The Degradation And Evolution Mechanism Of Wetlands At Southeast Of Mu Us Sandy Land

Wetlands are one of the three major terrestrial ecosystems, whose global area is about5×108~6×108hm~2, accounting for4%~6%of the Earth’s surface. Functioned as the "kidneys"of the earth, wetlands have significant effects on climate control, hydrological regulation, andpollution elimination. Large and small lakes and wetlands are distributed in the southeast ofMu Us sandy land. In recent years, due to climate change, resources exploitation, and humaninterference, the lakes shrink and disappear, and wetlands degrade. With the technology of3S,the paper interpreted the remote sensing satellite images of three periods of the study areasince1999, analyzed the landscape pattern and evolution process, and determined theecosystem stability. Besides, the distribution and response of biogenic elements in thedegradation process was studied through laboratory analysis of wetland soil samples withdifferent vegetation types and degradation stages. The results are as follows:(1) From a historical point of view, Mu Us sandy land already existed during the GreatIce Age (20000a BP). During the mid-Holocene (3000-8500a BP), climate warming andeastern desert moving westward made the deserts in Mu Su and the northeastern InnerMongolia disappear. The desertification became more serious in modern times due tointensifying drought and human war in Mu Us sandy land after several restoration anddesertification for3000years.(2) According to the wetland landforms and hydrological characteristics, the wetlands ofMu Us sandy land can be divided into4types: floodplain, lakeshore lowland, beach andinterdune depression. The study on the satellite images of the study area in1999and2010shows that the total areas of wetlands in1999and2010were respectively739286hm~2and496718hm~2. In1999the largest area was the floodplain (360709hm~2), accounting for48.79%of the total wetlands and mainly distributed in the south of the study area; the second waswater (184402hm~2),24.94%of the total area; the third was the interdune depression(88215hm~2),11.93%and mainly in the middle; then followed the beach (59514hm~2),8.05%and mainly in the south; the smallest was lakeshore lowland (46446hm~2), only covering6.28%. In2010, the largest area was the beach (152230hm~2), accounting for30.65%of thetotal wetlands and widely along two banks of Kuye River, Tuwei River, Yuxi River and Wuding River in the central north of the area； the second was the interdune depression(109102hm~2),21.96%and widely in the central north; the third was the lakeshore lowland(98061hm~2),19.74%； then followed floodplain (82301hm~2),16.57%and widely in thecentral south; the lakes in the study area were generally less than8m deep, and the rivers weremostly less than6m deep. Water occupied55024hm~2, accounting for11.08%of the totalwetlands, and mainly located in the central north. The statistical analysis shows that in thepast11years the water and floodplain areas decreased while the areas of beaches, interdunedepressions, and lakeshore lowland increased, indicating that the climate became increasinglydry and water resources less.(3) A net primary productivity model of natural vegetation was established with a view tothe ecophysiological features and regional evapotraspiration model relating water balanceequation with heat balance equation. It was calculated that the net primary productivity ofnatural vegetation in the southeast of Mu Su sandy land was3.63-5.57t/hm~2.a(0.99-1.52g/m~2.d), in line with the lower ecosystem productivity level (0.5-3.0g/m~2.d) in theOdum (Odum1959) classification. It will not decline to a lower level of natural system only ifit is above182.5g/m~2.a (0.5g/m~2.d). So it should be said that certain ecological carryingcapacity exists in the southeast of Mu Su sandy land. Under the current joint action of humaninterference, mineral exploitation and climate change, the sandy wetland ecosystem canmaintain certain dynamic balance as a whole and will not degenerate to a lower level ofecological system. Currently the sandy wetland ecosystem basically is stable.(4) The satellite image interpretation indicated that the area of Hongjiannao Lake, thebiggest lake in the southeast of Mu Su sandy land, shrank from4552.74hm~2in1999to3675.24hm~2in2010. It shrank at a relatively high rate before2006and degraded slowly from2006to2010. Shrub area had been increased and the ecological environment recovery wasgood. Interdune depression decreased before2006and increased afterwards. The shrink ofinterdune depression means the reduction of the water content and the transformation tofloodplain. The area of dune, decreasing before2006and slightly expanding afterwards, didnot change overall. With Hongjiannao Lake shrinking, the area of lakeshore lowlanddecreased as a whole. Through gray correlation analysis of natural and human factors, theinfluence order of Hongjiannao Lake degradation is as follows: arable area, humidity,temperature, population, rainfall, evaporation capability, sheep number, gross agriculturaloutput value, GDP and gross industrial output value.(5) There is no obvious variation pattern with depth for the organic carbon, total nitrogen,total potassium, total phosphorus, available phosphorus, and available potassium ofHongjiannao samples. With the emergence of the peat layer, the content fluctuated. The organic carbon, total nitrogen, total potassium, total phosphorus, available phosphorus,available potassium from Ba Xia Cai Dang samples showed a overall decreasing trend withincreasing depth and increased sharply with only one peak when the peat layer appears.Unlike general wetlands whose biogenic element content tends to decrease with depth, thebiogenic element content from Hongjiannao samples, from the profile characteristics, had noobvious decrease trend with depth, and multiple fluctuations appeared with the emergence ofthe peat layer, while the content from Ba Xia Cai Dang samples tended to decrease butincreased when the peat layer appeared. The features were unique to Mu Su sandy wetlands.Located in farming-pastoral ecotone, and water-wind erosion crisscross region, along with thesand deposition, underground water level falling, and alternation of drying and wetting, theaccumulation and distribution of the peat layer was quite different. Study on each sampleprofiles shows that peat layer of reed beach appeared roughly between60~80cm, peat layerof shrub and grass between100~120cm, and sandy peat layer between180~200cm. Fromthe perspective of vegetation succession and wetlands degradation, the more serious wetlanddegradation, the lower vegetation biomass and the underground water level, and the deeperpeat layer.(6)Comparison of the satellite images of Hongjiannao Lake samples in1999and in2010via GEO LINK indicates that the water surface of Hongjiannao Lake had shrank297.41m for11years. The analysis of degradation succession shows that the lake mud in1999haddegraded to the reed beach in2010, and the groundwater level had declined100cm. Thecomparison of biogenic elements distribution curves of the lake mud and the reed beachshows that there were three peaks in the organic carbon profiles of the reed beach, whichappeared respectively at the depth of30cm,45cm, and90cm. It can be conclude that in thewetland degradation process, organic carbon is gradually released. When water surfaceshrinks, the water content in the mud reduces, the surface redox environment changes fromanaerobic to aerobic, and organic carbon starts to decompose. With the passage of time, thesoil moisture content and the groundwater decreases further, and vegetation growth starts.Slow decomposition of organic carbon under40cm makes it possible to form the distributionpattern that currently the organic carbon content at the depth of90cm is equivalent to the mudsurface and the closer to the ground, the lower the organic carbon content is. In short, thedegradation process first is accompanied by water surface shrinking. When the originalanaerobic environment under waterlogged conditions changes, the biogenic elements tend todecrease. When the water surface stops shrinking, or even start to expand, the accumulationprocess begins and the fluctuation peak appears on the distribution curve. (7) The soil organic carbon reserve of the typical sandy wetland, Ba Xia Cai Dang, was30,300t, which was determined jointly by soil organic carbon density and area of eachvegetation type. The soil bulk density of the typical sandy wetland, Ba Xia Cai Dang, wassignificantly correlated to the organic carbon content (P<0.01).