| Desert grassland is one main utilizable land type in northern China and is seriously threatened by desertification.The main measures for protecting and stabilizing it are planting sand-fixing vegetation like Haloxylon ammodendron.Over the years,the effect of doing so on preventing and controlling desertification and stabilizing the grassland has gained the approval of governments,people and scholars.However,the effect of such vegetation on carbon sequestration has long been ignored and the full and reliable empirical data concerning it is still lacking.Shiyang River basin is located in an environmentally fragile and climatically sensitive area,one of the earliest desert grasslands to control desertification and grow sand-fixing vegetation.The desertification and growing sand-fixing vegetation in the basin are typical of arid areas in China.This research is mainly conducted on three levels,experiment,simulating with a model,and verification.Firstly,with the sand-fixing vegetation in the basin as the research objects,this study calculated the current carbon stock by such vegetation in the basin,and revealed the process,regularity and rate of sequestrating carbon by typical planted sand-fixing vegetation—H.ammodendron,by using plots.It also analyzed the impact of wind erosion and dust fall on carbon sequestration by planted H.ammodendron.In addition,based on the type II forest inventory data of Gansu province,the author used Carbon Benefits Project-Modelling Tools(CBP)to simulate and predict the carbon sequestration benefits of sand-fixing vegetation in the basin,its future change in carbon stock,and its potential in carbon sequestration against enclosure and reconverting cultivated land to forestry.At last,the simulation has verified by the measured results of carbon storage of artificial sand-fixing vegetation in Shiyang River Basin in field.The major results are as follows.Firstly,growing sand-fixing vegetation significantly increased the organic carbon stock of the vegetation and soil in arid desertified areas,with the most evident increase in that in the surface soil at the depth between 0 and 5 centimeters.The carbon stock of the 35th-year H.ammodendron reached 0.75 Kg C/m~2,and its average carbon sequestration rate was 0.02 Kg C/m~2/a,25 times that of migratory dunes.Its carbon sequestration rate was similar to that of the stable sand-fixing Nitraria tangutorum shrubs in the same areas,while its carbon stock was only 1/3 of that of the latter.The carbon stock in the soil of the 35th-year H.ammodendron reached 2.48 Kg C/m~2,and its average carbon sequestration rate was 0.07 Kg C/m~2/a,3.5 times that of migratory dunes.However,the rate was only 29.4%of that of stable sand-fixing Nitraria tangutorum shrubs.The stock of organic carbon in the soil system of the35th-year H.ammodendron was up to 3.23 Kg C/m~2,and its average rate for sequestrating carbon was 0.09 Kg C/m~2/a,3.4 times more than that of migratory dunes.However,the rate only accounted for 35.1%of that of stable sand-fixing Nitraria tangutorum shrubs.Evidently,typical planted sand-fixing H.ammodendron has greater capacity for carbon sequestration.Secondly,the carbon stock of the above-ground planted H.ammodendron increased first with the increasing afforestation years and decreased later.The stock in the soil increased with the increasing afforestation years,while that in the vegetation-soil ecosystem first increased,then decreased with increasing afforestation years,increased gradually later.Such stock of the planted above-ground H.ammodendron increased fast between the first and the tenth year,reaching its peak value of 2.74 Kg C/m~2 in the 10th year,3.65,3.01,and 6.68 times that in the 35th,25th and 3rd year respectively.By contrast,the carbon stock in the soil reached its peak value of 2.48 kg C/m~2 in the 35th year,1.09,1.56 and 2.41 times that in the 25th,10th and 3rd year respectively.The carbon stock in the vegetation-soil ecosystem reached its peak value of 4.34 kg C/m~2 in the 10th year,1.34,1.36 and 2.99 times that in the 35th,25th and 3rd year respectively.This indicated that the total carbon stock of planted H.ammodendron was significantly affected by that of sand-fixing vegetation,implying that the growth of planted H.ammodendron determined the carbon stock of its ecosystem.Thirdly,wind erosion and dust fall were the most frequent wind-sand activity in deserts and made up an important factor influencing the carbon sink of planted H.ammodendron.The carbon input rate by dust intercepted by the sand-fixing vegetation from wind erosion had a five-year average rate of 1.12±0.42 g C/m~2/a.With the mature 35th-year H.ammodendron as the reference,the carbon sequestration rate by sand-fixing vegetation in stopping the wind erosion only accounted for 1.58%of that of its soil,and 1.21%of that of its system.At the underlying surface of the sand-fixing vegetation,the carbon input rate caused by dust fall was only 2.47±0.47 g C/m~2/a.With the 35th-year H.ammodendron as the reference,the carbon input rate caused by dust fall only made up 3.48%of the yearly rate by the soil of sand-fixing vegetation,and 2.67%of the carbon sequestration rate of its system.Fourthly,the carbon flux in the soil was an important part of the carbon cycle in the soil.The carbon flux of typical sand-fixing vegetation—H.ammodendronis,showed evident seasonal and day-and-night variations,mainly impacted by the tree age,soil moisture and soil crust.The carbon flux of H.ammodendron of different age displayed evident seasonal changes,with the flux in the growing season(i.e.August)greater than that in non-growing season(i.e.January).The daily change in the carbon flux of H.ammodendron of different age in growing and non-growing seasons formed a clear unimodal curve and fluctuated to some extent.The highest daily emission rate appeared between 12:00 and 14:00 o'clock,while the lowest appeared around 8o'clock.Growing H.ammodendron and getting rid of soil crust significantly increased the carbon flux in the desert soil.The carbon flux of planted H.ammodendron of different age generally increased with the increasing afforestation years.Soil moisture was the key factor impacting its carbon flux.The flux in both growing and non-growing seasons were significantly related to the soil moisture at the depth from0 centimeter to 5 centimeters,and formed a quadratic curve.Fifthly,the prediction using GBP showed that the carbon stock of planted sand-fixing vegetation in the 39th year was 0.81 Tg C,and the carbon stock rate is0.045 kg C/m~2/a.GBP predicted that the maximal carbon sequestration capacity of the sand-fixing vegetation in the Shiyang River basin over 100 years was 2.19 Tg C,2.71times its current carbon stock,and the carbon stock rate is 0.19 kg C/m~2/a.From the35th year to the 100th,the GBP predicted that the carbon stock of the sand-fixing vegetation in the basin decreased gradually and became carbon emission after 80th year,reaching 0.44 Tg C in the 100th year.In the 35th year,a 34.5%difference was found betwwen the prediction using GBP and that of the sand-fixing vegetation calculated using plots.This verified the certain applicability and effectiveness of GBP in arid desert areas.The carbon stock in the Shiyang River basin was mainly distributed in Minqing Country and Liangzhou District,accounting for 92.4%of the total stock of sand-fixing vegetation in the basin.The stock of organic carbon of planted H.ammodendron reached 1.15 Tg C,84.8%of that of the organic carbon of the sand-fixing vegetation in the basin.Lowering the death rate of the vegetation,enclosure and growing vegetation could significantly improve the benefits of carbon sequestration of the sand-fixing vegetation.Sixthly,over the scale of 35 years,the carbon-sink benefits of the sand-fixing vegetation in the Shiyang River basin increased first and decreased later with the increase in the afforestation years.The mechanism for this was that in the first ten years,the H.ammodendron flourished gradually and the carbon sink of the vegetation increased fast and reached its peak in the 10th year.With the increasing afforestation years,the planted H.ammodendron started to decline and die and the carbon sink of the vegetation started to decrease,causing the carbon density of the ecosystem to decrease.The key in controlling and improving the carbon sequestration effects of the sand-fixing vegetation in the basin lies in making the soil moisture higher than the wilting point for vegetation and ensuring the normal growth of vegetation and maintaining the stability of the desert grasslands.In order to maintain the carbon sink benefit and avoid becoming a typical carbon source in the future,the planted sand-fixing vegetation in Shiyang River Basin needs to be conserved and restored.This study revealed the process,regularity and rate of carbon sequestration by the typical sand-fixing vegetation,H.ammodendron,and the changing tendencies of the carbon flux in soil and its impacting factors.This can make the important evidence for calculating the carbon stock in the ecosystem of desert grasslands in arid areas.It calculated the proportion of carbon sequestration by wind erosion and dust fall in that by the sand-fixing vegetation,an innovation in predicting the carbon sink in the arid desert areas.It also used CBP for the first time to simulate and predict the benefits and capacity of the sand-fixing vegetation in the Shiyang River basin for sequestrating carbon,and the results by using plots verified the effectiveness of the tool.The study can provide reference for accurately calculating and improving the carbon benefits of the sand-fixing vegetation in arid areas,significant for the ecological maintenance and scientific protection of the shrub grasslands in the arid deserts. |
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