carbon storage in the soil was higher in the upper ratherthan the lower soil layers. However, the increments of Carbon storage mainly changed in thelower soil layers. Soil water storage was not the key factor influencing carbon storage. Theresults suggested that changes to the Carbon storage were the result of the accumulation ofSOC and TN during forest succession and this capacity has shown to be positively related toforest succession on the Loess Plateau, China.（5） Carbon storage in vegetation is directly related to plant productivity; carbonstorage in the soil was not only related to plant productivity, but also to SOC and soilnutrients.The results of study the dynamics of plant and soil organic carbon pools following theconversion of sloping farmland to alfalfa fields over a period of30years show that soilcarbon storage（0-100cm）dynamics were consistent with belowground biomass storage withincreased planting years, but carbon storage always increased with the number of plantingyears in the0-5cm soil layer. Planted perennial alfalfa resulted in a decline in soil carbonstorage in the0-100cm soil depth in the early period（9yr）. During the late succession stageof alfalfa（13yr）soil carbon storage tends to recover, and after16years, storage values againdropped. However, it had recovered by30years at which time alfalfa productivity was verylow. Vegetation carbon storage was mainly decided by the belowground biomass andecosystem carbon storage dynamics was consistent with soil carbon storage. Vegetationbiomass, root/shoot ratio, SOC, soil total nitrogen and total phosphorus（TP）were the mainfactors affecting carbon storage in the entire alfalfa field ecosystem. The results suggest thatcarbon storage in vegetation is directly related to plant productivity; carbon storage in the soilthroughout the entire alfalfa field ecosystem was not only related to plant productivity, butalso to SOC and soil nutrients.（6） Long-term fencing to prevent overgrazing disturbance could greatly affectsoil carbon storages in comparison to grazed grasslands. The increment of soil carbonsequestration mainly occur in the deeper soil depths（30-100cm）due to long-termfencing.Overgrazing reduces plant species diversity, productivity and soil carbon storage due todegradation, especially in the arid and semi-arid ecosystems. Fencing increase vegetation coverage, height, plant diversity, biomass production and litter, resulting primarily from anincrease in the ratio of grasses functional group species, and increase the allocation ratio ofaboveground biomass owing to reduce the root/shoot（R/S）. There were significant effects offencing on soil bulk density（BD）, soil moisture（SW）and soil pH. Long-term fencing alsoled to marked increases in SOC, soil TN, C/P, N/P and soil carbon and nitrogen storages in the0-100cm soil depths, and C/N in top soils of0-5cm compared with grazed grasslands. Theincrement of soil carbon sequestration mainly occur in the deeper soil depths（30-100cm）dueto long-term fencing. Our study suggested that the long-term fencing to prevent overgrazingdisturbance could greatly affect soil carbon storages in comparison to grazed grasslands. Thefindings are important for assessing the resilience of these grazed-disturbed ecosystems anddeveloping a more effective strategy by means of fencing for the management of degradedgrassland from a long-term perspective in temperate grassland of the Loess Plateau, China. |
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