Table 1 Chemical analysis of all sands in deserts

    The SEDEX method was used for the measurement of particulate P speciation in the dust and sediment. The extraction procedure consists of five sequential leaching steps which extract the following: (1) exchangeable and loosely bound P (P_ex), (2) iron-bound P (P_Fe), (3) authigenic and biogenic apatite and calcium carbonate bound P (P_aut), (4) detrital apatite and any other remaining inorganic P phases (P_det), and (5) organic P (P_org). Details of the extraction were given elsewhere^[7] .
    In most solutions phosphate was analyzed spectrophotometrically by the standard phosphomolybdatre blue method. This method could not however be used directly for the CDB solutions, since the extraction interferes with the molybdate blue reagent. These solutions were first reacted with a 1% FeCl₃ solution, extracted into isobutanol and then analyzed by spectrophotometer^[8].

Table 2 SEDEX Procedure. The extraction was carried out sequentially, with 25ml of solution and 250mg of sample being used.

Table 3 Content of phosphate in sands at monitoring sites

    Most sands over 0.1mm will settle onto the river bed and that small than 0.1mm will deposit on the bank or float over the water. TP content of sand in western Inner Mongolia desert ranks between 390.38-1888.18mg/g. Heifengkou, Tuo county in Kubuqi desert, Xiangshawan of Kubuqi desert and Dengkou of Wulanbuhe desert run from high to low in terms of phosphorus content. And the smaller the size of sand, the higher is the phosphorus content. Heifengkou region used to be a lake area with high productivity. It is now part of Ejina oasis and locates next to Mongolia grasslands. That is why the sand phosphorus in this region ranks first. Whereas with the situation of Tuo County in Kubuqi desert, the soil along seasonal rivers is fertile and sands in the region is relatively small. When sands containing phosphorus settle down in the Yellow Rivea, P_ex will be released and phosphorus level in the upper water gets increased. P_ex refers to phosphorus absorbed by oxide, hydroxide and clay minerals in the sediments. With the combined effects of T, pH, water and bioprocess, P_ex will be released to join in re-circulation. According to calculation, each year Wulanbuhe desert will send 11,000 tons of TP to the Yellow River, out of which 4920 ton is P_ex. And Each year Kubuqi desert will send 9,200 tons of TP to the Yellow River, out of which 688 ton is P_ex.. P_ex is the weakest phosphorus in combinability. P_ex released in the water is either absorbed by aquatic plants and animals or transported to the ocean to promote primary productivity of sea waters. It is very convenient for the two deserts to contribute sand into the Yellow River, with Wulanbuhe being the first contributor of sediment TP and Kubuqi Xiangshawan being the first of dissolvable phosphorus. The content of bio-available phosphorus like P_Fe、P_org are also very high in the two deserts. These are released in the process of oxidation-reduction, organics mineralization and decomposition and therefore become the phosphorus source of sediment in rivers and oceans.
    Table 3 shows that P_au is the main specification of sand in western Inner Mongolia, and which accounts for 41.3 to 73.6% of the TP. P_det from igneous rock and metamorphic rock is also very high, taking a share of 16-45% of TP. In Kubuqi desert, P_aut, P_de, P_Fe P_ex and P_org run from high to low. In Wulanbuhe desert it takes an order of P_aut, P_det, P_{Fe ,} P_org, P_ex from high to low. The content of phosphorus is relevant with the size of sand, with P_det and P_aut being dominant for 130-150mm and <63mm sand and P_Fe being rich in bigger sands. This is because P_Fe is likely to co-precipitate with ferric oxide like szomolnokit, lepidocrocite and goethite, while the content of ferrous free oxygen is high in 130—150μm sand. Relatively more light, heat, water and efflorescence effect results in higher iron oxidation and P_Fe, which in turn explains high content of phosphorus in Kubiqi desert.
    The fact that P_det is over 130-150mm in<63mm sand demonstrates P_det high in the sands of western Inner Mongolia. The content of P_org displays the same characteristics as the content distribution of sand organism, which means that <63mm sand is higher than 130-150mm sand. That is because P_or forms a part of organics. And the smaller is the size, the wider is the specific area and the easier for organics to absorb.
3.2 Heifengkou Sand
Heifengkou is today's Xijuyanhai or Shungenuoer region. A very dry piece of land now, it locates in the northern part of Badanjilin desert and adjacent to the People's Republic of Mongolia.
    In 2005 Zhang Kai's^[9] research showed that 48.6% of dust occurred in the 12 sand storms in 2000 was originated from western Inner Mongolia desert, 38.0% of dust occurred in the 18 sand storms in 2001 was originated from western Inner Mongolia desert, and 25.0% in the 12 sand storms in 2002 was originated from the region. The research also found that the dust floated from western Inner Mongolia desert and loess plateau to the vast North China plain, Bohai Sea, Yellow Sea, sea of Japan and the Pacific. The western desert of Inner Mongolia has become one of the four high-frequency sand storm region. It is a very important component of central Asia dust area and the source of middle-route sand storm in the country.
    Table 3 shows TP in Heifengkou can be as high as 961.49μg/g, of which sands <63μm being the highest and reaching 1888.18mg/g，the next being natural size and last being sand of 130-150mm. Sands over 63μm is not likely to transport with wind. The majority part of sands, or 99.4%, in Heifengkou area falls in this category and indicates the potential expansion of deserts. The sands smaller than 63μm will become flying dust and run through the northern part of China to coastal regions. It is a source of phosphorus in the nearby sea water and the Pacific. Thus Heifengkou forms the transport mechanism of phosphorus to the ocean. Study conducted by Zhang Xiaoye^[10] in 2001 showed that in northwestern part of China 20% of dust containing phosphorus settled down on the loess plateau and 50% was transported to the faraway Pacific. In 1950s a belt of quartz-containing dust sediment were found in northern Pacific. The latitude, between 30°and 40°N, is almost the same as loess plateau's distribution^[11].
    P_ex and P_org are the dominant content of bio-available phosphorus in Heifengkou sands, with P_ex hits 19.78mg/g to 31.17mg/g and P_org hits 68.22mg/g to 90.22mg/g. Both are higher than those in Wulanbuhe desert and Kubuqi desert. When Heifengkou sands carrying phosphorus falls in the ocean, part of DIP and DOP will resolve in the water. Since DIP and DOP concentration is very low in the sea water, the sands from Heifengkou becomes the source of phosphorus. Research conducted by Dong Yemaixing^[12] in 1996 showed that phosphorous in the water was almost all absorbed by species and became bioclast, which would go further deep into the sea and be resolved into the sea by micro-organism. The part that is not dissolved will enter the deep-sea sediments. Phosphorus in surface water will participate in the species cycle, which is obviously a bioactive process.
    The sand storm is a key link in nature cycle and global biogeochemistry. It even plays an important role in protecting environment like purify air, reducing acid rain, promoting growth rate of marine biology and lower the pace of global warming. Sands is rich in Fe and P, which sea waters lacks.

    Because Wulanbuhe and Kubuqi deserts are the main source of Yellow River sediments in Inner Mongolia, 35% of the sediments is α- quartz，10-15% is clay minerals and 30% is plagioclase and orthoclase combined. This is a distinctive feature of surface sediments in the Yellow River.
    Table 5 displays the chemical and physical characteristics of water in different parts of the Yellow River’s mainstream. It shows high pH, high DO and high Eh value, which in turn means Fe(III) forms the dominant part in the Fe of P_Fe. After the section of Wuhai and Linhe, the Yellow River enters the Hetao area featuring broad irrigation. The degree of mineralization and electric conductivity get increased. The content of organic matter is relevant with the P_org content.

Table 5 The chemical and physical characteristics of water in the Yellow River

Table 6 The content of each species of phosphorus in surface sediments in the mainstream of Yellow River(μg/g)

    Table 6 shows the phosphorus content of each specification in surface sediments from the mainstream of Yellow River. The content of TP is between 1322.09mg/g and 1668.88mg/g, averaging at 1515.46mg/g and higher than the average amount of 638mg/g in loess plateau. The surface sediment of Yellow River as the major mechanism to transport phosphorus to the sea is proven. Phosphorus in the surface sediment of Yellow River is mainly IP amounting between 1226.75mg/g-1546.31mg/g and averaging at 91.95% of the TP. Of this P_aut is the main content. Phosphorus transported to the Yellow River is also mainly P_aut. The large amount of sands going into the water in Yellow River basin and great distribution of soil determine the high content of P_au in water sediments. The water sediment is rich in CaCO_3. In this area the soil is basicity, the climate is cold and dry, chemical weathering is weak. Together with little water and large amount of sand, the accumulation process is very fast. All these factors make it easy to form apatite mineral. In the sediment P_aut is the important storehouse of phosphate from waters of river mouth and Bohai sea^[14]. Like P_det, it is difficult for P_aut to turn to phosphate and has little effect on the enrichment of phosphate in small opening and top waters. These two specification of phosphorus cannot be used by species and will be buried in river courses and river mouth regions, or they will be transported to the sea for deposit and become the main source of P_aut and P_det on ocean edge and in ocean sediments.
    P_Fe content in surface sediment of Yellow River is around 1.49mg/g-21.26mg/g, averaging at 7.79mg/g or 0.51% of TP. This is because of its low content of ferric oxide, rich oxygen and high degree of mineralization. Under the Yellow River waters' Eh, Fe（III） is the dominant Fe. The P_Fe reactivity is low and likely to precipitate.
   P_ex content in surface sediment of Yellow River is around 6.58mg/g-27.84mg/g, accounting for 1.16% of TP. The value is determined under the solid/liquid effect of special chemical environment when the sands run into the river and a thermodynamic equilibrium of phosphate is reached. The low P_ex content is relevant with the mineral composition of surface sediments. 35% of Yellow River sediment is a-quartz，ie SiO₂, whose basic structure is SiO₄ formed with Si-O-Si-bond, because -Si-O-Si-bond is water repellent and layer charge is zero. The surface interaction is weak. The exchange process of surface sediment is mainly formed by clay minerals which are primarily crystalline aluminum or magnesium silicates with stacked-layer structure. Each unit layer is in turn one dimensional array. In illi, about 1/4 Si of is replaced by Al, which is TOT three-layer and chlorite is also three-layer clay. Clay layer distance is a special chemistry reaction place, which is characterized by layer exchange and layer absorption. Only 25% of the sediment is clay minerals like illi stone, out of which 105-125mm sand has a relatively higher clay mineral content and serves as the main place for P_ex existence. P_ex is bio-available and exchangeable phosphorus. It is also considered the most effect one. When phosphate content is very low in top waters, ion exchange can easily released P_ex who can be used by plankton or float into the ocean.
    P_ex, P_Fe and P_org, which total at the average level of 146.50mg/g and account only for 9.69% of TP, are potential effective phosphorus in surface sediment of Yellow River. The Yellow River contributes 1.6 billion tons of sand and soil to the Bohai sea each year^[15], Under the complicated effects of physics, chemistry and biology, if only 9.69% of phosphorus in the suspended surface sediment participates in the cycle of ecosystem, it will hold great impact on the eco-environment of Yellow River and the Bohai sea.
    It is not clear why the water of East Mediterranean is controlled by phosphorus. The coastal region of China shows a likely situation. Why is that？In 2002 Pan Gang^[16] studied phosphorus of Sahara dust, Nile particulate matter and East Mediterranean. The experiment and calculation found that when Sahara dust was put in the sea water of East Mediterranean, Sahara dust would release 3.3±0.3mmol P/g into the water and the phosphorus in the water stopped absorbing dust. Dust is the E·Med’s source of phosphorus. Therefore, Sahara dust might not be the limit factor of phosphorus in the region. When floods occurred before 1964, the Nile particulate matter would show reverse effect because the Aswan Dam hindered Nile's flood water from entering E·Med.Because the river Nile no longer floods into the Mediterranean basin after the completion of the Aswan dam, it is not possible to measure directly the past flux of P associated with particulates from the river into the basin. However P_au is possible to use the MEA model here to reconstruct the beharior of Nile river PM as it reached particulate flux was 25×10⁶ton/year. Using a water flux of 43×10⁹m³/year, Nile PM could adsorb P in river water(psink), and release it into the coastal water(source). The total P released from river PM into the Egyption coastal water during the Nile floods in this example was 4877 tons/year (can upper limit). The input of nutrients triggered a thick and continuous diatom blooms in the surrounding seawater, which supported large shoals of sardines. The sardine fishery collapsed in 1970s after the closure of the Aswan Dam. Thus, the damming of the Nile could reduce~8000tons/year P to the coastal water, which might play an important role in the P limitation in the region.
    Since 1954 many dams like Liujiaxia, Sanshengmo, Sanmenxia, Xiaolangdi and so on were built along the Yellow River with the purpose of lowering sand inflow. It remains a mystery whether this limited phosphorus in the inshore area of China because lack of historic data and difficulty of quantitative analysis. However, it is worthy of deep thinking and experiments.